Preface - bücher.de · heat loss (Chap.6), and corrective lenses to improve vision (Chap.11). The...

Preface

Physics explains everything from the beginning to the end of any completedescription of the human body. Such a comprehensive discussion should beginwith the basic structure of matter, as explained by quantum mechanics – thephysics at small dimensions, and end with the mechanics of human motion,the energetics of metabolism, the fluid dynamics of blood flow through vessels,the mechanisms for speaking and hearing, and the optical imaging systemwe call the eye. All of required combinations of atoms to form the complexmolecules and organs of organisms that live and reproduce can be explained byquantum mechanics; however, such explanations can get pretty complex. Thefields of chemistry and biology have been developed, in part, to explain thegap between the extremes – the microphysics and macrophysics of organismssuch as the human body.

This book focuses mostly on the macrophysics end of the human body. Wewill assume that atoms form molecules that form cells that form organs. Wewant to understand the physics of human organs and of humans themselves.We will apply and somewhat extend freshman level physics to see how thebody works. In addition to applying physical concepts to the body, we willtry to understand the body from a viewpoint that is more numerical than isoften adopted in biological and medical presentations.

One way to characterize this text is by saying what it is and what it is not.It is certainly about the physics of the human body. It is not about humananatomy, although we will need to use some basic anatomical concepts. Itis not about human physiology, although it can be called a book about thephysics of physiology. It is not a monograph in biomedical engineering per se,although about half of this volume concerns biomechanics, one important areain biomedical engineering. Medical physics is more closely related to healthphysics, the use of ionizing radiation, imaging, and instrumentation than tothe macrophysics of the body. Biophysics concerns how physics can be used tostudy biology and focuses much more on the molecular basis and the cellularbasis than will we (see Appendix E). One could say that the physics of thehuman body is synonymous with understanding the human machine.

VIII Preface

Our goal is to understand physical issues concerning the human body, inpart by solving problems to further this understanding. The focus is not atall on learning and memorizing medical terminology. Still some very basicconcepts in anatomy and physiology will be introduced and used. Several ofthe many excellent general anatomy and physiology texts are cited at the endof the chapter [11, 16, 21, 22, 23, 24, 25, 26, 27, 29].

One theme that runs throughout this text is developing and then usingsimple and subsequently more refined models of the macrophysics of the hu-man body [7, 13, 15]. Physicists tend to model concepts in as simple termsas possible at first. For example, to zero order a physicist would model a cowas a sphere. (This is sometimes used as part of a joke.) We will get a bitmore complex here, but not much more. Another theme is to address issues inhuman biology quantitatively that are often addressed only qualitatively. Thecall for more quantitative thinking in physiology by Burton in Physiology byNumbers [5] is much appreciated by the author. In addition, we will presentreal physiological data and tie them with quantitative analysis and modeling.

If there is an applied force, energy, fluid flow, a light ray, an electric current,or an electric or magnetic field associated with the body, we will call it physicsand we will analyze it. We will tend to avoid topics that delve into morechemistry and biology issues, but will briefly address physical chemistry issuesinvolving concentration gradients and such, as they relate to fluid exchangein capillaries and conduction in nerves. Although we emphasize the physicsof the body over the instrumentation used to make physical measurementson the body and probe body function, such instrumentation is addressed asneeded.

Our intent is to use basic physics and not to teach it, particularly fromscratch. Many chapters include a brief review of the physics principles neededin that chapter and subsequent chapters. Some topics are developed a bit fur-ther, and some even a bit further – and these are identified as advanced topics.More detailed overviews are given for topics seldom covered in detail duringa two-semester physics course, such as fluids (Chap. 7), acoustics (Chap. 10),and optics (Chap. 11) and for areas used in several contexts, such as harmonicmotion (Chap. 3). Some differential and integral calculus is used. (Partial dif-ferentiation is used sparingly, and mostly in sections labeled as advanced top-ics.) A brief review of the solutions to the simple differential equations usedhere is presented in Appendix C to help students with a limited backgroundin calculus.

We will start with a comparison of medical and physics-type terminology inChap. 1. The first chapter also includes a discussion of the “standard” humanand introduces the concept of scaling relations. We can group the topics insubsequent 11 chapters into four areas in human body physics. (1) In Chaps. 2–5, the mechanics of the static body (Chap. 2) and the body in motion (Chap. 3)are analyzed and are then linked to the mechanical properties of the materialsof the body (Chap. 4) and the body’s motors: muscles (Chap. 5); these topicscan be characterized as Locomotion on Land. (2) The second topic, Energetics

Preface IX

of the Body Metabolism, is discussed in Chap. 6 and is needed to understandthe discussions of body locomotion and function that precede and follow it. (3)Chapters 7–9 cover the Locomotion of Humans in Fluids (other than on land)and the Motion of Fluids in Humans. Chapter 7 overviews the physics of fluidsand addresses locomotion in water (swimming) and in air – above ground (atleast, the prospect for human flying). Chapters 8 and 9 respectively coverthe fluidics of blood (cardiovascular system) and air (respiratory system) inthe body. (4) Chapter 10 explores the acoustics of sound waves in speakingand hearing. The optics of eyes and vision are investigated in Chap. 11. Basicelectrical properties of the body are developed in Chap. 12, along with a briefdescription of the magnetic properties of the body. So these three chaptersrespectively address sound, electromagnetic, and electrical waves, which wecan collectively call Waves and Signals. (The electromagnetic nature of lightwaves is not discussed in Chap. 11.)

Chapter 13 examines how the body automatically uses the basic engineer-ing principle of feedback and control in regulating all aspects of function.The physics of sensation of three of the five senses are described: hearing,seeing, and touch – the last briefly in Chap. 2. Some connection is made be-tween the physics of sensation, biochemistry of sensation, and perception (psy-chophysics) in Chap. 1. The sense of taste and smell are purely chemical, withlittle basis in physics (other than the chemistry of the molecular interactionsin each being clear applications of physics), and are not covered – except fora brief discussion of the electrical properties of the taste and smell sensoryneurons in Chap. 12. The emphasis throughout is on how physics can explainthe functioning of the body under normal and unusual circumstances. Wewill concern ourselves with the human body with its common body coverings:footware to minimize stress during movement (Chap. 4), clothes to regulateheat loss (Chap. 6), and corrective lenses to improve vision (Chap. 11).

The chapters are set more to address specific areas in physics rather thanspecific parts or systems in the body. It is difficult to construct chapters withclean divisions because different areas of physics are needed to understandmany components of the body. For example, to understand the physics of theheart, you need to address its role in circulation (Chap. 8), the action of mus-cles (Chap. 5, which is more focused on skeletal muscle than the fairly similarcardiac muscle), and the electrical signals generated by the heart (Chap. 12).

This text concludes with five appendices. Appendix A overviews symbolsand units, and references tables of units presented in the chapters. Appen-dix B lists the figures and tables that describe the main features of humananatomical and anthropometric information, which are used throughout thistext. The types of differential equations used in the text are reviewed in Ap-pendix C. These same differential equations are used throughout the text inmechanical, fluid flow, and electrical models; the connections between thesemodels are made in Appendix D. Appendix E attempts to define the field ofbiophysics, and connects the contents of this text with this field.

X Preface

This text has been developed from the author’s lecture notes developedfor the course Physics of the Human Body, which is a “professional-level” re-stricted elective course he developed taken mostly by first and second yearundergraduates in the Columbia University Fu Foundation School of Engi-neering and Applied Science. This course was designed so it could be takenby all first year students in their second term (in conjunction with second-term physics and calculus). The author usually covers Chaps. 1–10 in somedetail and Chaps. 11–13 in less detail in a full semester.

Courses at different levels, including mid-level and upper-level undergrad-uate courses, can be taught by purposely including or excluding more detailedand advanced topics in the text and problems. Depending of the level of de-sired depth, material in about half to all the chapters can be covered in oneterm.

This text can also be used as a companion volume in introductory physicscourses, and assist premedical undergraduates in learning and reviewingphysics. It can also serve as a text in introductory biomedical engineeringor medical materials courses. Medical students interested in a more quan-titative approach to physiology and those doing medical research may alsoappreciate the approaches adopted here.

Many problems are presented at the end of each chapter, ranging fromsimple to more advanced problems (the latter are denoted as such). Severalproblems have multiple parts, and only a few of these parts can be assigned.Answers to selected problems are given after the appendices.

Usually SI (MKS, m-kg-s) units are used; when more convenient, othermetric units, including CGS (cm-g-s) units and mixed metric units are used.English FPS (ft-lb-s) units are sometimes purposely used to make a connectionto the real world (at least in countries such as the USA and UK). For example,it would be strange to hear a baseball announcer say, “This pitcher is reallythrowing some heat. The radar gun clocked his last pitch at 43.8 m/s (or158 km/h)”, as opposed to 98 mph. It would be stranger to hear a football(i.e., American football) announcer say, “They have first (down) and 9.144to go”, meaning 9.144 m instead of 10 yd. Similarly, it would be strange todiscuss the physics of the body in these sports, such as in throwing a baseball,in any but the usual units. Angles are given in radians, except when usingdegrees gives a more physical picture.

Several excellent texts cover material that overlaps topics covered here,each with a different focus. They are magnificent resources in their own right.Physics of the Body by Cameron, Skofronick, and Grant [6] spans most of thetopics in this book and provides excellent physical insight. It is at a level ofphysics that is lower than that used here and derives and presents fewer ofthe equations necessary for a more rigorous treatment, but it provides a verygood basic background in human physiology for nonexperts. In a way, theemphasis of The Human Machine by Alexander [2] coincides with ours, but,again, the explanations are more qualitative. The mode of physical thinking itpresents is impressive. Physics with Examples from Medicine and Biology by

Preface XI

Benedek and Villars [3] is a series of three comprehensive introductory physicstexts in which excellent examples and problems have been chosen concern-ing the physics of the body. The authors have taken several fairly complextopics and have made them utterly understandable. Many other first-yeargeneral physics texts commonly used nowadays have several examples andchapter problems dealing with the body. Intermediate Physics for Medicineand Biology by Hobbie [14] is a more advanced text that emphasizes bothphysics and physical chemistry. Medical Physics and Biomedical Engineeringby Brown et al. [4] is a bit more advanced and focuses also on classic ar-eas in medical physics, such as radioactivity and instrumentation. Many ofthe illustrative problems concerning human biology and related topics havebeen collected in the beautiful books: Biomedical Applications of Introduc-tory Physics by Tuszynski and Dixon [28], Physics in Biology and Medicineby Davidovits [9], Biophysics Problems: A Textbook with Answers by Maroti,Berkes, and T’olgyesi [17], Physics for the Biological Sciences: A Topical Ap-proach to Biophysical Concepts by Hallett, Stinson, and Speight [12], andTopics in Classical Biophysics by Metcalf [18]. Many of the issues in exercisephysiology, such as the metabolism during sporting activities, are describedin elementary terms in Fox’s Physiological Basis for Exercise and Sport byFoss and Keteyian [10] and Physiology of Sport and Exercise by Wilmoreand Costill [30]. Basic Biomechanics of the Musculoskeletal System, editedby Nordin and Frankel [20] is a comprehensive and clear overview of the bio-mechanics of structures, joints, and motion. The applications of physics at amore molecular and cellular level, more in the classical domain of biophysics,are described in Biophysics: An Introduction, by Cotterill [8] and BiologicalPhysics: Energy, Information by Nelson [19]. The more general application ofphysics to animals is addressed in the exciting and very comprehensive bookZoological Physics: Quantitative Models, Body Design, Actions and PhysicalLimitations in Animals by Ahlborn [1]. All of these texts are highly recom-mended for more details. They, along with the anatomy and physiology textscited earlier, have contributed to the preparation of this text.

The author thanks the many people who have made valuable commentscontributing to this book, including Marlene Arbo, Gerard Ateshian, Sarba-jit Benerjee, Alex Breskin, Bill Burdick, Yi-Ting Chiang, Kevin Costa, TedDucas, Yossi Goffer, Daniel Herman, Jonathan Herman, Steven Heymsfield,Jeffrey Holmes, Mark Langill, Barclay Morrison III, Elizabeth Olson, ThomasPedersen, Harry Radousky, Paul Sajda, Michael Sheetz, and Samuel Sia. Hewould also like to thank the Columbia University Library system.

This author began writing this text when he was a Lady Davis Scholar onsabbatical at Hebrew University in Jerusalem as a guest of Uri Banin, and hegratefully acknowledges this support.

New York, NY, November 2006 Irving P. Herman

1

Terminology, the Standard Human, and Scaling

Several concepts will appear throughout our discussion of the human body:medical terminology, the characteristics of a “typical” human, and how bodyproperties and responses scale with parameters. Much of the problem we havein comprehending specialists in any field is in understanding their jargon,and not in understanding their ideas. This is particularly true for medicine.Much of medical jargon of interest to us is the terminology used in anatomy,and much of that in anatomy relates to directions and positions. To makethings clearer for people who think in more physics-type terms, we will relatesome of the anatomical coordinate systems used in medicine to coordinatesystems that would be used by physicists to describe any physical system.We will also extend this terminology to describe the degrees of freedom ofrotational motion about the joints needed for human motion. In all of ourdiscussions we will examine a typical human. To be able to do this, we willdefine and characterize the concept of a standard human. The final concept inthis introductory chapter will be that of scaling relationships. We will examinehow the properties of a standard human scale with body mass and how theperception level of our senses varies with the level of external stimulus.

1.1 Anatomical Terminology

The first series of anatomical “coordinate systems” relate to direction, andthe first set of these we encounter is right vs. left. With the xyz coordinatesystem of the body shown in Fig. 1.1, we see that right means y < 0 andleft means y > 0. Right and left, as well as all other anatomical terms, arealways from the “patient’s” point of view. This was made perfectly clear to theauthor during a visit to his son’s ophthalmologist. When he tried to discusswhat he thought was his son’s right eye, it was pointed out to the author inno uncertain terms that he was really referring to the patient’s left eye andthat he was doing so in an improper manner. Case closed! (Stages in theatershave a similar convention, with stage left and stage right referring to the leftand right sides of an actor on stage facing the audience. This was evident in

2 1 Terminology, the Standard Human, and Scaling

Fig. 1.1. Directions, orientations, and planes used to describe the body in anatomy,along with common coordinate systems described in the text. We will assumeboth terms in the following pairs mean the same: superior/cranial, inferior/caudal,anterior/ventral, and posterior/dorsal, even though there may be fine distinctionsin what they mean, as is depicted here. (From [43], with additions. Used withpermission)

a funny scene in the movie Tootsie when a stagehand was told to focus onthe right side of the face of Dorothy Michaels, aka Michael Dorsey, aka DustinHoffman – and Dorothy heard this and then turned her (i.e., his) head so thecamera would be focusing on the left side of her (i.e., his) face. A comicaldebate then ensued concerning whose “right” was correct, that of a person onstage or one facing the stage.)

The second direction is superior (or cranial), which means towards thehead or above, i.e., to larger z. Inferior (or caudal (kaw’-dul)) means awayfrom the head, i.e., to smaller z – in an algebraic sense, so more and moreinferior means smaller positive numbers and then more highly negative valuesof z. (This is relative to a defined z = 0 plane. We could choose to define theorigin of the coordinate system at the center of mass of the body.) So, thehead is superior to the feet, which are inferior to the head. After supplyingthe body with oxygen, blood returns to the heart through two major veins, thesuperior and inferior vena cava (vee’-nuh cave’-uh), which collect blood fromabove and below the heart, respectively. (As you see, words that the authorhas trouble pronouncing are also presented more or less phonetically, with anapostrophe after the accented syllable.)

Anterior (or ventral) means towards or from the front of the body, i.e., tolarger x. Posterior (or dorsal) means towards or from the back, corresponding

1.2 Motion in the Human Machine 3

to smaller algebraic x. The nose is anterior to the ears, which is posterior tothe nose.

There is another pair of terms that relate to the y coordinate, specificallyto its magnitude. Medial means nearer the midline of the body, i.e., towardssmaller |y|. Lateral means further from the midline, i.e., towards larger |y|.

Other anatomical terms require other types of coordinate systems. Oneset describes the distance from the point of attachment of any of the twoarms and two legs from the trunk. Figure 1.1 depicts this with the coordinater, where r = 0 at the trunk. r is never negative. Proximal means near thepoint of attachment, i.e., to smaller r. Distal means further from the point ofattachment, or larger r.

The last series of directional terms relates to the local surface of the body.This can be depicted by the coordinate ρ (inset in Fig. 1.1), which is related tox and y in an x − y plane. ρ = 0 on the local surface of the body. Superficialmeans towards or on the surface of the body, or to smaller ρ. Deep meansaway from the surface, or towards larger ρ.

These directional terms can refer to any locality of the body. Regionalterms designate a specific region in the body (Tables 1.1 and 1.2). This is il-lustrated by an example we will use several times later. The region between theshoulder and elbow joints is called the brachium (brae’-kee-um). The adjec-tive used to describe this region in anatomical terms is brachial (brae’-kee-al).The muscles in our arms that we usually call the biceps are really the brachialbiceps or biceps brachii, while our triceps are really our brachial triceps ortriceps brachii. The terms biceps and triceps refer to any muscles with twoor three points of origin, respectively (as we will see) – and not necessarilyto those in our arms.

The final set of terms describes two-dimensional planes, cuts or sections ofthe body. They are illustrated in Fig. 1.1. A transverse or horizontal sectionseparates the body into superior and inferior sections. Such planes have con-stant z. Sagittal sections separate the body into right and left sections, andare planes with constant y. The midsagittal section is special; it occurs at themidline and is a plane with y = 0. The frontal or coronal section separatesthe body into anterior and posterior portions, as described by planes withconstant x.

Much of our outright confusion concerning medical descriptions is allevi-ated with the knowledge of these three categories of anatomical terminology.There is actually a fourth set of anatomical terms that relates to types ofmotion. These are discussed in Sect. 1.2.

1.2 Motion in the Human Machine

Anatomical terms refer to the body locally whether it is at rest or in motion.Since we are also concerned with how we move, we need to address humanmotion [32]. We will describe how we move by examining the degrees of freedom


Table 1.1. Anatomical terms in anterior regions

anatomical term common term

abdominal abdomen

antebrachial forearm

axilliary armpit

brachial upper arm

buccal cheek

carpal wrist

cephalic head

cervical neck

coxal hip

crural front of leg

digital finger or toe

frontal forehead

iguinal groin

lingual tongue

mammary breast

mental chin

nasal nose

oral mouth

palmar palm

pedal foot

sternal breastbone

tarsal ankle

thoracic chest

umbilical navel

of our motion and the means for providing such motion by our joints. We willsee that our arms and legs are constructed in a very clever manner. Becausejoints involve motion between bones, we will need to refer to the anatomy ofthe skeletal system, as in Fig. 1.2.

Table 1.2. Anatomical terms in posterior regions

anatomical term common term

acromial top of shoulder

femoral thigh

gluteal buttock

occipital back of head

plantar sole of foot

popliteal back of knee

sacral between hips

sural back of leg

vertebral spinal column


Fig. 1.2. Anatomy of the skeletal system, anterior view, with major bones andjoints listed. (From [59])

Think of a degree of freedom (DOF) of motion as a coordinate needed todescribe that type of motion. If you want to relocate an object, you are gener-ally interested in changing its center of mass and its angular orientation. Youmay want to change its center of mass from an (x, y, z) of (0, 0, 0) to (a, b, c).Because three coordinates are needed to describe this change, there are three


“translational” degrees of freedom. Similarly, you can change the angular ori-entation of the object about the x, y, and z axes, by changing the angles thisobject can be rotated about these three axes: θx, θy, and θz, respectively. So,there are also three rotational degrees of freedom. (Sometimes, these threeindependent rotations are defined differently, by the three Eulerian angles,which will not be introduced here.)

These six (three plus three) degrees of freedom are independent of eachother. Keeping your fingers rigid as a fist, you should be able to change inde-pendently either the x, y, z, θx, θy, and θz of your fist by moving your armsin different ways. You should try to change the x, y, and z of your fist, whilekeeping θx, θy, and θz fixed. Also, try changing the θx, θy, and θz of your fist,while keeping its x, y, and z constant.

We would like each of our arms and legs to have these six degrees offreedom. How does the body do it? It does it with joints, also known as articu-lations. Two types of articulations, fibrous (bones joined by connective tissue)and cartilaginous (bones joined by cartilage) joints, can bend only very little.There is a joint cavity between the articulating bones in synovial joints. Onlythese synovial joints have the large degree of angular motion needed for mo-tion. As seen in Fig. 1.3, in synovial joints cartilage on the ends of opposingbones are contained in a sac containing synovial fluid. The coefficient of fric-tion in such joints is lower than any joints made by mankind. (More on thislater.)

There are several types of synovial joints in the body, each with eitherone, two, or three degrees of angular motion. Each has an analog withphysical objects, as seen in Fig. 1.4. For example, a common door hingeis a model of one degree of angular freedom. Universal joints, which con-nect each axle to a wheel in a car, have two angular degrees of freedom.A ball-and-socket joint has three independent degrees of angular motion.The water faucet in a shower is a ball-and-socket joint. The balls and sock-ets in these joints are spherical. Condyloid or ellipsoidal joints are ball-and-socket joints with ellipsoidal balls and sockets. They have only two degreesof freedom because rotation is not possible about the axis emanating fromthe balls. A saddle joint, which looks like two saddles meshing into one an-other, also has two degrees of angular motion. Other examples are shown inFig. 1.4.

Now back to our limbs. Consider a leg with rigid toes. The upper leg bone(femur) is connected to the hip as a ball-and-socket joint (three DOFs) (as inthe song “Dry Bones” aka “Them Bones” in which “The hip bone is connectedto the thigh bones, . . . .” The knee is a hinge (one DOF). The ankle is a saddlejoint (two DOFs). This means that each leg has six degrees of angular motion,as needed for complete location of the foot. Of course, several of these degreesof freedom have only limited angular motion.

Now consider each arm, with all fingers rigid. The upper arm (humerus)fits into the shoulder as a ball-and-socket joint (three DOFs). The elbow is ahinge (one DOF). The wrist is an ellipsoidal joint (two DOFs). That makes


Fig. 1.3. The right knee synovial joint, with (a) anterior view with the kneecap(patella) removed and (b) in sagittal section (photo). Also see Fig. 3.2e. (From [59])

six DOFs. The leg has these six DOFs, but the arm has one additional DOF,for a total of seven. This additional DOF is the screwdriver type motion ofthe radius rolling on the ulna (Figs. 1.2, 2.7, and 2.8), which is a pivot with 1DOF. With only six DOFs you would be able to move your hand to a given x,y, z, θx, θy, θz position in only one way. With the additional DOF you can doit in many ways, as is seen for the person sitting in a chair in Fig. 1.5. Thereare many more degrees of freedom available in the hand, which enable thecomplex operations we perform, such as holding a ball. Figure 1.6 shows thebones of the hand, and the associated articulations and degrees of freedomassociated with the motion of each finger.


Fig. 1.4. Six types of synovial joints, including a: (a) hinge joint (1D joint), as in theelbow joint for flexion and extension, (b) pivot joint (1D joint), as in the atlantoaxialjoint in the spinal cord for rotation, (c) saddle joint (2D), which is both concaveand convex where the bones articulate, as in the joint between the first metacarpaland the trapezium in the hand, (d) condyloid or ellipsoidal joint (2D), as in themetacarpophalangeal (knuckle) joint between the metacarpal and proximal phalanxfor flexion and extension, abduction and adduction, and circumduction, (e) planejoint (2D), as in the acromioclavicular joint in the shoulder for gliding or sliding,and (f) ball-and-socket joint (3D), as in the hip joint (and the shoulder joint) forflexion and extension, abduction and adduction, and medial and lateral rotation. SeeFigs. 1.9 and 1.10 for definitions of the terms describing the types of motion aboutjoints and the diagrams in Fig. 1.11 for more information about synovial joints.(From [49]. Used with permission)


Fig. 1.5. Nonunique way of positioning the right arm. This is demonstrated bygrasping the armrest while sitting, with the six coordinates of the hand (three forposition and three for angle) being the same in both arm positions. This is possiblebecause the arm can use its seven degrees of freedom to determine these six coor-dinates. (From [32]. Copyright 1992 Columbia University Press. Reprinted with thepermission of the press)

We can also see why it is clever and good engineering that the knee hingedivides the leg into two nearly equal sections and the elbow hinge divides thearm into two nearly equal sections. In the two-dimensional world of Fig. 1.7this enables a greater area (volume for 3D) to be covered than with unequalsections.

Fig. 1.6. (a) Anatomy of the hand and (b) the degrees of freedom of the hand andfingers, with joints (spaces) having one (spaces with flat terminations) or two (curvedterminations, with a “2” below the joint) degrees of freedom. (From [32]. Copyright1992 Columbia University Press. Reprinted with the permission of the press)


Fig. 1.7. Range of hand motion in two dimensions for different lengths of the upperand lower arms. (From [32]. Copyright 1992 Columbia University Press. Reprintedwith the permission of the press)

In preparation for our discussion of statics and motion of the body, weshould consider the building blocks of human motion. There are four typesof components: bones, ligaments, muscles, and tendons. Each has a very dif-ferent function and mechanical properties. Bones are often lined with hyaline(high’u-lun) articular cartilage at the synovial joints. Ligaments hold bonestogether. Muscles, in particular skeletal muscles, are the motors that movethe bones about the joints. (There is also cardiac muscle – the heart – andsmooth muscle – of the digestive and other organs.) Tendons connect musclesto bones. Muscles are connected at points of origin and insertion via tendons;the points of insertion are where the “action” is. Figure 1.8 shows several ofthe larger muscles in the body, along with some of the tendons.

Muscles work by contraction only, i.e., only by getting shorter. Conse-quently, to be able to move your arms one way and then back in the oppositedirection, you need pairs of muscles on the same body part for each opposingmotion. Such opposing pairs, known as “antagonists,” are very common inthe body.

We now return to our brief review of terminology, this time to describe theangular motion of joints. It is not surprising that these come in opposing pairs(Figs. 1.9 and 1.10) as supplied by antagonist muscles. When the angle of a 1Dhinge, such as the elbow, increases it is called extension and when it decreasesit is flexion. When you rotate your leg away from the midline of your body, it


Fig. 1.8. (a) Anterior and (b) posterior views of some of the larger skeletal musclesin the body. Several muscles are labeled: S, sternocleidomastoid; T, trapezius; D,deltoid;, P, pectoralis major; E, external oblique; L, latissimus dorsi; G, gluteusmaximus. In (b), the broad-banded tendon extending from the gastrocnemius andsoleus (deep to the gastrocnemius, not shown) muscles to the ankle (calcaneus) isthe calcaneal (or Achilles) tendon. (From [49]. Used with permission)

is abduction, and when you bring is closer to the midline, it is adduction. Whenyou rotate a body part about its long axis it is called rotation. The screwdrivermotion in the arm is pronation (a front facing hand rotates towards the body)or supination (away from the body), and so supination is the motion of a righthand screwing in a right-handed screw (clockwise looking from the shoulderdistally) and pronation is that of a right hand unscrewing a right-handedscrew (counterclockwise looking from the shoulder distally). Examples of therotation axes for the synovial joints used in these opposing motions are givenin Fig. 1.11.

One example of opposing motion is the motion of the arm (Fig. 1.12). Thebiceps brachii have two points of origin and are inserted on the radius (asshown in Fig. 2.10 below). When they contract, the radius undergoes flexionabout the pivot point in the elbow. The triceps brachii have three points oforigin, and a point of insertion on the ulna. They are relaxed during flexion.


Fig. 1.9. Several antagonistic motions allowed by synovial joints. See other motionsin Fig. 1.10. (From [49]. Used with permission)

During extension they contract, while the biceps brachii are relaxed. Thisis an example of a lever system about a pivot point. (This is really a pivotaxis normal to the plane of the arm, as is illustrated in Fig. 1.11a for a hingejoint.)

A second place where there is such opposing motion is the eye. The threetypes of opposite motion in each eye (monocular rotations) are shown inFig. 1.13. During adduction the eye turns in to the midline, while during ab-duction it turns out. The eyeball can also undergo elevation (eye rotating


Fig. 1.10. More antagonistic motions allowed by synovial joints. See other motionsin Fig. 1.9. (From [49]. Used with permission)

Fig. 1.11. Rotation axes for four types of synovial joints are shown for each depictedrotation direction: (a) one axis for a hinge joint (1D joint), (b) two axes for a saddlejoint (2D), (c) two axes for an ellipsoidal joint (2D), and (d) three axes for a ball-and-socket joint (3D). (From [54])


Fig. 1.12. Opposing motions of the lower arm with antagonist muscles, with flexionby contraction of the biceps brachii and extension by the contraction of the tricepsbrachii. The axis of rotation is seen in Fig. 1.11a

upward, or supraduction) or depression (eyeball rotating downward, or infra-duction). Less common is the rotation of the eyeball about an axis normal tothe iris, in opposing intorsion (incycloduction) or extorsion (excycloduction)motions. There are three pairs of opposing muscles per eye, each attachedto the skull behind the eye, that control these motions (Fig. 1.14, Table 1.3).However, of these three pairs, only one is cleanly associated with only oneof these pairs of opposing motions. Adduction occurs with the contractionof the medial rectus muscle, while abduction occurs when the lateral rectuscontracts. The primary action of the superior rectus is elevation, while thatof the opposing inferior rectus is depression. The primary action of the su-perior oblique is also depression, while that of the opposing inferior obliqueis also elevation. These last two pairs of muscles have secondary actions inadduction/abduction and intorsion/extorsion that depend on the position ofthe eye. Binocular vision requires coordinated motion of the three opposingmuscle pairs in both eyes, as described in Table 1.4.

Fig. 1.13. Rotations of the right eye. A dashed line has been added across the iristo help view the rotations. (Based on [60])


Fig. 1.14. Ocular muscles, with the eyelid (palpebra) pulled up as shown. Thetendon of the superior oblique muscle (marked in two regions) passes through thetrochlea loop. (From [59])

Table 1.3. Ocular muscle functions. (Based on [60])

muscle primary action secondary action

lateral rectus abduction none

medial rectus adduction none

superior rectus elevation adduction, intorsion

inferior rectus depression abduction, extorsion

superior oblique depression intorsion, abduction

inferior oblique elevation extorsion, abduction

Table 1.4. Muscle combinations of both eyes for gaze directions. (Based on [60])

direction of gaze right eye muscle left eye muscle

eyes up, right superior rectus inferior oblique

eyes right lateral rectus medial rectus

eyes down, right inferior rectus superior oblique

eyes down, left superior oblique inferior rectus

eyes left medial rectus lateral rectus

eyes up, left inferior oblique superior rectus


1.3 The Standard Human

We will often, but not always, model humans assuming numerical values formass, height, etc. of a “standard” human, a 70 kg man with parameters similarto those in Table 1.5.

We will need details of human anatomy in some cases, and these are pro-vided now and in subsequent chapters as needed. We will also need to use the

Table 1.5. A description of the “Standard Man”. (Using data from [37, 44])

age 30 yr

height 1.72 m (5 ft 8 in)

mass 70 kg

weight 690 N (154 lb)

surface area 1.85 m2

body core temperature 37.0◦C

body skin temperature 34.0◦C

heat capacity 0.83 kcal/kg-◦C (3.5 kJ/kg-◦C)

basal metabolic rate 70 kcal/h (1,680 kcal/day, 38 kcal/m2-h, 44 W/m2)

body fat 15%

subcutaneous fat layer 5 mm

body fluids volume 51 L

body fluids composition 53% intracellular; 40% interstitial, lymph;

7% plasma

heart rate 65 beats/min

blood volume 5.2 L

blood hematocrit 0.43

cardiac output (at rest) 5.0 L/min

cardiac output (in general) 3.0 + 8 × O2 consumption (in L/min) L/min

systolic blood pressure 120 mmHg (16.0 kPa)

diastolic blood pressure 80 mmHg (10.7 kPa)

breathing rate 15/min

O2 consumption 0.26 L/min

CO2 production 0.21 L/min

total lung capacity 6.0 L

vital capacity 4.8 L

tidal volume 0.5 L

lung dead space 0.15 L

lung mass transfer area 90 m2

mechanical work efficiency 0–25%

There are wide variations about these typical values for body parameters. Also,these values are different for different regions; the ones in the table typify Americanmales in the mid-1970s. Values for women are different than for men; for exam-ple, their typical heights and weights are lower and their percentage of body fat ishigher.

1.3 The Standard Human 17

Table 1.6. Body segment lengths. Also see Fig. 1.15. (Using data from [63])

segment segment lengtha/

body height H

head height 0.130

neck height 0.052

shoulder width 0.259

upper arm 0.186

lower arm 0.146

hand 0.108

shoulder width 0.259

chest width 0.174

hip width/leg separation 0.191

upper leg (thigh) 0.245

lower leg (calf) 0.246

ankle to bottom of foot 0.039

foot breadth 0.055

foot length 0.152

aUnless otherwise specified.

findings of anthropometry, which involves the measurement of the size, weight,and proportions of the human body. Of particular use will be anthropometricdata, such as those in Table 1.6 and Fig. 1.15, which provide the lengths ofdifferent anatomical segments of the “average” body as a fraction of the bodyheight H.

Table 1.7 gives the masses (or weights) of different anatomical parts of thebody as fractions of total body mass mb (or equivalently, total body weight

Table 1.7. Masses and mass densities of body segments. (Using data from [63])

segment segment mass/ mass density

total body mass mb (g/cm3)

hand 0.006 1.16

forearm 0.016 1.13

upper arm 0.028 1.07

forearm and hand 0.022 1.14

total arm 0.050 1.11

foot 0.0145 1.10

lower leg (calf) 0.0465 1.09

upper leg (thigh) 0.100 1.05

foot and lower leg 0.061 1.09

total leg 0.161 1.06

head and neck 0.081 1.11

trunk 0.497 1.03


Fig. 1.15. Body segments length, relative to body height H. (From [38], as from[53]. Reprinted with permission of Wiley)

Wb). The mass and volume of body segments are determined on cadaver bodysegments, respectively by weighing them and by measuring the volume of wa-ter displaced for segments immersed in water. (This last measurement usesArchimedes’ Principle, described in Chap. 7.) The average density of differ-ent body segments can then be determined, as in Table 1.7. The volumes ofbody segments of live humans can be measured by water displacement (Prob-lem 1.37) and then their masses can be estimated quite well by using thesecadaver densities. Whole body densities of live humans can be measured usingunderwater weighing, as is described in Problem 1.40. A relation for averagebody density is given below in (1.3). This is closely related to determining thepercentage of body fat, as is presented below in (1.4) and (1.5).

The normalized distances of the segment center of mass from both theproximal and distal ends of a body segment are given in Table 1.8. (Prob-lems 1.42 and 1.43 explain how to determine the center of mass of the bodyand its limbs.) The normalized radius of gyration of segments about the cen-ter of mass, the proximal end, and the distal end are presented in Table 1.9.


Table 1.8. Distance of the center of mass from either segment end, normalized bythe segment length. (Using data from [63])

segment center of mass from

proximal distal

hand 0.506 0.494

forearm 0.430 0.570

upper arm 0.436 0.564

forearm and hand 0.682 0.318

total arm 0.530 0.470

foot 0.50 0.50

lower leg (calf) 0.433 0.567

upper leg (thigh) 0.433 0.567

foot and lower leg 0.606 0.394

total leg 0.447 0.553

head and neck 1.00 –

trunk 0.50 0.50

(The radius of gyration provides a measure of the distribution of mass aboutan axis, as described in (3.28) and Fig. 3.23b). Problem 3.9 describes how theradii of gyration in Table 1.9 are related.)

Note that all of the data in these different tables are not always consistentwith each other because of the variations of sources and the different rangesof subjects and methods used for each table.

We have a tremendous range of mobility in our articular joints, but notas much as in the idealized joints in Fig. 1.4. The average ranges of mobilityin people are given in Table 1.10 for the motions depicted in Fig. 1.16, along

Table 1.9. Radius of gyration of a segment, about the center of mass and eitherend, normalized by the segment length. (Using data from [63])

segment radius of gyration about

C of M proximal distal

hand 0.297 0.587 0.577

forearm 0.303 0.526 0.647

upper arm 0.322 0.542 0.645

forearm and hand 0.468 0.827 0.565

total arm 0.368 0.645 0.596

foot 0.475 0.690 0.690

lower leg (calf) 0.302 0.528 0.643

upper leg (thigh) 0.323 0.540 0.653

foot and lower leg 0.416 0.735 0.572

total leg 0.326 0.560 0.650

head and neck 0.495 0.116 –


Table 1.10. Range of joint mobility for opposing movements, with mean and stan-dard deviation (SD) in degrees. (Using data from [39], as from [33, 61])

opposing movements mean SD

shoulder flexion/extension 188/61 12/14

shoulder abduction/adduction 134/48 17/9

shoulder medial/lateral rotation 97/34 22/13

elbow flexion 142 10

forearm supination/pronation 113/77 22/24

wrist flexion/extension 90/99 12/13

wrist abduction/adduction 27/47 9/7

hip flexion 113 13

hip abduction/adduction 53/31 12/12

hip medial/lateral rotation (prone) 39/34 10/10

hip medial/lateral rotation (sitting) 31/30 9/9

knee flexion (prone) – voluntary, arm assist 125,144 10,9

knee flexion – voluntary (standing), forced (kneeling) 113,159 13,9

knee medial/lateral rotation (sitting) 35/43 12/12

ankle flexion/extension 35/38 7/12

foot inversion/eversion 24/23 9/7

The subjects were college-age males. Also see Fig. 1.16.

Fig. 1.16. Postures used for Table 1.10, for range of opposing motions. (From [38].Reprinted with permission of Wiley. Also see [33, 61])


with the standard deviations about these values. (For normal or Gaussiandistributions with an average, A, and standard deviation, SD, about 68%of all values are between A − SD and A + SD.) Three degrees of freedomare given for the shoulder and hip, two for the wrist and the foot (listedseparately as foot and ankle), and one each for the elbow and forearm. Theknee, as idealized above, has one DOF, but two are listed here: the flexion ina 1 D hinge and also some rotation of the upper and lower leg about the knee.

Table 1.11 gives the mass and volumes of different systems and parts ofthe body. The components of a typical human cell are given in Table 1.12.Although, most of our discussion will not concern these components of a cell,

Table 1.11. Mass and volume of the organs of the human body. (Using data from[42])

fluid, tissue, organ, or system total mass (g) total volume (cm3)

adult male body 70,000 60,000

muscle 30,000 23,000

fat 10,500 12,000

skin 2,000 1,800

subcutaneous tissue 4,100 3,700

skeleton 10,000 6,875

gastrointestinal track 2,000 1,800

contents (chyme/feces) ∼2,000 ∼2,000

blood vessels 1,800 1,700

contents (blood) 5,600 5,400

liver 1,650 1,470

brain 1,400 1,350

lungs (2) 825 775

contents (air) ∼7.7 ∼6,000

heart 330 300

chamber volume – 450

kidneys (2) 300 270

urinary bladder 150 140

contents (urine) ∼500 ∼500

digestive fluids ∼150 ∼150

pancreas 110 100

salivary glands (6) 50 48

synovial fluid ∼50 ∼50

teeth (32) 42 14

eyes (2) 30 27

hair (average haircut) 21 16

gall bladder 7 7

contents (bile) ∼50 ∼50

fingernails and toenails (20) 1.1 0.9


Table 1.12. Estimated gross molecular contents of a typical 20-µm human cell.(Using data from [42])

molecule mass molecular weight number of number of

(amu, daltons) molecules molecular entities

water 65 18 1.74 × 1014 1

other inorganic 1.5 55 1.31 × 1012 20

lipid (fat) 12 700 8.4 × 1011 50

other organic 0.4 250 7.7 × 1010 ∼200

protein 20 50,000 1.9 × 1010 ∼5,000

RNA 1.0 1 × 106 5 × 107 –

DNA 0.1 1 × 1011 46 –

this listing is instructive because it provides a quantitative assessment of thesecomponents.

So far we have considered only the physical specifications of a typical per-son. In subsequent chapters, we will address the physical aspects of how a typ-ical person responds to a wide range of physical conditions, for healthy andunhealthy people, and under normal and extreme conditions. The prospectof space exploration motivated extensive studies of how people respond to awide range of extreme physical conditions, such as extreme pressures, temper-atures, linear and rotary accelerations, collisions, vibrations, weightlessness,and sound [50].

We will also see that many processes can be described in terms of charac-teristic times or distances, such as the time needed for a muscle activation todecay or a molecule to diffuse in a cell. There are also more general character-istic times within the human body. Your heart beats and you breathe roughlyonce every second. Your blood flows throughout your body roughly once everyminute, and each ATP molecule (the molecule which is the ultimate form ofenergy usage in your body) is used and then regenerated roughly once everyminute.

1.4 Scaling Relationships

In human and animal biology there are diverse ranges of properties that scalewith either a property, such as mass or length, or a physical or chemical input.We summarize them here; they relate to many concepts developed and usedin subsequent chapters.

1.4.1 Allometric Rules

Some properties scale with body mass in a fairly predictable way, and arecharacterized by scaling relationships called allometric rules. For a property

1.4 Scaling Relationships 23

Table 1.13. Allometric parameters (1.1) for mammals. (Using data from [31, 55])

parameter a α

basal metabolic rate (BMR), in W 4.1 0.75

body surface area, in m2 0.11 0.65

brain mass in man, in kg 0.085 0.66

brain mass in nonprimates, in kg 0.01 0.7

breathing rate, in Hz 0.892 −0.26

energy cost of running, in J/m-kg 7 −0.33

energy cost of swimming, in J/m-kg 0.6 −0.33

effective lung volume, in m2 5.67 × 10−5 1.03

heart beat rate, in Hz 4.02 −0.25

heart mass, in kg 5.8 × 10−3 0.97

lifetime, in y 11.89 0.20

muscle mass, in kg 0.45 1.0

skeletal mass (terrestrial), in kg 0.068 1.08

speed of flying, in m/s 15 0.167

speed of walking, in m/s 0.5 0.167

f for animals with body mass mb (in kg), an allometric relation has the form

f(in a given set of units) = amαb . (1.1)

Technically, the relationship is allometric if α �= 1. Some examples are givenin Table 1.13. By the way, allometric means “by a different measure” from theGreek alloios, which means “different” – so how body height scales with bodymass is an allometric relationship. Isometric means “by the same measure” –so how leg mass scales with body mass is an example of an isometric rela-tionship. For a delightful discussion on allometry and scaling see [46]. Otherequally intriguing discussions have been presented in [31, 35, 36, 45, 47, 55, 56].

These relationships can hold for many species of a given type, such asland-based mammals, etc. Some are also valid within a species, such as forman – and as such would be called anthropometric relationships. Sometimesthey apply only to adults in a species, and not across all age groups. SeeProblems 1.54 and 1.55 for more on this.

The “predicted” values from a scaling relation would be the expected av-erage values only if the parameters for all species follow the relation exactly –and there is no reason why this must be so. Moreover, there is always a spread,or dispersion, about these average values. Some of these allometric relation-ships are empirical, and others can be derived, or at least rationalized, aswe will see for Kleiber’s Law of basal (6.19) (i.e., minimum) metabolic rates(BMRs) in Chap. 6.

One obvious example of such scaling is that the legs of bigger mammalstend to be wider in proportion to their overall linear dimension L (and massmb) than those for smaller mammals, and this is reflected in the larger ratio


of the width of their long skeletal bones, wbone, to the length, Lbone, forlarger mb. Such bones need to support their body weight or mass, whichare proportional to their volume ∼L3 because the mass densities are verynearly the same for all mammals. Such long bones in the different mammalscan withstand approximately the same force per unit cross-sectional area,which means that they have the same ultimate compressive stress or UCS.(The UCS is a measure of fracture conditions that will be discussed more inChap. 4.). If this maximum force is a fixed multiple of the weight, so it is ∝ L3,and the cross-sectional area is ∼w2

bone, then w2 ∝ L3 or w ∝ L3/2. Becausethe bone length Lbone ∝ L, we see that w/Lbone ∝ L1/2 ∝ m

1/6b . Body

mass mb increases by 104 from rat to elephant, and so this ratio increases by102/3 ∼ 4. This means that the body shapes and the bones themselves arenot geometrically similar.

In some cases more accurate scaling relations necessitate the use of para-meters in addition to the body mass. One example is the scaling of the surfacearea of a person A (in m2), which is empirically seen to depend on height H(in m), as well as body mass mb (in kg):

A = 0.202m0.425b H0.725. (1.2)

Accurate scaling relationships often involve the index Wb/Hp (or equiv-alently mb/Hp) with p ranging from 1.0 to 3.0. (Sometimes the reciprocalor square root or cube root of this parameter is used. Complicating mattersfurther, different sets of units are commonly used for these parameters.) Onesuch index is the specific stature or ponderal index S = H/m

1/3b (for which

p = 3), which is used in the Harris–Benedict versions of Kleiber’s Law spe-cialized for people (6.30–6.31). It is also used in the expression for the averagehuman density

ρ (in kg/L or g/cm3) = 0.69 + 0.9S (1.3)

where within S the units of H are m and those of mb are kg. Another scalingparameter is Quetelet’s index or the body mass index (BMI), Q = mb/H2

(for which p = 2.0). It is often considered the best index for epidemiologicalstudies. For example, with mb in kg and H in m, the average fat content ofthe body increases with this index:

Men: fat (% of body weight) = 1.28Q − 10.1 (1.4)Women: fat (% of body weight) = 1.48Q − 7.0. (1.5)

Normal or ideal body fat is 14–20% in men and 21–27% in women. Notcoincidentally, mortality increases as this index Q increases.

Some properties do not scale with mass. From mouse to elephant, a massrange of over 10,000, the maximum jumping height (of the center of mass) ofmost every mammal is within a factor of 2 of 2/3 m. Similarly, the maximumrunning speed of most mammals is within a factor of 2 of 7 m/s (15 mph).

1.4 Scaling Relationships 25

Table 1.14. Size-independent dimensionless groups in mammals. (Using datafrom [46])

parameter a α

breathing flow rate/blood flow ratea 2.0 0.00

mass of blood/mass of heart 8.3 0.01

time for 50% of growth/lifespan in captivity 0.03 0.05

gestation period/lifespan in captivity 0.015 0.05

breathing cycle/lifespan in captivity 3 × 10−9 0.06

cardiac cycle/lifespan in captivity 6.8 × 10−10 0.05

half-life of drugb/lifespan in captivity 0.95 × 10−5 0.01

The value of a in (1.1) is that for a 1 kg mammal. Also see Chaps. 8 and 9.a(tidal volume/breath time)/(heart stroke volume/pulse time).bMethotrexate.

The main reason behind these two relationships is that the force a musclecan exert is proportional to its cross-sectional area and therefore it varies asthe square of the characteristic linear dimension, such as height H. This isexplored in Problems 3.28, 3.29, and 3.41 in Chap. 3. The dimensionless ratiosof some physical and physiological properties are noteworthy in that they areessentially independent of size (and mass), as is seen in Table 1.14.

1.4.2 Scaling in the Senses

A very different type of scaling is exhibited by Stevens’ Law, which charac-terizes how the perceived strength P of a sense varies with the intensity of astimulus S for a given sensation. This scaling is

P = K(S − S0)n (1.6)

above a threshold S0 [57, 58]. As seen in Table 1.15, sometimes the perceptionof a sense, called the psychoperception, is sublinear with the strength of the

Table 1.15. Exponent n for perceived strength (P ) of a stimulus (S) above athreshold S0, with P = K(S − S0)

n in Steven’s Law. (Using data from [57, 58])

psychoperception n stimulus

brightness 0.33, 0.5 5◦ target, point source – dark adapted eye

loudness 0.54, 0.60 monoaural, binaural

smell 0.55, 0.60 coffee odor, heptane

vibration 0.6, 0.95 250 Hz, 60 Hz – on finger

taste 0.8, 1.3, 1.3 saccharine, sucrose, salt

temperature 1.0, 1.6 cold, warm – on arm

pressure on palm 1.1 static force on skin

heaviness 1.45 lifted weights

electric shock 3.5 60 Hz through fingers


stimulus and sometime it is superlinear. Three of the senses involve very sig-nificant physical aspects: hearing (Chap. 10, loudness) and seeing (Chap. 11),which we will examine in detail, and touch (Chap. 2, vibration, temperature,pressure on palm, heaviness; Chap. 12, electric shock), which we will studyin less detail. These three also have very important chemical and biologi-cal origins. The senses of taste and smell are essentially solely chemical andbiological in basis and will not be discussed any further, except for a briefdiscussion of the electrical properties of the membranes of the taste and smellsensory neurons in Chap. 12. For these three other senses, we will concen-trate mostly on the physical input and the beginning of the sensation process(detection). The nonlinearities inherent in Stevens’ Law are due in part tothe detection process. The final parts of detection process are the generationand transmission of neural signals sent to the brain – which we will cover –and the processing in the brain – which we will cover only briefly. There areapproximately 12 orders of magnitude sensitivity in hearing and vision.

1.5 Summary

Much about the body can be understood by learning the terminology of direc-tions and local regions in the body. Much about the motion of the body canbe explained by examining the rotation about bones about joints. In analyz-ing the physics of the body, reference can be made to anthropometric data onbody parts for a standard human. Many phenomena concerning anatomy andphysiology can be characterized and understood by using scaling relations.

Problems

Body Terminology

1.1. (a) Is the heart superior or inferior to the large intestine?(b) Is the large intestine superior or inferior to the heart?

1.2. (a) Is the navel posterior or anterior to the spine?(b) Is the spine posterior or anterior to the navel?

1.3. Is the nose lateral or medial to the ears?

1.4. Are the eyes lateral or medial to the nose?

1.5. Is the foot proximal or distal to the knee?

1.6. Is the elbow proximal or distal to the wrist?

1.7. Is the skeleton superficial or deep to the skin?

1.5 Summary 27

1.8. The blind spot in the eye retina is said to be nasal to the fovea (centerof the retina). What does this mean?

1.9. What would you expect the term cephalid to mean? What would be anequivalent term?

1.10. Which is the anterior part of the heart in Fig. 8.7? Is this a superior orinferior view of the heart?

1.11. Consider the directional terms ipsilateral and contralateral. One meanson the same side of the body, while the other means on opposite sides of thebody. Which is which?

1.12. The directional term intermediate means “in between.” What is inter-mediate between the upper and lower legs?

1.13. Encephalitis is the inflammation, i.e., “itis”, of what?

1.14. Presbyopia refers to disorders in vision due to old age, such as lack ofaccommodation in the crystalline lens (see Chap. 11). Presbycusis refers toold age-related auditory impairments (see Chap. 10.) What parts of these twoterms mean old age, vision, and hearing?

1.15. The three tiny bones in the middle ear, the malleus, incus, and stapesare interconnected by the incudomallear articulation and the incudostapedialjoint. Describe the origin of the names of these connections.

1.16. The quadriceps muscles in the upper leg attach to the kneecap (patella)through the quadriceps tendon. The kneecap is connected to the tibia byconnective tissue that is sometimes called the patellar tendon and sometimesthe patellar ligament. Explain why both designations have merit and whyneither designation completely describes the linkage perfectly well by itself.

1.17. Consider the drawing of the hand skeleton and the schematic of a handshowing joints with one or (labeled 2) two degrees of freedom in Fig. 1.6.(a) How many degrees of freedom does each hand have? (Ignore the wristjoint.)(b) Do we need so many degrees of freedom? Why? (There is no right or wronganswer to this part. Just think about what a human hand should be able todo (in clutching, etc.) and try to express your conclusions in terms of degreesof freedom.)

1.18. Estimate the angle of each of the joints in the hand for each of thefollowing functions [52]. (Define the angle of each joint as shown in the lefthand in Fig. 1.6b to be 0◦. Define rotations into the paper and clockwisemotions in the plane of the paper as being positive.)(a) lifting a pail (a hook grip)(b) holding a cigarette (a scissors grip)


(c) lifting a coaster (a five-jaw chuck)(d) holding a pencil (a three-jaw chuck)(e) threading a needle (a two-jaw pad-to-pad chuck)(f) turning a key (a two-jaw pad-to-side chuck)(g) holding a hammer (a squeeze grip)(h) opening a jar (a disc grip)(i) holding a ball (a spherical grip)

1.19. We said that the seven DOFs available for arm motion enabled nonuniquepositioning of the hand, but analogous nonunique positioning of the foot isnot possible because the leg has only six DOFs. Use Table 1.10 to explain whythis is not exactly correct.

1.20. Use Table 1.3 to show that the coordinated eye motions in Table 1.4 usethe muscles listed for primary motion.

1.21. Consider a limb, of length L, composed of upper and lower limbs withrespective lengths r1 and r2, with L = r1+r2. There is a total range of motionin the angles the upper limb makes with the torso and the lower limb makeswith the upper limb. Assume motion only in two dimensions (see Fig. 1.7.)(a) What area is subtended by the end of the lower limb (hand or foot) whenr1 = r2?(b) What area is subtended by the end of the lower limb when r1 > r2? Whatfraction of that in (a) is this?(c) What area is subtended by the end of the lower limb when r1 < r2? Whatfraction of that in (a) is this?

1.22. Redo Problem 1.21 in three dimensions, finding the volume subtendedby the end of the lower limb in each case.

The Standard Human

1.23. Qualitatively explain the differences of density in Table 1.7 in the differ-ent segments of the body. The average densities of blood, bone, muscle, fat,and air (in the lungs) can be determined from Table 1.11.

1.24. (a) Use Table 1.7 to determine the average density of the body.(b) Use this to determine the average volume of a 70 kg body.Your answers will be a bit different from the rough volume estimate given inTable 1.11.

1.25. (a) Calculate the range of segment masses alternatively using Tables 1.7and 1.16, for each type of segment listed in the latter table, for people withmasses in the range 40–100 kg.(b) Give several reasons why these ranges seem to be different.

1.5 Summary 29

Table 1.16. An alternative set of relations of weights of body segments (all in lb).(Using data from [48], which used unpublished data by [40])

segment segment weight

head 0.028Wb + 6.354

trunk 0.552Wb − 6.417

upper arms 0.059Wb + 0.862

forearms 0.026Wb + 0.85

hands 0.009Wb + 0.53

upper legs 0.239Wb − 4.844

lower legs 0.067Wb + 2.846

feet 0.016Wb + 1.826

1.26. (a) Show that (1.3) becomes

ρ(in kg/L or g/cm3) = 0.69 + 0.0297S , (1.7)

when S = H/W1/3b is expressed with H in inches and Wb in lb.

(b) Show that the average density for an adult of height 5 ft 10 in and weight170 lb is 1.065 g/cm3, with S = 12.64.(c) Show that the average density for an adult of height 1.78 m and mass77.3 kg is 1.066 g/cm3, with S = 0.418.

1.27. What percentage of body mass is fat, skin, the skeleton, blood, liver,the brain, the lungs, heart, kidneys, and eyes?

1.28. Use Table 1.11 to determine the mass density of blood, skin, the lungs,the air in the lungs, fat, liver, hair, eyes, and blood vessels.

1.29. In modeling heat loss in Chap. 6, a typical man is modeled as a cylinderthat is 1.65 m high with a 0.234 m diameter. If the human density is 1.1 g/cm3,what is the mass (in kg) and weight (in N and lb) of this man?

1.30. (a) If a man has a mass of 70 kg and an average density of 1.1 g/cm3,find the man’s volume.(b) If this man is modeled as a sphere, find his radius and diameter.(c) If this man is 1.72 m high, and is modeled as a right circular cylinder, findthe radius and diameter of this cylinder.(d) Now model a man of this height and mass as a rectangular solid withsquare cross-section, and find the length of the square.(e) Repeat this for a constant rectangular cross-section, and determine thesizes if the long and short rectangle dimensions have a ratio of either 2:1, 3:1,or 4:1.(f) In each above case calculate his surface area and compare it to that pre-dicted by (1.2) for a 1.72-m tall man. Which of the above models seems best?


Fig. 1.17. Cylindrical model of a man used in studies of convective cooling. (From[34], adapted from [51])

1.31. The cylindrical model of a man in Fig. 1.17 was once used in studiesof convective cooling. What are the volume, mass, and exposed surface area(including the bottom of the lower limb) for this person? Assume each fingeris 3.5 in long and has a diameter of 0.875 in and that the mass density of allcomponents is 1.05 g/cm3.

1.32. How much heavier is someone with a totally full stomach, small intes-tine, large intestine and rectum, than when each system is empty? Assumethe mass density of the contents is 1 g/cm3. Express your answer in mass (kg)and weight (N and lb). (Use Table 7.4.)

1.33. Use the anthropometric data to determine the average cross-sectionalarea and diameter of an arm and a leg of a 70 kg man. Assume the crosssection of each is circular.

1.34. Compare the surface area of the standard man given in Table 1.5, alter-natively as predicted by (1.1) and (1.2). Use the data given in Table 1.13.

2

Statics of the Body

The study of the force balance of an object at rest is called “statics.” Moreover,the study of very slow motion can usually be treated as a series of staticconditions – as if there were no motion; this is called “quasistatics.” Afterreviewing the conditions for static equilibrium in three dimensions, we willexamine the useful simplification to two dimensions, examples of which canoften be characterized as one of the three types of levers. We will then applythese equilibrium conditions to the lower arm, hip, and the spine (lower back).Statics is one important area in biomechanics [75, 82, 86, 94].

2.1 Review of Forces, Torques, and Equilibrium

Each force F can be resolved into components in the x, y, and z directions(Fx, Fy, Fz). In a static condition the sum of the forces F in each the x, y,and z directions is zero:

∑Fx = 0,

∑Fy = 0,

∑Fz = 0. (2.1)

The speed of the center of mass of the object in each direction is then constant,and will usually be assumed to be zero here. These forces can be in balanceeither for the entire body or for any part of the body.

Similarly, each torque τ can be resolved into components in the x, y, andz directions (τx, τy, τz). In a static condition, the torques τ about the x-, y-,and z-axis also each sum to zero for the entire body and for any body part:

∑τx = 0,

∑τy = 0,

∑τz = 0. (2.2)

The speed of angular rotation of the object about each axis is then constant,and will usually be assumed to be zero here.

What actually is a torque? Forces describe changes in linear motion – whichmeans changes in velocities, while torques describe how these same forces

38 2 Statics of the Body

Fig. 2.1. Torques and relevant axes

can change angular motion – which means changes in angular velocities. Thediagram in Fig. 2.1b shows that a force F applied in the positive y direction(with component F ) a distance +r from the z-axis, leads to a torque aboutthe z-axis τy of magnitude rF . This leads to motion in the counterclockwisedirection, caused by an angular acceleration that increases the angle θ. Thisis defined as a positive torque about this axis. A negative torque would occur,for example, if the force were applied in the negative y direction. This wouldlead to motion in the clockwise direction, caused by an angular accelerationthat decreases the (signed) angle θ.

In general, the torque (vector τ) about any axis is defined as the vectorcross product between the distance vector from that axis to the point wherethe force is applied r and the force vector F (Fig. 2.1e)

τ = r × F. (2.3)

(You do not need to understand or use this vector cross product, just theresults that are given below.)

Because vectors can be translated anywhere, things may be clearer if wemove both r and F, as in Fig. 2.1b, d, so they originate from where the forceis applied, as in Fig. 2.1c, e, respectively. We will call the angle from the rvector to the F vector θ. The torque τz about the upward axis is

τz = rF sin θ, (2.4)

where r is the magnitude of vector r (the distance from the axis to the pointwhere the force is applied) and F is the magnitude of vector F. For 0◦ < θ <180◦ (or 0 < θ < π in radians), sin θ is positive and the torque is positive(Fig. 2.2a), while for 180◦ < θ < 360◦ (π < θ < 2π), sin θ is negative and thetorque is negative (Fig. 2.2b). When θ = 90◦(= π/2), sin θ = 1 and the torque

2.1 Review of Forces, Torques, and Equilibrium 39

Fig. 2.2. Direction of torques, showing (a) positive and (b) negative torques

is rF , as above. When r and F are either parallel (θ = 0◦(= 0)) or antiparallel(θ = 180◦(= π)), the torque is zero.

Clearly, only the component of the r normal to the F, which we will callr′, contributes to the torque action. In fact, as Fig. 2.3 proves, τz = r′F .Equivalently, only the component of F normal to r, i.e., F ′, contributes to thetorque action, and τz = rF ′. As we will see, sometimes information is providedwhere these normal components of displacement or force are provided, and thetorques can be calculated without explicitly determining the angle betweenthe displacement and force vectors. Consequently,

τz = rF sin θ = r′F = rF ′. (2.5)

This is true for any axis. The axis can be chosen cleverly for a particularproblem to simplify analysis.

In linear motion, a force leads to an acceleration a = d2r/dt2, which isequivalent to a change in velocity v = dr/dt (the magnitude of which is thespeed v) or momentum p = mdv/dt, by

F = ma = mdvdt

=dpdt

, (2.6)

Fig. 2.3. Determining torques from using components of the displacement and forcevectors that are normal to the force and displacement vectors, respectively


where m is the mass of the object. Similarly, a torque leads to an analogouschange in the angle θ and angular frequency Ω = dθ/dt, and orbital angularmomentum L

τ = IdΩ

dt=

dL

dt, (2.7)

where I is the moment of inertia and L = IΩ. In static situations the sumof the forces and torques on the object is zero, so the right-hand side of (2.7)equals zero.

By the way, what we are defining as torques are indeed commonly called“torques” in connection to the rotational and twisting motions of objects, asin this chapter and Chap. 3, but are instead called “moments” in connectionto the bending of objects, as in Chap. 4.

2.2 Statics: Motion in One Plane and Levers

Many problems involve motion in one plane, say the xy plane – for which zis a constant. For example, the motion of knees and elbows is in one plane.Some problems involving motion of the leg about the hip can be treated inthese two dimensions. The six equations in (2.1) and (2.2) then reduce tothree equations:

∑Fx = 0,

∑Fy = 0,

∑τz = 0. (2.8)

(We will adopt this xyz coordinate system because it is conventionally used intwo-dimensional problems, even though it differs from the coordinate systemconvention we adopted for the body in Fig. 1.1.)

These types of problems can be classified as one of the three types of levers(Fig. 2.4). There are examples in the body of each. They can be described byhow a weight W and a force M , provided by a muscle, act on a solid object,say a bone resting on a fulcrum; this represents an articular joint. The weightcan include that of parts of the body as well as external weights. The weightand muscle act at distances dW and dM from the joint. For each type of leverthe total torque is zero when

MdM = WdW, (2.9)

so

M =dW

dMW. (2.10)

The relative directions of the forces and the relative distances of the weightand muscle forces from the joint are different for each type of lever.

In a first class lever, the weight and muscle act on opposite sides of thefulcrum and are in the same direction (Fig. 2.4a). This is the least commontype of lever in the body. Using the x, y coordinate system shown, there are

2.2 Statics: Motion in One Plane and Levers 41

Fig. 2.4. Three types of levers, (a) first, (b) second, and (c) third class levers. Thelarge increase in the distance the weight moves over the change in muscle length inthe third class lever is also seen in part (c)

clearly no forces in the x direction so the first equation in (2.8) is automati-cally satisfied. Since the weight and muscle both act in the same direction –downward – force balance in the y direction requires that the fulcrum providesan upward force of W + M . Balancing torques in the z direction requires achoice of a z-axis. Any axis normal to the xy plane can be chosen. The sim-plest one is an axis at the fulcrum. The weight provides a torque of WdW,while the muscle provides a torque of −MdM. The signs are consistent withthe above discussion. The fulcrum provides no torque about this axis becausethe distance from the fulcrum to the axis is zero. So

∑τz = WdW − MdM = 0. (2.11)

This leads to (2.9), which tells how large the muscle force must be to maintainequilibrium. If the muscle cannot provide this large of a force, there can be nostatic condition. (Example: The lead ball is too heavy to hold up.) If the muscleprovides more than this force, there is motion. (Example: The baseball is beingthrown, as we will see later.) In these two cases,

∑τz in (2.11) is not zero.


Fig. 2.5. Displacing the axis for calculating torques to the right of the weight by adistance x, as shown for a first class lever. For the axis chosen at the fulcrum x = dW.The axis can be laterally displaced anywhere, to the left or right (as shown) of thelever, above or below it, or in it

It may seem that we cheated by choosing the axis at the fulcrum. Actually,we could have chosen the axis anyway in the xy plane. To prove this let uschoose the axis anywhere along the bone, say a distance x to the right of theweight (Fig. 2.5). The torques provided by the weight, fulcrum, and muscleare now Wx, (W +M)(dW −x), and −M(dW +dM−x), respectively. Balancerequires

∑τz = Wx + (W + M)(dW − x) − M(dW + dM − x) = 0, (2.12)

which reduces to (2.11) again.One type of the first class lever is a seesaw or teeter totter. A second type

is the head atop the spinal cord, where the weight of the head is balanced bythe downward effective force of the muscles (Fig. 2.6a). In a third example, the

Fig. 2.6. Examples of first (a,b), second (c), and third (d) class levers in the body

2.3 Statics in the Body 43

triceps brachii pull on the ulna about the elbow pivot balanced by the forceson the forearm. With the upper arm down, the triceps brachii can balance anupward force pushing the hand up (Fig. 2.6b). (With the upper arm pointedup, the triceps brachii can also balance the hand holding a weight.) BecausedM � dW here, the force that the triceps brachii needs to exert is much greaterthan the forces exerted at the hand.

In a second class lever, the muscle and weight act on the same side of thefulcrum, and the weight is nearer to the fulcrum, so M < W (Fig. 2.4b). Thistype of lever is the second most common in the body. One example is standingon tiptoes: the rotation of the foot about the toes (the fulcrum), which wouldbe caused by the weight of the foot, is balanced by the muscle force transmittedby the Achilles tendon (Fig. 2.6c). Another example is pushing down with thetriceps brachii.

In third class levers, the muscle and weight are again on the same sideof the fulcrum, but now the muscle is nearer to the fulcrum than the weight(Fig. 2.4c). This is the most common example in the body. Because oftendM � dW, we see that M = (dW/dM)W � W . This arrangement means thatvery large forces must be exerted by the muscles because of this dW/dM ampli-fication, which seems to be a big disadvantage (and is literally a mechanicaldisadvantage). However, something else is gained in this tradeoff in design.As seen in the Fig. 2.4c, when the bone rotates a given angle, causing a ver-tical displacement yM at the muscle, there is an amplification of the distancetraveled at the position of the weight by dW/dM. As we will see in Chap. 5,muscles are able to contract only a small fraction of their length – whichamounts to at most several cm in many muscles. The length of the biceps isabout 25 cm, and the maximum contraction is by ∼7–8 cm. With this ampli-fication, the weight can now move much more than this. One example of athird class lever is the balancing of the lower arm by the biceps brachii insertedon the radius (Fig. 2.6d). Another is holding a weight with an outstretchedarm.

2.3 Statics in the Body

We will examine the planar forces in the static equilibrium of the lower arm,at the hip, and in the back. In analyzing the lower arm, we will choose suc-cessively more complex and realistic models. We will see that the forces inthe hip and back are quite large, much more than one would expect, andexplains why people often have problems in these parts of the body, prob-lems that can lead to hip replacements and life-long lower back pain. Theapproach for these problems is the same. We consider all elements in oneplane and examine the forces in the (as defined) x and y directions andthe torque in the z direction. Some of the approaches of [65] and [86] arefollowed.


2.3.1 The Lower Arm

We will examine the equilibrium of the forearm balanced by the contractionof the biceps brachii inserted on the forearm long bone called the radius; thisis a continuation of the discussion of third class levers. The relevant bones areshown in Figs. 2.7 and 2.8. In equilibrium, the biceps brachii force countersthe potential rotation about the elbow joint by the weight held in the hand(Fig. 2.9a). We will examine this example for different models, using Fig. 1.15and Tables 1.6 and 1.7 to provide anthropometric information. The forearmis 0.146H long and the hand length is 0.108H, where H is the body height,so the weight held in the hand is about (0.146 + 0.108/2)H = 0.2H from thepivot. (The ball is in the middle of the hand.)

Case 1

The biceps brachii insert about 4 cm from the pivot axis. Say there is a weightWW held in the hand, which is dW = 36 cm from the pivot. (With H =180 cm, 0.2H = 36 cm.) Therefore M = (dW/dM)WW = (36 cm/4 cm)WW =9WW. So for a weight of 100 N the muscle must provide a force of 900 N forbalance. Here N stands for the MKS/SI unit of newtons. Since 1 N � 0.225 lb,equivalently, a 22 lb weight is balanced by 200 lb of force exerted by the bicepsbrachii (Fig. 2.9b).

We have made several assumptions and approximations in this examplewithout explicitly stating them. It is always good to start with simple models.It is equally important to understand exactly what assumptions and approx-imations are being made. Then, the model can be made more realistic. Here,we have assumed that the forearm and upper arm make a 90◦ angle. We havealso neglected the mass of the forearm.

Case 2

Now let us improve the model by including the weight of the forearm WF

(Fig. 2.9c). This is about 0.022Wb, (where Wb is the body weight) (Table 1.7).For a 70 kg (700 N, 160 lb) person, this is ≈15 N (3.4 lb). We can treat the effectof the weight of the forearm as if it were acting at its center of mass, whichis approximately in the middle of the forearm, dF = 0.146H/2 = 13 cm fromthe pivot:

∑τz = MdM − WWdW − WFdF = 0 (2.13)

MdM = WWdW + WFdF (2.14)

M =dW

dMWW +

dF

dMWF. (2.15)

The ratio dF/dM (= 13 cm/4 cm), so now M = 9WW +3.25WF and the muscleforce required to maintain equilibrium has increased to 900 N + 3.25 (15 N) =950 N (210 lb).


Fig. 2.7. Bones of the arm, anterior view. (From [78]. Used with permission)


Fig. 2.8. Bones of the arm, posterior view. (From [78]. Used with permission)


Fig. 2.9. (a) Weight held in the hand, showing the biceps brachii muscles. (b–d)Forces for the equilibrium of a weight held in the hand for Cases 1–3

Case 3

What happens if we no longer assume that the forearm and upper arm makea 90◦ angle? Let us keep the upper arm vertical and let the forearm make anangle θ, which can range over 142◦ (Table 1.10). The force due to the muscleis then still vertical, and those due to the weights of the forearm and ball are,of course, downward. From Fig. 2.9d we see that the torque caused by each ofthese three forces is multiplied by sin θ. Now

∑τz = MdM sin θ − WWdW sin θ − WFdF sin θ = 0 (2.16)

and we arrive at the same result that

MdM = WWdW + WFdF. (2.17)

Actually, we made additional assumptions in this example that we willre-examine later. The distance from the pivot where the biceps brachii in-sert on the radius really changes with θ (Fig. 3.42). Also, while this analysissuggests that the muscle force M required for equilibrium is the same for allangles, there is a subtlety in this result. Equation (2.16) gives the muscle forceneeded to maintain equilibrium. As we will see in Chap. 5, muscles can exert


forces up to a maximum value. If the M from (2.17) can be achieved, thenthere can be equilibrium; if it cannot, then the static condition cannot beachieved. The maximum force that a muscle can exert depends on its length,which, from Fig. 2.9, is clearly a function of θ. So the M in (2.17) may beachievable at some angles (nearer 90◦, where the maximum force turns out tobe greatest) and not at others.

Case 4

The biceps brachii are not the only muscles used to flex the elbow. What hap-pens if we also include the contributions of these other muscles? Figure 2.10shows that the biceps brachii, the brachialis, and the brachioradialis all con-tribute to this flexing. Assuming that θ = 90◦ (which may not be a goodassumption for each muscle), (2.13) is modified to

∑τz = M1dM1 + M2dM2 + M3dM3 − WWdW − WFdF = 0 (2.18)

M1dM1 + M2dM2 + M3dM3 = WWdW + WFdF (2.19)

where M1, M2, and M3 represent the forces exerted by the three musclesMi, respectively. If the physiological cross-sectional areas of the three mus-cles are A1, A2, and A3, respectively (which we usually call PCA), and themuscle force for each can be assumed to be proportional to this area (whichis a pretty good assumption), then Mi = kAi, for i = 1, 2, 3. (We will see

Fig. 2.10. Sketch of the elbow joint for analyzing the statics of the lower arm forCase 4, with the three muscles, the biceps (brachii), brachialis, and brachioradialis,shown in (b–d). (From [76])


Table 2.1. Data for the three elbow muscles used for flexion. (Using data from[76, 95])

muscle moment arm physiological cross-section

di (cm) (PCA) (cm2)

biceps (muscle 1) 4.6 4.6

brachialis (muscle 2) 3.4 7.0

brachioradialis (muscle 3) 7.5 1.5

that this is a good assumption with k reaching a maximum of ∼40 N/cm2

or so.) So,

kA1dM1 + kA2dM2 + kA3dM3 = WWdW + WFdF (2.20)

k =WWdW + WFdF

A1dM1 + A2dM2 + A3dM3

(2.21)

M1 = kA1 = A1WWdW + WFdF


(2.22)



(2.23)



. (2.24)

Using the parameters from Table 2.1, we get M1 = 262 N (biceps), M2 =399 N (brachialis), and M3 = 85 N (brachioradialis) when we generalize Case 2.This compares to the M1 = 696 N that we would obtain for Case 2 with thebiceps alone, using d1 = 4.6 cm (instead of the 4 cm used before, which led to800 N). The total muscle force is 746 N, which is greater than 696 N becausethe brachialis has a relatively small moment arm.

Life is a bit more complex than this result suggests because we assumedthat k has the same value for each muscle. Really Mi = kiAi, and all the ki’sneed not be the same, as long as ki is less than the maximum that can beexerted by the muscles. Unfortunately, if all the kis are not assumed to beequal, we do not have enough information to solve this problem uniquely asposed. The body may solve the indeterminate nature of this problem (withmore variables than conditions) by minimizing energy or optimizing the forcedistribution (shifting the load from one muscle to another) to rest specificmuscles or to keep the weight balanced better (so it will not tip in the hand).

2.3.2 Hip Problems

The hip (pelvis) is not a single bone, but several bones that are fused together(Figs. 2.11 and 2.12). The pelvis is composed of the pelvic girdle and two


Fig. 2.11. Right hip bone in adult. (From [65])

parts of the spinal cord, the sacrum and coccyx. The pelvic girdle itself iscomposed of the right and left coxal (or hip) bones. Each coxal (hip) bone iscomposed of three bones: the ilium (at the top = superior), pubis (bottomfront = inferior, anterior), and ischium (iss-kee’-um) (bottom back = inferior,posterior). The acetabulum (a-si-tab-yoo’-lum) is the socket area where thefemur of the leg (Fig. 2.13) is attached (“hip joint”). Actually, the head of thefemur is in this socket, and is maintained there by the muscles attached atthe greater trochanter. These muscles are collectively called the hip abductormuscles. (The hip abductor muscles are not the only one attached at thegreater trochanter (see below), but they are the ones that contribute to theforce needed for the equilibrium condition in this problem.)

First we will determine the force on the head of the femur and in thehip abductor muscles while the subject is standing on one leg, say the right

Fig. 2.12. Front and side views of the hip. (From [65])


Fig. 2.13. Anterior view of right femur. (From [65])

leg. This is actually a good model for determining these forces during slowwalking. The upper and lower leg and the foot are treated as a rigid body.This problem is solved in two steps. First, the forces in the whole body areanalyzed and then the rigid leg is treated as a free rigid body, whose onlyinteraction with the rest of the body will be the normal force from the hip[65, 85]. The bones in the leg are shown in Figs. 2.14 and 2.15.

Total Body Equilibrium

There are only two forces on the body. The body weight Wb acts downward,and as if it all originated at the center of mass of the body, which is in themidline in the hip (Fig. 2.16). The foot feels an upward normal force fromthe floor of magnitude N . There are no forces in the x direction, and thesetwo forces in the y direction must balance in equilibrium, so N = W . Inequilibrium the body cannot start to rotate, so the torques are zero. It is clearfrom the Fig. 2.16 that this occurs when the foot is directly below the hip, inthe midline. If we choose the pivot axis at the center of mass, the torque fromthe center of mass is zero because the distance term (from the axis to thecenter of mass) is zero and the torque from the normal force is zero becausethe normal force is antiparallel to the distance vector (θ = 180◦). (You canprove for yourself that the total toque is zero for any other axis normal to thexy plane.)

Equilibrium of the Individual Body Component

There are four external forces on the leg (Figs. 2.16 and 2.17):

(a) N is the normal force on the leg from the floor, and we know that N = Wb.


Fig. 2.14. Bones of the leg and hip, anterior view, with names of bones in bold.(From [79]. Used with permission)


Fig. 2.15. Bones of the leg and hip, posterior view, with names of bones in bold.(From [79]. Used with permission)

Fig. 2.16. Anatomical diagram of the leg and hip for someone standing on oneleg, or during slow walking, showing the forces on them and relevant dimensions,including the force exerted on the head of the femur by the acetabulum R and thenet force exerted by the hip abductor muscles. (From [65])

Fig. 2.17. Force diagram for a leg for someone standing on one foot, using Fig. 2.16.(From [65])


(b) Wleg is the weight of the leg. Table 1.7 shows that this is about 0.16Wb.It acts as if it were applied at the center of mass of the leg, which isapproximately halfway down the leg.

(c) R is the reaction force on the leg from the hip, and it is normal to thehip socket. We will define the x and y components Rx and Ry so they arepositive (Fig. 2.17). Equivalently, we could define the magnitude of R andthe angle it makes with the vertical, ϕ.

(d) M is the force (of magnitude M) due to the hip abductor muscles. Thereare actually three muscles involved here: the tensor fascia (fash-ee’-uh)latae (la-tuh) (see Fig. 3.2a), gluteus (gloo’-tee-us) minimus (see Fig. 3.3c),and the gluteus medius (see Fig. 3.3a, b). (The gluteus maximus muscle iswhat the author is sitting on as he is typing this.) These three muscles havea mass ratio of about 1:2:4 and, as we will see, this is roughly the ratio ofthe forces each can exert. The hip abductor muscle structure we consideris a composite of the three muscles. It has been shown that the effectiveaction of this system is � 70◦ to the horizontal, acting on the greatertrochanter.

We have three equations with three unknowns: Rx, Ry, and M . The two forceequations are

∑Fx = M cos 70◦ − Rx = 0, (2.25)

∑Fy = M sin 70◦ − Ry − 0.16Wb + Wb = 0, (2.26)

where Wleg has been replaced by 0.16Wb.We will choose the rotation axis to emanate from the center of the head of

the femur because the reaction force from the acetabulum passes through thispoint. This makes the analysis easier, but, of course, the solution would be thesame if we chose any other parallel axis. The relevant distances of interest areshown in the diagram (obtained from anatomical dimensions and geometry),as needed for torque analysis.

(a) The component of the distance vector perpendicular to the normal force(r′) is 10.8 cm, so the normal force causes a torque of (10.8 cm)Wb. Thisis a positive torque because the normal force induces a counter clockwiserotation about the chosen z-axis (see Fig. 2.2).

(b) The component of the distance vector normal to the force of the weightof the leg is 3.2 cm and this force tends to induce a clockwise rota-tion, so it contributes a torque of −(3.2 cm)Wleg = −(3.2 cm)(0.16Wb) =−(0.5 cm)Wb.

(c) With the choice of the axis, the torque from the reaction force from thehip is zero, because the distance vector and normal force are antiparallel.

(d) The component of the force from the hip abductor muscles normal to thehorizontal distance vector (of magnitude 7.0 cm) is M sin 70◦. Since this


causes a clockwise rotation, the torque is −(7.0 cm)M sin 70◦. So we seethat∑

τz = (10.8 cm)Wb − (3.2 cm)(0.16Wb) + 0 − (7.0 cm)M sin 70◦ = 0,

(2.27)

M =10.8 − 0.57.0 sin 70◦

Wb = 1.57Wb. (2.28)

We see that torque provided by the hip abductor muscles is needed to counterthe torques from the normal force from the floor and the weight of the leg.This normal force torque is much more important than that due to theleg, because of the greater magnitude of the force and the larger momentarm.

Using this value for the muscle force, the force balance in the x directiongives Rx = M cos 70◦ = 0.54Wb. From the balance in the y direction, Ry =M sin 70◦+0.84Wb = 2.31Wb. The magnitude of R = (R2

x+R2y)1/2 = 2.37Wb,

and tan θ = Rx/Ry = 0.54/2.31 = 0.23, so θ = 13◦.Because M � 1.6Wb and R � 2.4Wb, for mb = 90 kg we have Wb = 880 N

(200 lb), and so M � 1,400 N (320 lb) and R � 2,100 N (470 lb). The origin ofhip problems is clear: The force from the hip is much greater than the bodyweight because of the large moment arms.

We next examine a variation of this problem. The person now uses a caneto provide support on the left side while standing on his or her right leg(Fig. 2.18). As shown in Fig. 2.19, the cane is 30.5 cm (1 ft) from the bodymidline. It is supported by and pushed down by the left arm or shoulder.Consequently, there is a normal force Nc from the floor. We assume Nc =Wb/6. We will see that this has two immediate consequences. The right footis no longer directly in the body midline but is displaced a distance L to theright (in the reference of the body) and the normal force felt by the right footNf is no longer the body weight.

Whole body equilibrium gives

Nf + Nc − Wb = Nf + Wb/6 − Wb = 0 (2.29)

or Nf = 5Wb/6. Using the same axis as before, the torque balance is∑

τz = (30.5 cm)Nc − L(Nf) = (30.5 cm)(Wb/6) − L(5Wb/6) = 0 (2.30)

or L = (Nc/Nf)30.5 cm = (1/5)30.5 cm =6.1 cm.For the same leg as in Fig. 2.17, the corresponding distances are differ-

ent because the angle of the leg is now different (as obtained from anatom-ical dimensions and the new geometry) (Fig. 2.20). We will examine thewhole leg equilibrium again. The cane is not explicitly involved, but implic-itly through the changes in the leg position and the load borne by the leg.


Fig. 2.18. Forces on entire person for someone walking with a cane. (From [65])

Fig. 2.19. Force diagram for a person using a cane for some support. (From [65])


Fig. 2.20. Free-body force diagram of the leg for someone walking with a cane.Note that the center of mass of the leg is now 0.33 cm to the left of the verticalfrom the center of the head of the femur, whereas without the cane it was 3.2 cmto the right of the vertical. Consequently, the leg center of mass is 6.65 cm from thevertical line from the greater trochanter, whereas the center of the head of the femuris 6.98 cm from it. (From [65])

Now∑

Fx = M cos 70◦ − Rx = 0, (2.31)∑

Fy = M sin 70◦ − Ry − 0.16Wb + (5/6)Wb = 0, (2.32)∑

τz = +(4.7 cm)(5/6)Wb+(0.33 cm)(0.16Wb)+0−(6.98 cm)M sin 70◦ = 0.

(2.33)

The first equation is unchanged. (We are assuming that the effective angleof the hip abductor muscles with the x-axis is still 70◦, even though this is nolonger rigorously true.) The only change in the second equation is the smallernormal force on the leg. There are two changes in the torque equation. Thefirst term, due to the normal force from the floor, is much smaller due to thechange in the moment arm and the normal force of the leg – the former effectbeing much larger than the latter. The second term, due to the weight of theleg, is much smaller in magnitude and is now a positive torque instead of anegative torque, because the center of mass of the leg is now to the left of thevertical drawn down from the axis (Figs. 2.2 and 2.20).


Table 2.2. Analytic estimates of peak hip forces. (Using data from [82])

activity magnitude/body weight, Wb

walking 4.8–5.5

walking slowly with/without a cane 2.2/3.4

stair ascending/climbing 7.2–7.4

stair descending 7.1

chair raising 3.3

Now M = 0.61Wb, Rx = 0.21Wb and Ry = 1.24Wb, with R = 1.26Wb

and ϕ = 9.5◦. With Wb still 880 N (200 lb), the muscle force M is now 540 N(120 lb) (instead of 1,400 N (320 lb) without the cane) and the reaction forceat the hip R is now 1,100 N (250 lb) (instead of 2,100 N (470 lb)). This is avery big effect, considering that only about 145 N (35 lb) (≈ Wb/6) is restingon the cane. By far the major consequence of the cane is the change of themoment arm of the normal force from the floor from 10.8 to 4.7 cm, becausethe foot shifted to the right by 6.1 cm.

Another variation of this situation is presented in Problems 2.8 and 2.9.Instead of using a cane with the left hand, you will examine the consequencesof carrying a weight on the left side. This greatly increases the moment arms,thereby increasing M and R. Table 2.2 shows estimates for peak hip forcesfor several activities from more detailed analyses.

Excessive hip forces for extended periods of time can thin cartilage andcause other damage; this leads to pain during walking that can sometimes onlybe alleviated with a hip replacement. Total hip replacements entail replacingthe ball of the femur and the acetabulum. The femoral component is a highlypolished ball of a high-strength alloy, such as cobalt–chromium or titanium,with a step that is placed in the canal of the femur. It is often fixed with anacrylic plastic, such as poly(methyl methacrylate), PMMA, “bone cement.”The acetabulum component is a socket made from ultrahigh molecular weightpolyethylene, and may have a metal backing.

2.3.3 Statics of Other Synovial Joints

Other synovial joints can be examined in similar ways by using models ofthe muscles and tendons. They are briefly outlined here; see [86] for moredetails.

Shoulder

The anatomy and musculature of the shoulder are shown in Figs. 1.2 and1.8. Figure 2.21 shows a model of the static equilibrium of a horizontal armthat holds a weight by means of the deltoid muscle. This weight and muscle


Fig. 2.21. Forces on the arm and shoulder, when the arm is abducted to the hor-izontal position and the hand holds a weight, along with the force diagram. (From[86])

are in equilibrium with the weight of the arm and the reaction force on thejoint. Problems 2.15–2.17 address this with and without a weight held in thehand. For reasonable parameters, the component of the muscle force in the xdirection is about 4× that in the y direction, suggesting that holding the armhorizontally is not stable – as we all know.

Dislocation of the shoulder (glenohumeral) joint is common because it isshallow. The large degree of rotational motion of the head of the humerusabout the articulating surface of the glenoid fossa enables this large mo-tion, albeit with little stability. The shoulder joint angles range by ∼249◦

during flexion/extension, ∼182◦ in abduction/adduction, and ∼131◦ for in-ward/outward rotation (Table 1.10). Fracture of the humerus is also relativelycommon.

Knee

The knee joint is really two joints (Fig. 1.3). The tibiofemoral joint is locatedbetween the medial and lateral condyles of the femur and tibia, which areseparated by cartilaginous regions called menisci. (A condyle is the roundedprominence at the end of a bone, often at an articulation joint.) The secondis the patellofemoral joint between the kneecap (patella) and the anterior endof the femoral condyles. Muscle control of the knee is mostly through thequadriceps and hamstring muscles. The quadriceps attach to the quadricepstendon, which attaches to the kneecap, which attaches to the patellar tendon,which attaches to the tibia.

The forces on the static lower leg loaded with an ankle weight, as duringexercise (Fig. 2.22), are shown in Figs. 2.23 and 2.24. The forces shown are


Fig. 2.22. Exercising muscles near and at the knee. (From [86])

due to this added weight, the weight of the lower leg, the quadriceps muscleforce transmitted by the patellar tendon M (of magnitude M), and the jointreaction force R (of magnitude R), while the angle between the horizontaland the leg is β. In equilibrium, the muscle force and the x and y components

Fig. 2.23. Forces on the lower leg, while exercising the muscle around the knee.(From [86])


Fig. 2.24. Resolution of the forces on the lower leg in Fig. 2.23. (From [86])

of the joint force are

M =(bW1 + cW0) cos β

a sin θ, (2.34)

Rx = M cos(θ + β), (2.35)Ry = M sin(θ + β) − W0 − W1. (2.36)

These forces are very large [84, 86]. For a = 12 cm, b = 22 cm, c = 50 cm,W1 = 150 N, W0 = 100 N, θ = 15◦ and β = 45◦, we see that the muscleforce M = 1, 381 N and the joint force R = 1, 171 N. This is examined fur-ther in Problem 2.18. Problems 2.21–2.24 address a related condition, that ofthe crouching position, as occurs during ascending and descending stairs orjumping.

One function of the kneecap is to increase the moment arm (Fig. 2.25).We can analyze the equilibrium of the kneecap at the patellofemoral jointbetween the reaction force on the kneecap from the anterior end of the femoralcondyles, the patellar tendon and the quadriceps tendon. This is shown inFig. 2.26. The compressive force applied on the kneecap is

FP =cos γ − cos α

cos φM (2.37)

at an angle

φ = arctan(

sinα − sin γ

cos γ − cos α

). (2.38)

Ankle

We now examine the equilibrium of the foot. The anatomy of the ankle andfoot are depicted in Figs. 2.14 and 2.15 (also see Figs. 1.8 and 3.4). Figures 2.27


Fig. 2.25. The presence of the kneecap (patella) increases the moment arm in thelever. (From [86])

Fig. 2.26. Force diagram of the kneecap (patella) in equilibrium. (From [86])


Fig. 2.27. Force diagram of the foot on tiptoe, showing that they form a concurrentsystem. (From [86])

and 2.28 show the forces when someone stands on tiptoes on one foot. Thereaction force on the talus bone of the foot is in balance with the normal forcefrom the floor (equal to the body weight) and the muscle force transmittedby the Achilles tendon on the calcaneus (heel). (The mass of the foot itself isneglected here.) The muscle and reaction forces are

M = Wbcos β

cos(θ + β), (2.39)

R = Wbcos θ

cos(θ + β). (2.40)

Fig. 2.28. Components of the forces acting on a foot on tiptoe. (From [86])


Fig. 2.29. The masseter and temporal muscles in the jaw, about the temporo-mandibular joint. (From [88])

These are both much greater than the body weight Wb. For θ = 45◦ andβ = 60◦, we see that M = 1.93Wb and R = 2.73Wb.

Jaws and Teeth

Forces on teeth arise from several sources [72]. Figure 2.29 shows how themasseter muscles provide the force in the lever system involved in chewingand biting. Lever models can be used to examine the quasistatics of chewingand biting. (See Problem 2.27.)

Orthodontics is the practical application of biomechanics to move teethusing forces applied by appliances, such as wires, brackets, and elastics [81,87, 90]. Each tooth has a center of mass, but since teeth are not free bodies –they are restrained by the periodontium – a more useful position in the toothis defined, the center of resistance. This is the balance point for the tooth.Figure 2.30 shows how forces and torques (moments) applied to the crown of atooth, can be designed to create a lateral force at the center of resistance, butno torque about it. Appliances can affect several teeth, such as the intrusion

Fig. 2.30. In orthodontics forces and torques are applied to the crown (left), leadingto forces (and in this case no torques) about the center of resistance. (Reprinted from[81]. Used with permission of Elsevier)


Fig. 2.31. Photo of an intrusion arch used in orthodontics. (Reprinted from [81].Used with permission of Elsevier)

arch shown in Fig. 2.31, which leads to the application of forces and torquesshown in Fig. 2.32.

2.3.4 Lower Back Problems

Most people eventually develop chronic lower back problems. The best wayto avoid such a persistent and annoying problem is to try to avoid situationsthat might trigger your first back problem, such as bending over and liftingheavy objects, or sleeping on very hard beds. We will examine a simple staticsmodel that will show why you should never bend and lift. (You should usethe muscles in your legs to lift and not those in your back.) We will examinethe force on the fifth lumbar vertebra when you bend and lift.

The spinal cord consists of 33 vertebra with 26 bones, and is classified intofive sections. Starting from the top (superior) in Fig. 2.33, there are sevencervical, twelve thoracic – which rhymes with “Jurassic” – and five lumbarvertebra, and then five fused vertebra in the sacrum (sae’krum) and four fusedvertebra in the coccyx (koak’-sis) (tail bone). Figure 2.34 shows two lum-bar vertebra with the central regions of each, the centrum, separated by theintervertebral disc; more detail about the vertebrae is shown in Fig. 2.35. Note

Fig. 2.32. Forces and torques applied by the intrusion arch in Fig. 2.31. (Reprintedfrom [81]. Used with permission of Elsevier)

Fig. 2.33. The vertebral column (spine). (From [65].) The thoracic and sacral curvesare primary curves, while the cervical and lumbar curves are secondary curves

Fig. 2.34. Side view of two vertebrae separated by a vertebral disc. (From [65])

Fig. 2.35. Vertebra viewed from above. (From [65])


Fig. 2.36. Cross-section of two vertebrae and a vertebral disc with nerve for (a) anormal intervertebral disc and (b) one that has degenerated and is compressing thenerve root. (From [65])

the spinal nerve root (in Fig. 2.36) (pain region) and the spinous processes.The distinct vertebrae become successively larger down the spinal cord, be-cause of the additional load they bear. This combination of vertebrae andintervertebral discs provides flexibility in the spinal cord, but also causes po-tential problems.

The spinal cord is not straight; each section is curved. At birth, only thethoracic and sacral curves are developed. These primary curves are in thesame direction and lead to the “fetal position.” At three months, the cervicalcurve develops, so the baby can hold his/her head up. When the baby learnsto stand and walk, the lumbar curve develops. These secondary curves havecurvature opposite to that of the primary curves (Fig. 2.33). Figure 2.37 showsthe lumbosacral angle between the fifth lumbar vertebra and the sacrum.

Fig. 2.37. The lumbosacral angle is defined as that between the horizontal and thetop surface of the sacrum. (From [65])


Deviations in the angle from ∼30◦ can lead to lower back pain. We will modelthe spinal cord as a rigid bar even though this description of the spinal cordcurves would suggest a more complex model.

There is a series of massive muscles from the posterior parts of the il-iac crest and sacrum to the skull (occipital bone), called the erector spinae(spy’nee) (or sacrospinalis (sae-kro-spy-na’-lis)) (Figs. 2.38 and 2.39). Theywill be modeled as a single muscle inserted 2/3 from the center of mass of thehead and arms, at a 12◦ angle (Fig. 2.40).

Consider the spinal cord at an angle θ to the horizontal (Fig. 2.40); initiallywe will take θ = 30◦, corresponding to a deep bend. It is hinged at the lum-bosacral disc, just below the fifth lumbar vertebra and the sacrum (Figs. 2.37and 2.40). We will choose the axis for torque analysis right there. There isa reaction force R (of magnitude R) from the sacrum with components Rx

and Ry. The weight of the trunk (above the hips, excluding arms and head)W1 acts half way down the spinal cord. The weight of the arms, head, andany object lifted, W2, act at the top of the spinal cord. These are shown inFig. 2.40 along with the erector spinae force M (of magnitude M ). Using Ta-ble 1.7, it is reasonable to approximate W1 = 0.4Wb and W2 = 0.2Wb, withnothing being lifted.

Figure 2.40 shows that M acts at an angle that is θ − 12◦ relative to thehorizontal; for our first example θ = 30◦ and so this angle is 18◦. The forcebalances are

∑Fx = Rx − M cos 18◦ = 0, (2.41)

∑Fy = Ry − M sin 18◦ − 0.4Wb − 0.2Wb = 0. (2.42)

With this choice of rotation axis, the torque due to the reaction force is zero.The component of M normal to the spinal cord is F ′ = M sin 12◦, leading toa torque (2L/3)(sin 12◦)(M). The fraction of each weight force normal to thespinal cord is θ = cos 30◦. Torque balance requires

∑τz =

2L

3sin 12◦(M) − L

2cos 30◦(0.4Wb) − L cos 30◦(0.2Wb) = 0. (2.43)

The torque equation gives M = 2.5Wb or 2,200 N (500 lb) for the 880 N(200 lb) body weight of a 90 kg body mass. The reaction force parameters areRx = M cos 18◦ = 2.38Wb, Ry = 1.37Wb, φ = arctan(Ry/Rx) = 30◦ (whichis the angle the reaction force makes with the horizontal), and R = 2.74Wb,which is 2,400 N (540 lb) here. Clearly, the muscle forces and reaction forceon the lower spinal cord are much larger than the body weight. The momentarms of the weights are about the same as that of the muscle. However, thedirection of weight forces lead to large torques at large bending angles, whilethe direction of the muscle force does not lead to a large torque to balancethe weights – at any bending angle.


Fig. 2.38. Intermediate (left) and deep (right) layers of back muscles – showing theerector spinae muscles. The erector spinae consists of lateral columns (the iliocostalislumborum, thoracis, and cervicis muscles), intermediate columns (the longissimusthoracis, cervicis, and capitis muscles), and a medial column (spinalis thoracis).(From [93])

Matters are even worse if you hold a weight in your arms; this simulatesbending and lifting an object of this weight. All this does is to increase W2.Let us increase W2 by 0.2Wb (180 N (40 lb) in our example) to 0.4Wb. Now

∑Fx = Rx − M cos 18◦ = 0, (2.44)


Fig. 2.39. Diagram of the erector spinae muscles used to control the trunk whenbending. (From [65])

Fig. 2.40. Free-body diagram of the vertebral column while bending, with the spinemodeled as a straight bar at an angle θ to the horizontal, which we will take to be∼30◦ – a bit steeper bend than is depicted here and shown in the inset. The angleof R to the horizontal is φ. With nothing being lifted, we will take W1 = 0.4Wb andW2 = 0.2Wb. (From [65])


Table 2.3. Forces in the body during bending and lifting

θ M M R R

(no lifting) (extra 0.2Wb) (no lifting) (extra 0.2Wb)

30◦ 2.50 3.74 2.74 4.07

60◦ 1.44 2.16 1.93 2.81

80◦ 0.50 0.75 1.08 1.53

90◦ 0 0 0.60 0.80

All forces are in units of the body weight. For a body mass of 90 kg, multiply eachnumber by 880N (200 lb)

∑Fy = Ry − M sin 18◦ − 0.4Wb − 0.4Wb = 0, (2.45)

∑τz =

2L

3sin 12◦(M) − L

2cos 30◦(0.4Wb) − L cos 30◦(0.4Wb) = 0. (2.46)

The force the erector spinae muscles need to exert increases to 3.74Wb

or 3,300 N (740 lb). The muscle must exert an additional 1,100 N (250 lb) tobalance only an additional 180 N (40 lb). We see that Rx = 3.56Wb, Ry =1.96Wb, and R = 4.07Wb. This is 3,600 N (810 lb), an additional 1,200 N(270 lb) of reaction force on the fifth lumbar vertebra.

Table 2.3 shows these forces for several bending angles. We can see whybending itself, and bending and lifting can lead to problems with the backmuscles and the lower vertebra discs.

What does this mean for the lumbosacral (intervertebral) disc? Let us con-sider our initial example of bending by 60◦ from the upright position, to 30◦

from the horizontal, without any lifting. The reaction force on this disc is2,400 N (540 lb). This force pushes down on the top and up on the bottomof this cylindrical disc. We are assuming a load that is normal to the axis,which is not exactly correct at these angles. Figure 2.41 shows how much theheight of the disc decreases (fractionally) with this type of load. The weightcorresponding to 250 kg is 2,400 N (540 lb), so by using this figure a 20% con-traction is expected parallel to the spine. If the disc is a cylinder of radius rand height H, then its volume is πr2H. If the material in the disc is incom-pressible, its volume will not change with this compression and if H decreasesby 20%, r will increase by about 10%. The bulging disc can press against thenerve, as seen in Fig. 2.36b. With lifting, the reaction force goes up more, thecompression of the disc increases, and there is more bulging and more irrita-tion of the nerves. (We have compared a 245 kg (2,400 N, 540 lb) load on thedisc to a situation with no force on this disc. In the upright position, thereis 0.6× the body mass (and weight), 54 kg (530 N, 120 lb) already supportedby this disc due to body weight, so the load is really changing from 530 N(120 lb) to 2,400 N (540 lb). (How does this affect our numerical results andconclusions?) Also the loading is not uniform when you lift or bend, and therecan be tension and compression and bulging due to this, as in Fig. 2.42.)


Fig. 2.41. Loading of wet lumbar vertebral discs of persons 40–59 years of age vs.percent compression. For the loading in N, the ordinate scale needs to be multipliedby 9.8. (From [65]. Based on [96])

This teaches us two things. (1) Understanding the mechanical propertiesof the parts of the body is essential to understand the implications of forceson the body. (2) You should never bend and lift. Problems 2.33–2.35 examinethe torques during lifting of objects of different sizes, with different upperbody positions (as above), and with different techniques.

These lower back forces depend not only on spine angle and load, but onknee bending during lifting and where the load is positioned relative to thebody. During lying the lower back forces are 20–50% of those while standingupright relaxed (Table 2.4), and reach values over double the weight and over

Fig. 2.42. Disc compression for a person who is (a) vertical, (b) bending forward,and (c) bending backward. Bending forward leads to disc compression anteriorlyand tension posteriorly, bulging on the compressive side, and the shifting of the discnucleus posteriorly. (From [88])


Table 2.4. Values of intradiscal pressure for different positions and exercise, relativeto that during relaxed standing. (Using data from [80, 83, 92])

position or activity %

still

lying supine 20

side-lying 24

lying prone 22

lying prone, extended back, supporting elbows 50

relaxed standing 100

standing, bent forward 220

sitting relaxed, no back rest 92

sitting actively straightening back 110

sitting with maximum flexion (bent forward) 166

sitting bent forward, thigh supporting the elbows 86

sitting slouched in a chair 54

motion

standing up from chair 220

walking barefoot or in tennis shoes 106–130

jogging with shoes 70–180

climbing stairs, one at a time 100–140

climbing stairs, two at a time 60–240

walking down stairs, one at a time 76–120

walking down stairs, two at a time 60–180

lifting

lifting 20 kg, no bent knees 460

lifting 20 kg, bent knees, weight near body 340

holding 20 kg near body 220

holding 20 kg, 60 cm from chest 360

3× the torso weight during fast walking (Fig. 2.43). Table 2.4 shows that thepressure between vertebral discs for people sitting is minimized when theyslouch, so when your parents tell you to stop slouching you can respond thatyou are trying to minimize intradisc pressure for long-term care of your spinalcord.

How can you maintain your back and relieve lower back pain if you haverelatively minor lower back damage? Stretch your back muscles. Always bendyour knees and use your leg muscles (and not your back) when you lift. Makesure that you maintain the curvature of your lower back at all times. Youshould sit on chairs with lower back support (see Fig. 2.44 and [91]), butwhen you must sit on flat back chairs, you may want to use a rolled-up towelor a pillow at the bottom of your back to help maintain the curvature. Whenyou sleep, maintain one of two positions: on your back with your knees bent


Fig. 2.43. Axial load on the disc between the L3 and L4 vertebra while walking atdifferent speeds. LHS and RHS are left and right heel strike, respectively. (Based on[68, 83])

and feet pulled up (bent hips and knees) or the fetal position; never sleep onyour front. (One reason for this is that in the supine position with straightlegs, the vertebral portion of the psoas muscle puts a load on the lumbarspine, whereas this muscle relaxes and this load decreases when the hips andknees are bent and supported [83].) Only sleep on beds that allow your back

Fig. 2.44. The pressure on the third lumbar disc is decreased with backward back-rest inclination and with lumbar support. Also, it is increased with support in thethoracic region, which is not shown here. Chairs with some backward inclinationand lumbar support provide needed support, while those with an upright flat backor that curve toward the body can cause painful pressure. (Based on [64, 69])


Fig. 2.45. Hyperextension exercises recommended (at the Tientzin Hospital inChina) to strengthen the back of lumbago patients (i.e., those with mild to severepain or discomfort in the lower back); each is performed with the patient’s backkept hollow. Several other exercises are also recommended for those with lower backpain. If you have lower back pain, please consult your physician before attemptingany of these exercises. (From [70])

to maintain its natural curvature; beds should not be too firm or too soft.Some hyperextension exercises that can strengthen muscles to help your backare shown in Fig. 2.45. (Guess, who has a lower back problem?)

2.3.5 Three-Force Rule

Consider the foot–lower leg combination shown in Fig. 2.46a, which is a modelfor a person walking upstairs. There are three forces acting on this isolated

Fig. 2.46. (a) Illustrating the three-force rule on the free lower leg, with reactionforce N, patellar tendon force M, point where these extended vector lines meetP , and the point where the joint force acts on the tibia Q . (b) Vector diagram ofthree-force rule with R being the joint reaction force at Q . (From [76])


system. The normal force N acts vertically at the front of the foot and forslow stair climbing has a magnitude equal to the body weight Wb. This nor-mal force tries to rotate the lower leg clockwise about the knee. The patel-lar tendon transmits the force of the thigh quadriceps muscle M, as shown,and tries to counter this rotation. The direction of this force is along thistendon and the direction of this tendon is known. There is also a reac-tion force R exerted by the femoral condyles on the top of the tibia, act-ing as shown at point Q. In this two-dimensional problem, there are threeequations (two for force balance and one for torque balance) and three un-knowns: the magnitudes of M and R, and the direction of R. As such, thisproblem can be solved in the straightforward way illustrated earlier. How-ever, there is a graphical procedure called the three-force rule that offerssome additional insight and, of course, the same answers as the straightfor-ward method. (For simplicity, we are ignoring the mass of the leg and foothere.)

Extending the N and M vectors, we see that they intersect at point P. If wechoose the torque rotation axis perpendicular to this point, we see that thesetwo forces contribute nothing to the torque since their r and F vectors are,respectively, antiparallel and parallel to each other. Since in static equilibrium(or the quasistatic situation here) the net torque is zero, the reaction forcevector R must also pass through P when it is extended, and so R is parallelto the QP line segment. Because the directions of M and R are now known,we can place them at the head and tail of the N, respectively, as shownin Fig. 2.46b (always with the head of one vector to the tail of the other,as in adding vectors). The lengths of both vectors are now determined bylengthening them until they hit each other. Then the sum of the force inthe x and y directions is zero (because of the closed triangle), and we haveobtained all of the needed information.

By the way, the reaction forces at the knee are ∼3–4Wb during walking[82] and are much greater during stair climbing.

2.3.6 Multisegment Modeling

Several parts of the body are often important in modeling the body, bothin static situations and for those involving motion. As an example, Fig. 2.47shows how an anatomical model of leg can be modeled by three segments,describing the upper leg, lower leg, and the foot. Each segment is labeled byits mass (m), center of mass (dot), and moment of inertia (I). In this examplethere are also three joints (freely rotating hinges or pins), between the hipand upper leg, the upper leg and lower leg (the knee), and the lower leg andfoot (the ankle), each denoted by an open circle. The forces by muscles andnormal forces at the joints and other places (such as the floor) can be addedto this link segment model, and the forces on torques on the entire objectcan be analyzed or those on each segment can be analyzed by itself. (This is


Fig. 2.47. Relationship between an anatomical model of the leg and a link segmentmodel of the upper leg, lower leg, and foot. (From [95]. Reprinted with permissionof Wiley)

somewhat similar to examining the whole body and then the leg by itself inthe above analysis of the hip.)

In analyzing each segment in Fig. 2.47 by itself, you arrive at a free bodydiagram for each segment, as in Fig. 2.48. In this example, the reaction forces

Fig. 2.48. Relationship between the link segment of the leg with a free-body di-agram, with individual upper leg, lower leg, and foot. (From [95]. Reprinted withpermission of Wiley)

2.4 The Sense of Touch 79

at every body joint are shown (R), and at each particular joint the forces areequal in magnitude and opposite in direction due to Newton’s Third Law, suchas R2 shown for the knee. The forces due to each muscle (or effective set ofmuscles) could be added, acting with equal magnitudes but opposite directionsat the origin and point of insertion (again Newton’s Third Law). (This isexamined in Problem 2.37.) In the free-body diagram shown in Fig. 2.48, thetotal torque (which is called moment M in the figure) about each joint isshown.

2.4 The Sense of Touch

The body also “feels” force. This is the sense of touch. The somatic senses,include the sense of touch by the skin (cutaneous sensations), the sense ofposition of the limbs (proprioception), and the sense of movement of the limbs(kinesthesis). There are sensors in the skin for tactile perception, perceptionof temperature, and the perception of pain [73].

There are four types of tactile receptors in glabrous (hair-free) skin(Fig. 2.49), each sensitive to stresses and displacements at different frequen-cies. Merkel receptors (or disks), located near the border between the epi-dermis (outermost skin layer) and dermis (the underlying layer), are mostsensitive to pressure disturbances which vary in the 0.3–3 Hz range. Theother receptors are deeper within the dermis. Meissner corpuscles sense light

Fig. 2.49. Cross-section of (a) hair-free and (b) hairy skin, showing tactile sensors.(From [89])


tapping, which is characterized by 3–40 Hz variations, and sensed as flutter.Ruffini cylinders (or endings) sense faster vibrations, 15–400 Hz, such as thosecaused by stretching of the skin or joint movement, and sensed as buzzing.Pacinian corpuscles sense a range of rapid vibrations, from 10 to 500 Hz. Mo-tion of hair also stimulates these receptors.

The spatial density of mechanical receptors on the skin varies from 25/cm2

at the tip of the tongue to 0.02/cm2 on the back [66]. The movement of hairon hairy skin contributes to the sense of touch, along with sensing by thesefour receptors. The mechanism for converting these impulses into electricalsignals is discussed in Chap. 12.

The skin also has two types of receptors that sense temperature. Warmfiber thermoreceptors increase their firing rate only when it is warmer and coldfibers only when it is colder; they are not sensitive to mechanical stimulation.Warm fibers respond from 30 to 48◦C, and best at about 44◦C, while coldfibers respond from 20 to 45◦C, and best at 30◦C. There are on the order of5–10 thermal receptors per cm2 over most of the skin. This sense of temper-ature is important in the body’s drive for temperature control (Chap. 13).

Nociceptors in the skin sense excessive pressure, extreme temperature, andcorrosive chemicals, each which can damage the skin. Some pain receptorstransmit signals very rapidly to the brain, on myelinated axons in neuronswith conduction speeds up to 30 m/s (see Chap. 12). Some pain receptorsindicate persistent pain and have signals that are transmitted on very slowunmyelinated axons with speeds of 2 m/s and slower.

As characterized by Stevens’ Law ((1.6), Table 1.15), the variation of theperceived stimulus is slightly sublinear with stimulus strength for vibration,somewhat superlinear for sensing pressure on the palm and the heaviness oflifted objects, linear for coldness, and superlinear for warmth.

A problem in Chap. 7 (Problem 7.14, Fig. 7.24) addresses how to use thesense of touch to estimate the internal pressure in an elastic vessel. This is anexample of palpation, which is the use of a physician’s hands to examine partsof the body during a medical examination. (A physician also taps the body inthe diagnostic known as percussion. A third physical diagnostic is listening tosounds emanating from the body, which is called auscultation; it is discussedin Chap. 10.)

2.5 Diversion into the Units of Force and Pressure

2.5.1 Force

We should be clear on the correct units of force, as in Table 2.5. In the English(or FPS) system it is pounds (lb). In the metric MKS-SI system it is newtons(N) and in the CGS systems it is dynes. Most technical work is in MKS-SI;however, much medical work is done using the English system (in the US). We

2.5 Diversion into the Units of Force and Pressure 81

Table 2.5. Units of force

1 N = 1kg-m/s2

1 N = 105 dyne

1 N = 0.225 lb (∼2/9 lb)

1 lb = 4.45 N

will usually use lb or N, with lb used when referring to body weights. (Alsosee Appendix A.)

One kilogram (kg) is a mass (m) that at sea level (on the planet Earth) hasa weight of 9.8 N (or 2.2 lb). This is a consequence of gravitational accelerationon earth and Newton’s Second Law of motion

F = mg, (2.47)

where g = 9.8 m/s2. “g” is the acceleration of any freely falling object dueto gravity. In CGS units, a mass of 1 g has a weight of 980.7 dynes becauseg = 980.7 cm/s2. In the English system g = 32.2 ft/s2. The unit of mass inthe English system is the slug; it is rarely used in the US. One slug has aweight of 32.2 lb. (Sometimes in the biomechanics literature the loading massis given, and it is possible for the unit “kg” to be mistakenly referred to as aforce. What is really meant is the weight of an object with a mass of 1 kg. Tolessen confusion, this should be expressed as 1 kg (force). Even better, when1 kg (force) is seen it should be replaced by 9.8 N (∼10 N).)

2.5.2 Pressure

We will encounter the concept of pressure later. Pressure (P ) is simply a forceper unit area. (More generally, a force per unit area is a stress (Table 2.6).Pressure or more precisely hydrostatic pressure is a stress that is the same inall directions.) The units of pressure (or stress σ) are the units of force dividedby the units of area. Because work (or energy) is force × distance, the unitsof pressure are the same as those of energy/volume. (Also see Appendix A.)

The standard MKS-SI unit of pressure is a pascal (Pa), with 1 Pa =1 N/m2. We will find that often the numbers we will encounter are simplerin units of N/mm2, which are the same as the units MPa. Common units inEnglish units are pounds per square inch (psi).

These units are independent of the planet we happen to be on. Some com-mon units of pressure are specific to Earth (at sea level, at 0◦C temperature).A column of a liquid of mass density ρ and height h exerts a pressure

P = ρgh. (2.48)

This is the same as (2.47), after both sides have been divided by the columnarea A, because P = F/A and m = ρ × volume = ρAh. At sea level on


Table 2.6. Units of pressure

1 N/m2 = 1 Pa = 9.87 × 10−6 atm. = 0.0075 mmHg = 0.102 mmH2O

1 N/mm2 = 106 N/m2 = 106 Pa = 1MPa = 145 psi = 9.87 atm. � 10 atm.

1 psi = 0.0069 N/mm2 = 6,894.8 N/m2 (or Pa) = 1/14.7 atm. = 0.068 atm.

1 bar = 105 N/m2 (or Pa) = 0.1 N/mm2 �1 atm.

1 atm. = 1.013 × 105 Pa = 1.013 bar = 0.103 N/mm2 (or MPa) = 14.7 psi

= 760mmHg = 29.9 inchHg = 1,033 cmH2O = 407 inchH2O

1 mmHg = 0.00132 atm. = 133 N/m2 = 13.6 mmH2O

1 mmH2O = 0.1 cmH2O = 9.68 × 10−5 atm. = 9.81 N/m2 = 0.0735 mmHg

1 kg (force)/cm2 = 9.8 N/cm2

We will often use MPa (= N/mm2), but occasionally use other units, such as mmHgwhen discussing blood pressure

Earth (0◦C) the air pressure is 1 atmosphere (1 atm.), which is the pressureexerted by the air column above it. The same pressure is exerted by 760 mmof Hg (mercury) or 1,033 cm of water. The units of mmHg and cmH2O (orinches of H2O) are very commonly used even though they really refer to hin (2.48) and not pressure. (In a calculation, they need to be multiplied byρHg = 13.6 g/cm3 or ρH2O = 1.0 g/cm3 and then by g. Note that ρblood =1.0 g/cm3.) In the US, air pressure is commonly expressed in weather reportsin units of inches of Hg, as in “the air pressure is 29.8 in and dropping.” Bloodpressure is commonly reported in mmHg, as in 120/80, which means that thesystolic and diastolic pressures are, respectively, 120 mmHg and 80 mmHg. Airpressures in the body, such as in the lungs, are sometimes expressed in termsof cm or inches of water, because it is a smaller and much more convenientunit. Pressure is often referenced to atmospheric pressure, so a blood pressureof 120 mmHg really means an absolute pressure that is 120 mmHg above at-mospheric pressure (with 1 atm. = 760 mmHg), and as such is called a gaugepressure.

It is an amazing coincidence that the Earth-based unit of atmosphericpressure (1 atm.) is within 1% of the Earth-independent MKS-SI-based unitof a “bar” (1 bar = 105 N/m2 (or Pa)), with 1 atm. = 1.013 bar.

2.6 Summary

Force and torque balance can often be analyzed in terms of levers, such asfor the lower arm. The equilibrium of a part of the body can be analyzed byexamining the forces due to gravity, muscles, and reaction forces in the wholebody and that body part separately. Muscle and internal reaction forces canexceed the body weight, as was seen in analyzing the forces in the hip andlower back, and this can have serious consequences.

2.6 Summary 83

Problems

Statics of the Arm and Levers

2.1. A person holds a weight with her arm extended horizontally, using herdeltoid muscles to balance the weight about the shoulder joint. What type oflever is this? Sketch and label this lever.

2.2. A person holds himself up in a pushup position. Consider one of theperson’s arms, with the pivot point being the hand on the ground. Are thebiceps brachii or triceps brachii involved? What type of lever is this? Sketchand label this lever.

2.3. For each of the following, identify the type of lever, and show in a sketchthe locations of the applied force, the fulcrum, and the load being applied:(a) cutting with a pair of scissors(b) lifting a wheelbarrow(c) picking up something with a pair of tweezers.

2.4. Analyze the force balance in the x and y directions and the torque balancein the z-direction for the lower arm, with a vertical upper arm and a hori-zontal lower arm (length 35 cm) (Fig. 2.9). As with Case 1 ignore the forearmweight and other muscles. Now the effect of the reaction force N on the joint(with components Nx and Ny) is explicitly included (because it contributesno torque it did not have to be included in the earlier Case 1 analysis) andassume the biceps are attached 4 cm from the pivot at an angle θ to the lowerarm. With θ = 75◦, find the muscle force M and the magnitude of the reactionforce in terms of the weight W and the angle of this reaction force relative tothe x (horizontal) axis.

Statics of the Hip and Leg

2.5. Using Fig. 1.15, what can you say about the height of the person whoseleg is depicted in Fig. 2.17?

2.6. Show how the dimensions given in Fig. 2.16 change to those in Fig. 2.20when the person holds a cane.

2.7. In examining the hip forces during the equilibrium of a man standing onone foot, we assumed that the effective angle of the hip abductor muscles to thex-axis was the same, 70◦, without or with a cane. Would using more realistic(and different) values of this angle without or with a cane, significantly affectthe conclusions concerning the effect of using a cane?


Fig. 2.50. Forces on hip and femoral head while standing on one leg and liftinga weight with the opposite hand. (Reprinted from [75]. Used with permission ofElsevier.) For Problem 2.8

2.8. A 200 lb man stands on his right foot while carrying a 100 lb bag in hisleft hand. The center of mass of the bag is 12 in from his center of mass (seeFig. 2.50).(a) Show that the placement of the foot (as shown) leads to no net torque inthe body.(b) Find the force (its magnitude and direction) on the head of the sup-port femur and the force in the hip abductor muscle by examining theright leg.(c) Compare your answers in parts (a) and (b) with what was found for theman holding no mass – without and with a cane (for Wb = 880 N = 200 lb).Are the forces here greater than for a (200 lb + 100 lb =) 300 lb man (with nocane). Why? (The muscle angle and leg mass are only trivially different forthe problem given here and those analyzed above.)

2.9. Redraw Fig. 2.50, changing all distances and forces into metric units andthen do Problem 2.8.

2.6 Summary 85

2.10. Calculate the force on the hip abductor muscles for a person standingsymmetrically on two feet, as a function of foot separation. For what positionis this force zero?

2.11. In the arabesque position an initially upright gymnast kicks one legbackward and upward while keeping it straight (mass mleg), pushes her torsoand head forward (mass mtorso+head), and propels her arms backward andupward while keeping them straight (each marm). They, respectively, makeacute angles θleg, θtorso+head, and θarms, to the horizontal. The centers of massof the extended leg and vertical balancing leg are xextended leg and xbalancing leg

behind the vertical from the center of mass (in the midsagittal plane), andthat of her upper body (torso/head/arms combination) is xupper body in frontof this vertical (so all of these distances are defined as positive). In achievingthis arabesque position her center of mass drops vertically from height ybefore

to yafter.(a) Draw a diagram showing the gymnast before and during this maneuver.(b) Find the equilibrium condition in terms of these masses and distances.(Hint: Analyze the torques about her center of mass in the arabesque position.You can ignore the contribution from her arms. Why? You may not need allof the information that is presented.)(c) Assume the gymnast is 1.49 m (4 ft 11 in) tall and has a mass of 38 kg(weight 84 lb) and her free leg and arms make a 30◦ angle with the horizontal.What angle does her upper body make with the horizontal? (Assume theanthropometric relations for a standard human.)

2.12. Redo Problem 2.11c if the free leg of the gymnast is horizontal and herarms are vertical. (This is the cheerleading position.)

2.13. (a) Calculate the torque of the diver of mass mb about the axis throughher toes (normal to a sagittal plane) when she is on a diving board and leaningover and about to dive, so the vertical axis through her center of mass is adistance x in front of her toes.(b) Now say that the diver has a height H and that her body is proportionedas per the data given in Chap. 1. Calculate the torque in terms of H, withher body straight with arms stretched parallel to her torso. Assume that herbody can be straight (and so everything in her body can be approximated asbeing in one plane) and ignore the change in position because she is on hertoes.(c) Redo this if her arms are instead along her sides. How does this torquediffer from that in part (b)?

2.14. The split Russel traction device is used to stabilize the leg, as depictedin Fig. 2.51, along with the relevant force diagram [86]. The leg is stabilizedby two weights, W1 and W2, attached to the leg by two cables. The leg andcast have a combined weight of W1 = 300 N and a center of mass 2/3 of theway from the left, as shown. The cable for W2 makes an angle β = 45◦ with


Fig. 2.51. The split Russel traction device. (From [86].) For Problem 2.14

the horizontal. For equilibrium, find the tension in the cables T1 and T2 andangle the cable for W1 makes with the horizontal, α.

Statics of the Shoulder, Knee, Ankle, and Jaw

2.15. You are able to hold your arm in an outstretched position because ofthe deltoid muscle (Fig. 2.52). The force diagram for this is shown in Fig. 2.53.Use the three equilibrium conditions to determine the tension T in the deltoidmuscle needed to achieve this equilibrium, and the vertical and horizontal com-ponents of the force exerted by the scapula (shoulder blade) on the humerus.Assume the weight of the humerus is mg = 8 lb and the deltoid muscle makean angle of α = 17◦ to the humerus. (From [65].)

2.16. Solve the more general shoulder problem with a weight in the hand,depicted in Fig. 2.21, by finding M , R, and β. Now evaluate the x and ycomponents of the muscle force, the magnitude of the joint reaction force,and its angle for the following parameters: a = 15 cm, b = 30 cm, c = 60 cm,θ = 15◦, W = 40 N, and W0 = 60 N.

Fig. 2.52. Deltoid muscle during lifting with an outstretched arm. (From [65].) ForProblem 2.15

2.6 Summary 87

Fig. 2.53. Force diagram for the deltoid muscle and reaction forces during liftingwith an outstretched arm. (This is a simpler version of Fig. 2.21.) (From [65].) ForProblem 2.15

2.17. (a) A gymnast of mass mb suspends himself on the rings with his bodyupright and straight arms that are horizontal with which he clutches the rings.Each ring is suspended by a rope with tension T that makes an acute angleθ with his arms, and the rings are separated by a distance d. Solve for the Tand θ. Assume symmetry.(b) If the gymnast weighs 600 N, θ = 75◦, and d = 1.8 m, find T . (From [74].)

2.18. Derive (2.34)–(2.36) for the equilibrium of the lower leg with an ankleweight.

2.19. Determine the angle between the leg and the reaction force at the kneefor the conditions given in the text for Fig. 2.23.

2.20. Analyze how the patellar tendon and reaction forces depend on the ankleweight (for a fixed leg weight) and leg angle for Fig. 2.23. During exercise, whatare the advantages of varying this weight vs. this angle?

2.21. Consider the equilibrium of the foot during crouching for a 200-lb per-son, with the force through the Achilles tendon, the reaction force of the tibia,and the normal force from the floor in balance, as in Fig. 2.54 – neglecting theweight of the foot for simplicity. Take the angle α = 38◦.(a) Why is the normal force from the floor 100 lb?(b) Find the magnitude of the Achilles tendon tension T and the magnitudeand direction of the reaction force F.

2.22. The topic of Problem 2.21 is similar to the discussion of forces on thefoot in Chap. 3, where we will assume that all forces are parallel or antiparallelto each other and normal to a bar, as in a lever. Is this totally valid? Why orwhy not?

2.23. Redo Problem 2.21, now including the mass of the foot. If the distancefrom the bottom of the tibia (where the normal force emanates) is 4 in andthe center of mass of the foot is halfway between it and the ground. Use thedata for the mass of the foot in Chap. 1.


Fig. 2.54. Forces on the foot during crouching. (From [65].) For Problem 2.21

2.24. In the crouching position, the lower leg is held in equilibrium throughthe action of the patellar ligament, which is attached to the upper tibia andruns over the kneecap. As depicted in Fig. 2.55, the forces acting on the lowerleg are N, R, and T. If the lower leg is in equilibrium, determine the magnitudeof the tension T in the patellar ligament, and the direction and magnitude ofR. Assume that the tension acts at a point directly below the point of actionof R. Take the normal force equal to 100 lb (half the body weight), the weightof the leg Wleg as 20 lb, and the angle α = 40◦ (for the leg at a 45◦ angle).(From [65].)

2.25. Derive (2.37) and (2.38) for the equilibrium of the kneecap.

2.26. Derive (2.39) and (2.40) for the equilibrium of the foot.

Fig. 2.55. Forces on the lower leg during crouching. (From [65].) For Problem 2.24

2.6 Summary 89

2.27. Laterally from the midline in either jaw, we have eight teeth: two in-cisors, one canine, two bicuspids (or premolars), and three molars (the lastmolar being a wisdom tooth). (This tooth order can be remembered fromits acronym: ICBM – which itself is easy to remember because it is also theacronym for Intercontinental Ballistic Missiles.) The lateral distances fromthe temporomandibular joint to the insertion of the masseter muscles, thefirst bicuspids, and the central incisors, are 0.4L, L, and 1.2L, respectively.(a) What type of lever is involved in biting and chewing with the massetermuscles?(b) For biting in equilibrium with a masseter muscle force of 1,625 N, showthat the force on the first bicuspid of 650 N, assuming there is no force on thecentral incisors [67]. Draw a force diagram for this.(c) Under these conditions, show that the needed counter force on the centralincisors is 540 N, now assuming no force on the first bicuspid. Draw a forcediagram for this.

2.28. When you bite an apple with your incisors only, you exert a force of650 N on it. When you bite an apple with your bicuspids only, you exert aforce of 540 N on it. Find the force per unit area (which is called the stress)on the apple for both cases if the effective contact areas of the incisors andbicuspids are 5 mm2 and 1 mm2, respectively.

Statics of the Back

2.29. In analyzing bending, we assumed that the weight of the trunk (abovethe hips, excluding arms and head) is W1 = 0.4Wb and the weight of the armsand head is W2 = 0.2Wb. Is this reasonable. Why?

2.30. Consider a woman of height 1.6 m and mass 50 kg.(a) Calculate the reaction force on her lower vertebrae and the force in hererector spinae muscle when she is either upright or bent at 60◦ (and conse-quently 30◦ to the horizontal).(b) Recalculate these forces when she is pregnant. Assume that during preg-nancy the mass of her torso increases by 15 kg, but the center of mass of thetorso is the same.(c) The forces in part (b) are equivalent to those for the same nonpregnantwoman who lifts a weight of what mass?

2.31. Describe the designs of the back of a chair that could lead to pain inthe lumbar vertebrae and those that would give good lumbar support.

2.32. We showed that when the force on the lumbosacral (intervertebral) discincreases from 0 to 2,400 N, the disc height H decreases by 20% and thedisc radius r increases by about 10%. When it is recognized that the load


on the disc for a vertical person is really 530 N (and not 0), how do the discdimensions really change when the person bends to an angle of 30◦ (and thento a load of 550 lb)?

2.33. Why is it more difficult to lift bulky objects? A person lifts a package ofmass 20 kg in front of her so the back of the package touches her abdomen.The horizontal distance from the person’s lumbar-sacral disc to the front ofher abdomen in 20 cm. Calculate the bending moments (in N-m) about thecenter of mass of her disc caused by the lifted loads, assuming the package isalternatively 20 or 40 cm deep [83]. Draw force diagrams for these two cases.The other dimensions of the packages are the same and they both have uniformdensity. How does this show that the size of the lifted object affects the loadon the lumbar spine?

2.34. Why is it better to standard erect when you hold an object? A personholds a 20 kg object while either standing erect or bending over. The mass ofthe person above his lumbar-sacral disc (his torso) is 45 kg. When upright, thecenter of mass of the torso is (horizontally) 2 cm in front of his disc and thatof the object is 30 cm in front of his disc. When bent, the center of mass ofthe torso is 25 cm in front of his disc and that of the object is 40 cm in frontof his disc. Draw force diagrams for these two cases. Calculate the bendingmoments (in N-m) about the center of mass of his disc caused by holding thisload while either being upright or bent over [83]. How does this show thatbending when lifting an object affects the load on the lumbar spine?

2.35. Why is it best to lift an object with bent legs and the object very close toyou? A person lifts a 20 kg object while either bending over with legs straight,with bent knees and the object near to her body, or with bent knees and theobject far from her body. The mass of the person above her lumbar-sacral disc(her torso) is 45 kg. When bent over with straight legs, the center of mass ofher torso is (horizontally) 25 cm in front of her disc and that of the object is40 cm in front of her disc. When bent over with bent knees and the objectnear her body, the center of mass of her torso is (horizontally) 18 cm in frontof her disc and that of the object is 35 cm in front of her disc. When bentover with bent knees and the object far from her body, the center of mass ofher torso is (horizontally) 25 cm in front of her disc and that of the object is50 cm in front of her disc. Draw force diagrams for these three cases. Calculatethe bending moments (in N-m) about the center of mass of her disc for thesethree lifting methods [83]. Which position is the worst? How does this showthat the position when lifting an object affects the load on the lumbar spine?

2.36. One position during shoveling snow or soil is shown in Fig. 2.56. Assumethis is an equilibrium position.(a) If the shovel and contents have a mass of 10 kg, with a center of mass 1 mfrom the lumbar vertebra, find the moment about that vertebra.(b) If the back muscles are 5 cm behind the center of the disc, find the mag-nitude and direction of the muscle force needed for equilibrium.

2.6 Summary 91

Fig. 2.56. Shoveling. (From [71].) For Problem 2.36

(c) Find the force on the intervertebral disc.(d) If the abdominal muscles of the person are strong and can provide someupward force, would that help relieve stress to the back muscles and the disc?Why?

Multisegment Modeling

2.37. (a) Sketch an (in-plane) multisegment model of the leg showing theforces on the upper leg, lower leg, and ankle – using the resultant forces inFig. 2.57. Show the center of mass gravity forces on each segment, along withthe normal forces (at each body joint and with the floor).(b) Label all distances and angles needed to analyze the in-plane forces andtorques. For each segment, label the distances starting from the proximal end.Label each angle between muscle and bone; use the angles as shown, with theacute angle when possible.(c) Write the equilibrium force balance and torque equation for each of thesegments.

Fig. 2.57. Idealized resultant forces for six muscle groups in the leg. (Based on[77].) For Problem 2.37


(d) For each of these equilibrium conditions, sum the equations – such as thetorque equations – for the upper leg, lower leg, and foot. Show that the threeresulting balance equations are the correct equations for the entire leg.

Sense of Touch

2.38. (a) A 50 kg person stands on her fingertips. Assuming each finger makesa 1 cm2 contact area with the ground, find the pressure on each finger tip.(b) Which tactile sensors are sensing this pressure?

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