MUSCLE SYSTEM 8d.pdfSYSTEM There are three types of muscle in the human body. The most abundant type...

8MUSCLESYSTEM

There are three types of muscle in the humanbody. The most abundant type is called skeletalmuscle because virtually all these muscles are at-tached to the bones of the skeletal system. Skel-etal muscle makes up the more than 600 musclesin the body, most of which are close to the sur-face of the body, between the integumentary sys-tem and the bones (Fig. 8.1). Many muscles bulgewhen they contract; therefore, they are visible andcan be felt as firm lumps under the skin. Thischapter is concerned mainly with skeletal muscle.

Cardiac muscle is found exclusively in theheart (Chap. 4). Visceral muscle, or smooth muscle,is found within organs in many body systems.

MAIN FUNCTIONS FORHOMEOSTASIS

The muscle system performs three functions thathelp maintain homeostasis: movement, support,and heat production.

Movement

The movement produced by muscles allows aperson to carry out the last step in negative feed-back systems: making an adjustment to a changein conditions. Movement is used to get away fromimpending danger (e.g., fire, falling objects), es-cape from unfavorable conditions (e.g., intensesunlight), and eliminate wastes and unwantedmaterials (e.g., carbon dioxide, splinters).

Movement is also important in taking positiveactions. It allows a person to move toward, ob-tain, and use items and conditions that promotethe welfare of the body and quality of life. Theseneeds include basic physical needs (e.g., food,water, shelter) and other needs (e.g., social inter-actions, recreational activities). Movement allowspeople to rearrange their environment and con-struct and repair useful and decorative artifactsto suit human requirements and desires.

Support

The muscle system provides support whenmuscle contractions prevent the movement of apart of the body. Support maintains proper po-sitional conditions of parts of the body so thatthey function well. For example, muscle contrac-tions can maintain an upright posture. This ac-tivity includes holding the bones in place and

ill

174 Human Aging: Biological Perspectives

FIGURE 8.1 Skeletal muscles in the muscle system. preventing the protrusion of the organs in thelower trunk. With proper posture, circulation isimproved because blood vessels are open ratherthan pinched shut, and respiration is assistedbecause the lungs have room to inflate easily.Holding the head up positions the eyes for view-ing the surrounding environment.

Heat Production

Heat production is essential for maintaining aproper and fairly stable body temperature be-cause most people live in environments that arecooler than normal body temperature. Therefore,the body is always losing heat to the environment,just as any warm object or substance, such aswarm food, loses heat and becomes cool. How-ever, if the body is allowed to become cool, thiswill make the rate of its chemical reactions too

slow to sustain life functions (e.g., heartbeat, res-piration, brain activities) and perform effectivenegative feedback responses. Therefore, to pre-vent cooling of the body, the amount of heat lossmust be balanced by an equal amount of heat pro-duction. .

Heat is produced by many chemical reactionsin the body, but the muscle system is the mainheat producer. One reason for this is that themuscle system is one of the largest systems, usu-ally accounting for one-third or more of bodymass. Second, the muscle system is one of themost active systems in the body. When a personis awake but resting, this activity involves steady ~

muscle contractions (muscle tone) that help main-tain posture. This system is especially active andproduces much more heat when a person is force-fully contracting muscles during vigorous exer-cise. Still, the muscle system performs manychemical reactions even when the muscles are re-

laxed; this is why a sleeping person remainswarm.

AGE CHANGES VERSUS OTHERCHANGES

The functioning of the muscle system depends onthe nervous, circulatory, and respiratory systems.As a result, many age-related changes in this sys-tem derive from age-related changes in the othersystems, which vary greatly among individuals.Furthermore, alterations in exercise received bymuscles quickly and dramatically affect the

Chapter 8 - Muscle System 175

muscle system. Though there is an average age-related decline in exercise, changes in the totalamount of exercise of the body and of each musclevary greatly both from time to time and from oneperson to another. Two consequences of thesevariables are that they add to the age-related in-crease in heterogeneity among people and makeit quite difficult to identify true age changes inthe muscle system. Therefore, in this chapter thecauses of each age-related change will be noted.

MUSCLE CELLS

The muscles are composed primarily of musclecells, which perform the three functions of themuscle system. Other materials in each muscleinclude nerve cells, collagen and elastin fibers, fat,and blood vessels (Fig. 8.2).

Structure and Functioning

The main activity of muscle cells is contraction,which produces both the force needed for move-ment and support and most of the heat derivedfrom the muscle system. Muscle cells have manyspecializations that permit them to perform con-traction.

Muscle cells are very long and thin, reachinglengths of up to several centimeters. Usually, thesecells are as long as the muscle in which they arecontained. Because of their shape, muscle cells arealso called muscle fibers (Fig. 8.2).

Cell Membrane (Sarcolemma) The muscle cellmembrane (sarcolemma) is modified in threeways (Fig. 8.3). First, the spot on the membranethat receives stimulatory messages from a somaticmotor neuron is highly convoluted. This modi-fied area (motor end plate) apparently providesmore surface area and receptor molecules to re-ceive and respond to molecules of acetylcholinefrom the somatic motor neuron. Second, the cellmembrane can carry messages in the form of ac-tion potentials, just as axons do. Third, the mem-brane has many penetrating indentations (T tu-bules), which deliver action potentials deepwithin the cell.

Myoglobin, Oxygen, and Energy The musclecell cytoplasm (sarcoplasm) contains a proteincalled myoglobin, which is found only in musclecells and causes muscles to appear red in color.

FIGURE 8.2 Components of a muscle.

Muscle

Bone (scapula)

Tendon

Nerve .

Blood v",rel

PERIMYSIUM

(fibers sum>undingf.sicle)

Nerves

PERIMYSIUM

«ibers ..",ounding

fascicle)

Myoglobin attracts oxygen from the blood into themuscle cells and stores oxygen. As soon as amuscle cell uses some of the oxygen, its myoglo-bin quickly attracts more (Fig. 8.4).

The muscle cell uses the oxygen to obtain en-ergy from sugar and other nutrient molecules. Aslong as the cell has enough oxygen, it can obtainmuch energy from nutrients while producing onlycarbon dioxide (CO) and water as waste prod-ucts. The CO2 and water are easily removed fromthe cell and can be eliminated from the body bythe respiratory and urinary systems, respectively.

When a person engages in vigorous activity, theamount of oxygen required to produce the energyneeded by a muscle cell often rises above the sup-ply of oxygen to the cell. The cell can continue towork because some energy can be obtained bybreaking nutrients down partially. One of themain waste products from this process is lacticacid (Fig. 8.4), which tends to accumulate inmuscle cells and causes them to become acidic. A

result of lactic acid accumulation is weakening ofthe muscle cell's contractions. The affected per-son experiences fatigue in the forms of muscleweakness and muscle pain. The person also feelsout of breath.

If activity decreases, the circulatory and respi-ratory systems can again deliver oxygen to themuscle cell faster than oxygen is consumed. Theextra oxygen is used to complete the breakdownof lactic acid into CO2 and water; this not onlyeliminates the lactic acid but also makes a large


FIGURE 8.3 Components of a muscle cell.

MUSCLE FIBER

Actin and myosin ImyofilamentsMyofibrils

T tubule

Sarcoplasmicreticulum

amount of energy available to the muscle cell. Theaffected person's sensation of fatigue subsidesand he or she may claim, "I have caught mybreath." The oxygen used to eliminate the lacticacid produced by vigorous exercise is called theoxygen debt.

Much of what has just been said is also true ofcardiac muscle cells. For example, when cardiacor skeletal muscle cells accumulate lactic acid,they become weak. However, unlike cardiacmuscle cells, skeletal muscle cells are rarely seri-ously injured or killed by lactic acid. These cellscan continue to work with lactic acid present aslong as the acid concentration does not becometoo severe. Still, exceedingly high levels of lactic acidwill prevent skeletal muscle cells from contracting.

Contraction The membranes of the endoplas-mic reticulum within muscle cells are arrangedin the form of lacy tubes that extend over thelength of the cell (Fig. 8.3). These membranes are-

FIGURE 8.4 Obtaining energy in muscle cells.

Myoglobin

---?>

Musclecell Mitochondrion

~

called sarcoplasmic reticulum and regulate themovement of calcium ions needed for contraction.

The region in the cell surrounded by each tubeof sarcoplasmic reticulum contains an array oftiny fibers called myofilaments. Clusters of thickmyofilaments alternate with clusters of thin myo-filaments (Fig. 8.5). The alternating clusters over-lap to form units called sarcomeres, which extendfrom one end of the cell to the other like links in a

chain. Each chain of sarcomeres is surrounded bysarcoplasmic reticulum and is called a myofibril.

When the cell is stimulated and action poten-tials pass over the sarcolemma, the sarcoplasmicreticulum releases calcium, which causes the thickmyofilamentsto pull on the thin myofilamentsand slide farther among them. The pulling andsliding cause the muscle cell to become shorter;contraction has occurred. The contraction appliesa pulling force to the bone or other structure towhich the muscle is attached, and the structure iseither moved or held in place.

Recall that energy for contraction comes fromthe breakdown of nutrient molecules. Only someof the energy released from these molecules isconverted into movement of the myofilaments;the remainder is converted into heat. This is whymuscles produce so much heat when they contract I

Types of Muscle Cells The proportions of musclecell components are different among muscle cells,so the cells have different characteristics. TypeIfibers contract more slowly and can work longer


FIGURE 8.5 Myofilaments and the process ofcontraction: (a) Contraction begins. (b) Contrac-tion ends. Sarcomeres and cells are shorter.

Sarcomere Myosinmyofilamentsr

Cross- A band

m~ ~ ~

(a) Contraction begins

~

---~ ~

t ' nends(b) Contrac 10

before becoming fatigued. Type IIAfibers contractmore quickly and resist becoming fatigued, also.Type IIB fibers contract quickly, but they becomefatigued quickly. Type IIC fibers are intermediatebetween Type IIA and Type lIB. Type IIA and TypelIBfibers are most important for fast and power-ful movements. Different muscles have different

combinations of these types of muscle cells, and thecombinations change gradually during adulthood.

Age Changes in Muscle Cells

Internal Components As muscle cells age, theconvolutions in the motor end plate decrease andthe sarcolemma becomes smoother. The resultingdecrease in surface area diminishes the ability ofthe muscle cell to be stimulated by the motor neu-ron. Other changes in the sarcolemma cause theaction potentials that lead to contraction to be-come weaker, slower, and more irregular. Becauseof the changes in the action potentials, the celltakes longer to begin to contract and is less ableto recover from one contraction and prepare forthe next. Age-related slowing of calcium releaseand retrieval by the sarcoplasmic reticulum con-tribute to these effects.

The large-scale results of these cellular agechanges include a longer time to respond when aperson wants to move suddenly and a diminishedability to perform rapidly repeated movementssuch as playing fast music on a piano. Musclecomposed of aging cells also has a weaker maxi-mum strength when used for activities requiringrapid and very strong contractions, such as grasp-ing a handrail to stop a fall.

Another change in aging muscle cells is a de-crease in the substances used to supply energyfor contraction (ATP, creatine phosphate, glyco-gen). Much of this change seems to be caused bya decrease in exercise rather than by aging, Lackof exercise also seems to cause most of the de-

crease in the enzymes that extract energy fromnutrients. There is even a decrease in the number

and size of mitochondria, which perform most ofthe energy extraction. Many remaining mitochon-dria have been damaged, so they are less efficientand produce more free radicals (*FRs). Somemuscle cells seem to accumulate damaged mito-chondria and become sources of *FR damage tosurrounding cells. All these changes leave the cellswith less energy, especially for tasks requiring aprolonged effort.

The final substantive change inside musclecells is a decrease in the number of sarcomeres

within the myofibrils. This tends to cause the cellsand the muscles they compose to become shorterand have a reduced distance through which theycan move. The affected person experiences stiff-ness and diminished freedom of movement. The

loss of sarcomeres also reduces the strength of thecells and muscles.

Cell Thickness Since muscle cells that get littleexercise lose parts of their internal components,they decrease in thickness. This shrinkage isprevalent among the elderly because of the gen-eral reduction in physical activity as people age.Regularly exercised muscles show little change incell thickness until age 70 or beyond. Even then,there is only a slight thinning of cells in musclesthat receive plenty of exercise. Therefore, reduc-tion in exercise rather than aging is the main causeof muscle cell thinning and much of the conse-quent decrease in muscle thickness and strengththat usually accompanies advancing age.

Cell Number Most of the decrease in the thick-

ness of muscles with aging is caused by the death


of muscle cells. Up to half the muscle cells in amuscle may be lost by late old age. This loss oc-curs in exercised muscles and in muscles receiv-

ing little use. Lost muscle cells are not replacedby new ones because except in very unusual cir-cumstances, adult muscle cells cannot form newmuscle cells.

Type II fibers become thinner and are lost fasterthan Type I fibers. The ratios of loss are differentfor different muscles. Some muscles may loseType II fibers more than twice as fast as they loseType I fibers. Type lIB fibers are lost faster thanType IIA fibers. Some of the loss of Type II fibersmay be from conversion to Type I fibers. Most age-related decreases in strength and speed resultfrom thinning and loss of Type II fibers.

In muscles receiving much regular strenuousexercise, the space left by the lost cells may belargely filled by the remaining cells. This occursbecause muscle cells pulling against heavy loadson a regular basis adapt by synthesizing moreinternal components. The additional componentsincrease the thickness and strength of these cells,which encroach on the vacant areas. As a result,the decline in thickness and strength of exercisedmuscles is slow.

Aging muscles that receive little strenuous ex-ercise have the spaces left by lost cells filled withfibrous tissue and fat. Such muscles become thin-

ner and considerably weaker as time passes.

Cell Repair Though muscle cells are unable toreproduce, they can repair themselves after aninjury. One common cause of injury is contract-ing against a load much heavier than that nor-mally encountered by muscle cells. This type ofinjury can be sustained when a person who nor-mally lifts objects weighing less than 30 poundstries to support a 60-pound object.

A muscle containing muscle cells injured by anexcessive force, such as lifting a heavy object, isweakened and causes the sensations of musclesoreness and stiffness. If the muscle is rested, theinjured muscle cells will repair themselves withina few days and the soreness and stiffness will sub-side. As was mentioned above, the cells will adaptto the heavy demands previously placed on themby becoming thicker and stronger. They will thenbe more resistant to injury caused by excessiveloads. For muscle cells receiving regular strenu-ous exercise, the ability to repair injury and re-

NERVE-MUSCLEINTERACTION

cover from such weakness and soreness is not

al tered by aging.It is uncertain whether muscle cells that receive

little exercise can repair themselves as quickly asexercised muscle cells do. Still, muscle cells inexercised or unused muscles retain the ability toadapt to heavier loads by manufacturing internalcomponents. Thus, the thickness and strength ofmuscles can be increased by strenuous exerciseregardless of age. However, muscle cells in olderindividuals make the compensatory increase inthickness more slowly.

Motor Units

Recall from Chap. 6 that skeletal muscle cells arestimulated to contract by nerve cells called so-matic motor neurons. The axon from each motor

neuron branches as it passes through its muscle.Some motor neurons have only a few branches,while others have several hundred.

Each branch from a motor neuron axon endson a muscle cell, and each muscle cell receives abranch from only one motor neuron (Fig. 8.6).Thus, the muscle cells in a muscle are organizedinto groups, with all the cells in each group beingcontrolled by one motor neuron. The.combinationof one motor neuron and all the muscle cells itcontrols is the functional unit of the muscle andis called a motor unit.

When an impulse travels down a motor neu-ron, it passes along every branch of its axon.Therefore, every muscle cell in the motor unit isstimulated to contract; it is not possible to causeonly some of these cells to contract.

The strength of each contraction is determinedby which motor units and how many motor unitsare activated at a given time. Since more variedcombinations of numbers of muscle cells can be

selected in muscles with small motor units, a per-son has more control over the amount of strengthprovided by each contraction in such muscles. Itis more difficult to select precise levels of strengthfrom muscles with large motor units because themuscle cells contract in larger groups. The differ-ence in the degree of control is similar to the dif.ference between the ability to pay an exact amountwhen one has many one dollar bills and small


FIGURE8.6 Motor units.

SpinalcordAxonal terminals at

neuromuscular junctions

change and the ability to pay an exact amountwhen one has only bills of large denominations.

Changes in Motor Units

Since motor units change in many ways as peopleget older, the functioning of a muscle alsochanges. Some of these changes and their conse-quences were described in Chap. 6.

One change is an exponential decrease in thenumber of motor neurons. The loss may reach 50percent by age 60. This is the main reason for thedecrease in the number of muscle cells because a

muscle cell degenerates and dies if it does not re-ceive stimulation from a motor neuron. As moremotor neurons and their muscle cells are lost, themaximum strength of contraction the muscle canproduce diminishes.

Fortunately, many surviving motor neuronsproduce additional axon branches that connect tothe orphaned muscle cells. These adopted musclecells survive and function. This compensatoryprocess helps slow the decline in the strength ofthe muscle. Note, however, that the size of theremaining motor units increases. This means thatthere is a decrease in control of the strength ofeach contraction. This may be one reason peoplehave a reduced ability for fine movements as ageincreases. Also, Type II fibers are often "adopted"by motor neurons from Type I fibers. This alter-

ation speeds up the conversion of Type II fibersto Type I fibers.

A second age change is a slowing in the pas-sage of impulses to muscle cells. There is a vari-able amount of slowing among the motor neuronscontrolling a muscle. As mentioned in Chap. 6,three alterations in the overall contraction of the

muscle result. First, it takes longer for the muscleto reach its peak strength of contraction. Second,the peak amount of strength is lower. Third, theentire contraction takes more time. These alter-ations further reduce the maximum amount of

strength a muscle can pr.oduce and make it moredifficult to perform very quick movements.

Another change in motor neurons is a decreasein the frequency with which impulses are sent tothe muscle. Normally, a motor neuron sends avolley of impulses in rapid succession so that themuscle cells contract rapidly. A rapid series ofcontractions-incomplete tetany-provides afairly smooth and strong contraction that can bemaintained for a long time. Since age changes inmuscle cell action potentials decrease the fre-quency at which muscle cells can contract, reduc-ing the frequency of neuron impulses may be com-pensatory. Sending impulses faster than themuscle cells can respond would be wasteful ofneuron energy and neurotransmitter materials.

Other Nerve-Muscle Interactions

Several other changes in the nervous system al-ter the operation of muscles as people age. Recallfrom Chap. 6 that age changes in sensory neurons,synapses used by reflex pathways, and other ar-eas of the central nervous system involved in con-trolling voluntary movements all affect adverselythe ability of the muscle system to maintain ho-meostasis and the quality of life.

BLOOD FLOWIN MUSCLES

Muscle cells depend on the circulatory system tosupply oxygen, nutrients, and other needed ma-terials and to remove wastes. Service of musclecells diminishes somewhat even when no disease

of the circulatory system is present. Exactly howmuch of this decrease is due to age changes andhow much is due to a reduction in exercise is notknown.

Reasons for the reduced ability of the circula-tory system to meet the needs of muscle cells


include the decrease in the density of capillariesamong muscle cells and age changes in capillarystructure. These changes may contribute to thedecline in the maximum rate of working and thefaster onset of muscle fatigue as people advancein age. These and other age changes in and dis-eases of the circulatory system that can affect themuscle system were discussed in Chap. 4.

CHANGES IN MUSCLE MASS

The many changes at the cellular and microscopiclevels in the muscle system combine to reduce thethickness of each muscle and therefore the totalamount of muscle mass. Serious loss of muscle

mass is called sarcopenia. On the average,sarcopenia begins during the third decade. Therate of loss is low at first, but the rate increaseswith age, rising quickly after age 50. Muscle massmay decrease as much as 50 percent by age 80.This increasingly rapid loss seems to be due pri-marily to the decline in physical activity that usu-ally accompanies advancing age. Most of the lossof muscle mass and thickness is due to the loss of

muscle cells rather than to thinning of the cells.

Effects of Mass on Strength

The decline in muscle mass produces several ef-fects, one of which is a decline in muscle strength.This loss of strength is related to the total thick-ness of a muscle since the amount of strength perunit of cross-sectional area of muscle cells remains

fairly stable regardless of age. However, the re-duction in muscle strength that accompanies ag-ing is only partially due to thinning of themuscles. Other important factors include changesin muscle cell structure and functioning and in-creases in fat and fibrous material among themuscle cells. Changes in factors outside themuscle system (e.g., nervous system, joints, mo-tivation) also play an important role in the de-cline in strength with age.

In general, strength peaks during the third de-cade and declines little during the fourth decade.Age-related decrease in strength becomes morerapid and significant during the fifth decade. Thedecline in strength becomes faster as age increasesafter that. However, the decline in muscle strengthvaries considerably from person to person andfrom muscle to muscle. There is variation with

respect to the age at which a substantial reduc-

tion in strength can first be detected and the rateat which strength declines afterward. Musclesused for quick strong contractions show a greaterdecline in strength than do muscles used to main-tain posture or perform other actions requiringlong-lasting mild contractions.

It seems that the most important reason forheterogeneity in loss of strength is the increasedvariability in the amount of strenuous exerciseperformed regularly by each person and eachmuscle. For example, individuals whose dailyroutines include gripping objects or tools lose gripstrength slowly, but these individuals may havefairly rapid loss of leg strength if their activitiesinclude little use of the legs.

The amount of strength lost over a period ofyears may impair an individual's ability to carryout ordinary activities such as shopping, garden-ing, cleaning, climbing stairs, and breathingheavily during exertion. It becomes increasinglydifficult to continue in certain lines of employ-ment, such as those requiring lifting or movingheavy loads. It may be necessary to forsakestrenuous recreational activities such as sailing.Still, many aging individuals can tolerate declin-ing strength by using methods requiring less brutestrength, substituting power tools and appliancesfor muscle power, and enlisting aid from others.

The unevenness in loss of strength among dif-ferent muscles creates an additional problem inthe form of reduced coordination. This occurs

because the balance in strength among themuscles used to perform an action is altered. Animportant effect of dwindling strength and de-cline in coordination is an increase in the risk of

falling. Reduced and unbalanced muscle strengthalso modifies posture. Detrimental outcomes fromdeteriorating posture may include biological ef-fects (e.g., restricted ability to inhale, impinge-ment of bones on nerves), social and psychologi-cal effects of altered appearance, and economiceffects from the need to obtain different clothingor furniture.

Other Effects

The reduction in muscle mass accompanying ag-ing can have effects other than changes instrength. A change in body proportions can havesocial, psychological, and economic consequencesfor the reasons noted above related to altered pos-ture. Another effect is the need to modify one's


diet. With less muscle mass, there is a decrease inthe basal metabolic rate and a consequent de-crease in the amount of calories needed per day.Though the diet consumed by most aging peopleshould contain fewer calories, it should be richerin protein to maintain the remaining muscle masswhile preventing an undesirable gain in weight.The declining metabolic rate, along with a rela-tive decrease in the proportion of body mass com-posed of lean muscle, also necessitates adjust-ments in the doses of medications.

MUSCLE SYSTEM PERFORMANCE

Reaction Time and Speed of Movement

All the changes in the muscle system alreadymentioned, combined with aging in the nervous,circulatory, respiratory, and skeletal systems, leadto other noticeable alterations in the actions pro-duced by muscles. One is an increase in the timeneeded to begin a voluntary motion in responseto a stimulus (reaction time). For example, it takeslonger for a driver to move his or her foot fromthe gas pedal to the brake when a traffic signalturns red. Most of the increase in reaction time is

caused by slowing of the processing of impulsesin the central nervous system.

Note that by definition, reaction time endswhen the person begins to move. The time fromthe beginning of a motion to the end of that mo-tion also increases with age. This second alter-ation, a decrease in the speed of movement, iscaused by decreasing muscle strength.

Both the increase in reaction time and the de-

crease in speed of movement make the perfor-mance of rapid movements difficult. As can beseen in the example of driving, these changes in-crease certain risks. There is also an increased risk

of falling. The probability of sustaining greaterinjury from a fall also rises because it takes longerto grasp a handrail or piece of furniture or tochange body position to break or cushion the fall.

Longer reaction times and slower movementsalso make it more difficult to perform rapidly re-peated movements such as those used in playingfast music or dancing. The effects of these changesbecome greater when individuals attempt morecomplex or less familiar movements. As with de-clining strength, changes in reaction time andspeed of movement occur faster as age increases.

Skill

A third aspect of muscle activity that changes withage is skill in performing tasks. Though changesin reaction time and speed of movement haveprofound adverse effects on a person's skill inperforming novel activities, they have much lessof an impact on activities that have been per-formed routinely for many years. Skill in well-practiced actions can even improve with age ifrepetition of the movements involved continues.

Practice also reduces the frequency of errorsin performing an intended movement and select-ing sequences of movements to complete compli-cated tasks. New strategies are formulated, andthe efficiency of energy use improves with prac-tice. Therefore, experienced older individuals mayperform better than do younger individuals inactivities requiring both strength and speed.

Stamina

The advantage of experience can be overshad-owed by a gradual drop in stamina. Stamina maybe defined as the ability to perform vigorous ac-tivity continuously for more than a few seconds.The effect of dwindling stamina on overall musclesystem performance is proportionately greaterthan is the effect of the age-related decrease inspeed of movement. Stamina declines faster as ageincreases.

The decrease in stamina is manifested in four

ways. First, there is a decline in the maximum rateat which vigorous activities can be performed. Forexample, the maximum speed at which a bicyclecan be ridden diminishes. Second, the length oftime such activities can be performed withoutstopping to rest becomes shorter. This decreasein endurance is evident whether a person is work-ing as fast as possible or at a rate somewhat lowerthan the maximum rate. As will be explained be-low, important causes of the reduction in endur-ance include a more rapid accumulation of lacticacid in muscles and a faster and more intense

onset of discomfort at a given rate of vigorousactivity.

The third indication of reduced stamina is a

lengthening of the time needed to recover afterending an activity such as running. For example,it may take longer for respiration and heart rateto return to resting values. One reason for the in-crease in recovery time is the faster accumulation


of lactic acid caused in part by a decline in theefficiency of movement. Another factor is a slow-ing in the rate at which the heat produced bymuscle contraction is released from the body.

The fourth indication of dwindling stamina isa rise in muscle stiffness and soreness experiencedhours or days after a vigorous activity has ended.Lactic acid buildup also seems to be a main rea-son for this indication.

The decline in the maximum rate of perform-ing physical activity has been studied intensively.Therefore, this age-related change will be dis-cussed in detail below.

'Vozmax The maximum rate at which a personcan use muscles to perform an activity is com-monly determined by measuring the rate at whichthat person uses oxygen while engaging in anactivity at the fastest rate possible. The maximumrate of working is expressed as the "V°2max. Aperson's 'V02max is the amount of oxygen usedper kilogram of body weight per minute while aperson is exercising at the fastest rate attainable.Exercises commonly used for determining 'V02maxinclude riding a stationary exercise bike and walk-ing or running on a treadmill. 'V02max is alsocalled aerobic capacity.

'V02max declines with age. The decline beginsat about age 20 for men and about age 35 forwomen. These are average values, however. Aswith many other age-related changes, there isgreat variability among individuals of the sameage in regard to actual "V°2maxvalues and the rateof decline in "V°2max values.

A main reason for differences in the levels and

rates of change of 'V02max is variation in theamount of exercise a person gets. For example,the 'V02max for people who have a rather seden-tary lifestyle drops about twice as fast as does the'V02max of individuals whose jobs, home lives,and recreational activities include large amountsof physical activity. Also,'V02max begins to de-cline faster when a person's activity decreases.By contrast, when a person's participation in regu-lar vigorous exercise increases, the decline maybe delayed and become slower or even be tempo-rarily reversed. Still, some reduction in 'V02maxeventually occurs in all people, including indi-viduals who engage in highly demanding physi-cal activities throughout life. When vigorousphysical training continues, 'V02max declines 5

percent per decade.Much of the decline in 'V02max is due to the

age-related decrease in total muscle mass com-bined with a relative increase in the proportionof body fat. The rate of oxygen consumption ofeach kilogram of muscle may be the same despiteage.

Many other factors seem to contribute to thedecline in 'V02max. One factor is a reduction inthe ability of muscles to extract oxygen fromblood. Other factors include changes and diseasesthat limit the functioning of the circulatory, res-piratory, and skeletal systems. A person may beaffected by more than one factor, and many peopleare affected by most or all of them. Therefore, itis extremely difficult to identify how much of thedecline in .Vo2max is due to aging of the musclesystem rather than to other factors.

Consequences of Lowered 'Vozmax Since.V02max is an indicator of the maximum rate atwhich a person can perform activities, a smalldecline means a drop in the maximum rate atwhich a person can run, climb stairs, and carryout other vigorous activities. Individuals withlowered values tend to stop physical activitiessooner because of the discomfort such activities

induce. As "V°2maxdecreases further, limitationsin less demanding activities, such as walkingbriskly, become evident. When very low valuesare reached, individuals may have trouble walk-ing slowly or even getting up from a chair or bed.

Since a substantial decline in .Vo2max ad-versely affects the performance of all types ofphysical activity, it can reduce a person's ef-fectiveness and participation in occupational,recreational, and social activities. When'V02max becomes very low, the performance ofordinary daily activities needed to maintain aperson becomes difficult or impossible. Ex-amples include shopping, dressing, and bath-ing. Serious losses in the sense of independenceand other negative psychological consequencesoften develop. Undesirable alterations in onearea can cause detrimental effects in other ar-

eas, leading to a synergistic spiral of decline.As mentioned previously, the decline in

'Vo2max can be slowed or even reversed when anadult of any age begins a program of exercise orincludes vigorous activity in his or her daily life.Individuals with relatively high "V°2max valuesneed to engage in activities with high intensity


and frequency to derive beneficial alterations in'V02max.People whose 'V02max is fairly low canslow the decline or increase this parameter withless strenuous activities. For many, substitutingmuscle power for convenience can achieve realgains. For example, parking farther from storesand walking to reach them or climbing stairsrather than using an elevator can significantlyincrease a person's amount of exercise.

STAYINGPHYSICALLY ACTIVE

Many age-related changes in the muscle systemare caused or greatly increased by a decrease inphysical activity. Conversely, many of these ad-verse changes can be greatly slowed or even ne-gated by continuing to engage in regular exercise.It is also possible to delay, slow, reduce, or pre-vent many undesirable changes and diseases inother body systems by living a physically activelifestyle. In general and within reasonable limits,the more exercise a person gets, the greater thebenefits.

Specific Effects

Many effects from maintaining a high level ofphysical activity are listed in Table 8.1.

Muscle Mass Besides retaining the strength toperform both heavy and ordinary tasks, maintain-ing muscle mass helps stabilize body proportions.

It also reduces detrimental changes in the abilityof the hormone insulin to regulate blood sugarand certain metabolic activities in the body (Chap.14). The effects of ongoing exercise on the ner-vous system help slow both the increase in reac-tion time and the decline in speed of movement.

.Vo2max The impact of ongoing exercise on slow-ing the decline in 'V02max is so great that veryactive elderly people have values equal to orgreater than those of sedentary individuals ofabout age 30. However, no amount of exercise cancompletely stop the decrease in 'V02max as ageincreases. Therefore, younger people who exercisewill have values greater than those of older indi-viduals who get in the same amount of exercise.

Overview

In considering the beneficial effects of years ofphysical activity, it is important to realize thatthese benefits are obtained only by persons whocontinue to lead active lives. People who are veryactive or athletic during youth but then becomesedentary for many years lose most of the benefitthey acquired in their previously active lives.

Getting regular exercise throughout life hasbeen shown to increase life expectancy, possiblybecause exercise reduces the risks of certaincauses of death. Exercise has not been shown to

increase maximum longevity. Finally, exerciseundoubtedly improves the quality of life. Long-

TABLE 8.1 EFFECTS OF MAINTAINING A HIGH LEVEL OF PHYSICAL ACTIVITYTHROUGHOUT LIFE

Muscle systemSlower decline in energy molecules (ATP, creatine phosphate, glycogen), oxidative enzymes, muscle cell

thickness, number of muscle cells, muscle thickriess, muscle mass, muscle strength, blood supply, speed ofmovement, stamina, endurance, 'Vozmax

Slower increase in fat and fibers, reaction time, recovery time, development of muscle soreness

Nervous system

Slower decline in processing impulses by the CNS

Slower increase in variations in speed of motor neuron impulses

Circulatory system

Maintenance of lower levels of LDLs and higher HDL/ cholesterol and HDL/LDL ratios

Decreased risk of high blood pressure, atherosclerosis, heart attack, strokeSkeletal system

Slower decline in bone minerals

Decreased risk of fractures and osteoporosis


term exercise enables an individual to participatemore fully and with greater pleasure in manymore life activities. Years of regular exercise alsomarkedly reduce the risk of developing many dis-abling diseases. Those who exercise and still de-velop a disease are often less affected.

STARTING OR INCREASINGEXERCISE

Clearly, elderly individuals who have been in-volved in vigorous physical activity throughouttheir lives benefit from such a lifestyle. Youngpeople who adopt active lifestyles can expect toreap the same benefits when they become elderly.Furthermore, people of any age who have livedsedentary lives and begin to get exercise and thosewho have been getting only low or moderateamounts of exercise for many years and increasetheir exercise can improve their well-being. Wewill now examine outcomes in older people whobegin vigorous exercise or substantially increasetheir level of physical activity.

Effects

Many effects on older people who begin or in-crease physical activity are listed in Table 8.2.

Circulatory System The rise in maximum car-diac output is evident within a few days to weeksof initiating an exercise program. The more in-tense the exercise program, the sooner a signifi-cant increase in maximum cardiac output appears.This rise begins to be reversed within days of end-ing the exercise program. The final maximum car-diac output of those leaving an exercise programwill be about the same as that which existed when

the exercise program began. Altering blood lipo-protein levels requires a decrease in body fat alongwith the effects of the exercise.

Respiratory System There is disagreement aboutwhether increasing exercise increases respiratoryvolumes and speed of airflow, but long-term par-ticipation in exercise programs slows the declinein respiratory functioning. Therefore, in the long

TABLE 8.2 EFFECTS OF STARTING OR INCREASING EXERCISE

Circulatory systemIncreases cardiac output, cardiac efficiency, HDL/ cholesterol and HDL/LDL ratios, blood vessel diameter,

muscle capillariesSlows decline in heart functioning

Decreases resting blood pressure, blood pressure during exercise, heartbeat abnormalities

Respiratory system

Increases clearance of mucus and respiratory efficiency

Slows decline in respiratory functioning and closing of airwaysNervous system

Increases formation of new axon branches to orphaned muscle cells, speed of impulse processing by the CNS,balance, short-term memory, sleep, mental abilities (possibly)

Decreases variability in speed of action potentials in motor neurons and risk of fallingMuscle system

Increases oxidative enzymes, stored glycogen, capillary numbers, blood flow, uptake of oxygen from blood, cellthickness, muscle strength, muscle mass, speed of movement, stamina, endurance, 'Vo2max

Slows decline in efficiency of movement and increase in recovery timeSkeletal system

Increases ease of movement, range of movement, joint flexibilitySlows bone demineralization

Decreases risk of falling and sustaining fracturesEndocrine system

Increases glucose tolerance and sensitivity to insulinSlows decline in growth hormone


run elderly individuals who exercise will even-tually have better respiratory system functioningthan they will if they remain sedentary.

The respiratory system changes caused by aproper exercise program are of special importanceto persons who have chronic obstructive pulmo-nary diseases (COPDs) such as chronic bronchi-tis and emphysema.

Nervous System The mechanisms by whichstrenuous exercise increases strength in olderpeople are different from those in younger people.At younger ages the increase in strength fromtraining with heavy weights is caused almost ex-clusively by thickening of the muscle cells ratherthan alterations in the nervous system. Perhapsthe change in mechanisms for increasing strengthis a way the aging body partially compensates fora decreased ability of the muscle cells to adapt tolifting or moving heavy loads.

Muscle System The gain in strength achievedby older individuals is proportionately the sameas that which younger adults attain with the sametype of exercise. For example, consider an olderperson who has had little exercise for many yearsand a younger adult who has had the same levelof activity. The older person will not be as strongas the younger adult because the older adult hasbeen losing strength for a longer period. If bothindividuals begin an exercise program designedto double the strength of an adult, both will endthe program with twice the strength they hadwhen they started. Of course, since the youngerperson was stronger at the start of the program,he or she will be the stronger person at the end.However, an older person who participates insuch a program can become stronger than ayounger adult who remains sedentary.

Older individuals whose exercise is not strenu-

ous enough to cause an increase in strength stillbenefit because they at least have a slower declinein strength. Therefore, after long-term involve-ment in physical activity these individuals willbe stronger than they would have been if they hadremained sedentary. They will also have retainedmore total muscle mass. Keeping a high musclemass helps the functioning of insulin.

Alterations in 'Vo2max caused by increasedexercise are similar in three ways to exercise-induced changes in strength. First, the increasein 'Vo2max attained by an older person is propor-

tionately the same as that achieved by a youngeradult who starts with the same capability andparticipates in the same exercise program. Second,elderly people who increase their physical activ-ity enough can develop values that are greaterthan those of much younger adults who remainsedentary. Third, elderly people whose increasein exercise is not enough to produce an increasein 'Vo2max will still benefit because even smallincreases in physical activity slow the decline in'Vo2max. Therefore, these individuals will even-tually have a greater 'Vo2max than they wouldhave had if they had remained sedentary.

There is an important difference between theeffects of exercise on alterations in conditions such

as blood lipoproteins, body composition, percentbody fat, functioning of insulin, and strength andthe effects on alterations in 'Vo2max. Though vig-orous activity is needed to effect substantialchanges in the first five parameters, for very sed-entary older people even low levels of easy ac-tivities such as walking can substantially increase'Vo2max. The resulting improvements can restorethe ability of very sedentary elderly individualswith extremely low 'V°2max values to perform theordinary activities of daily living.

All the exercise-induced alterations in the ner-

vous and muscle systems just described combineto produce several other benefits in the elderly.These benefits include the perception that lesseffort is needed to perform demanding tasks; im-proved mood and sense of well-being; improvedsocial interactions; and increased independence.

Skeletal System Aging of the skeletal systemraises the risk of sustaining fractures by causingdemineralization of bones and reducing the easeof movement and range of motion of joints. Someforms of the joint disease called arthritis exag-gerate these changes.

Noone knows the best exercise program forslowing or reversing bone demineralizationcaused by aging or osteoporosis; different pro-grams may be effective for different individuals.Also, different individuals can tolerate differentamounts and types of exercise. The causes of thesedifferences include physical condition, presenceof diseases, lifestyle, and motivation.

The possible benefits of slowing bone deminer-alization and deterioration of joint functioningthrough a large increase in strenuous or vigorousphysical activity must be weighed against the

II


added risk of injury. Some more common prob-lems include; increased risk of fracture of thebones in the spinal column caused by lifting orholding heavy loads; increased risk of fracturinghip, leg, or arm bones by falling; traumatic injuryto the bones, muscles, ligaments, and tendons inthe lower legs from walking or running on hardsurfaces or with improper footwear; and injuryto the joints from excessive movements or forces,including impact forces.

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Endocrine System Exercise leading to a decreasein body fat significantly improves glucose toler-ance and insulin sensitivity in elderly people whohave a reduced glucose tolerance or non-insulin-dependent diabetes mellitus (NIDOM). Individu-als who improve their glucose tolerance and in-sulin sensitivity have a substantially reduced riskof developing the complications associated withdiabetes.

These beneficial effects of exercise begin todevelop within days or even hours after a personincreases the level of vigorous physical activity.Improvement increases as body fat decreases.However, the beneficial effects of the exercise be-gin to dwindle within a few days of ending in-volvement in the exercise program. Therefore, tosustain the benefits of exercise, a person with re-duced glucose tolerance or NIOOM must engagein the exercise at least once every three days.

By contrast, individuals with type I (insulin-dependent) diabetes mellitus (IDOM) have veryunstable blood sugar levels. Therefore, theamount of exercise they get must be carefullymonitored and adjusted according to factors suchas the severity of the disease, diet, and insulintreatments.

Other Effects The effects of increasing exercisementioned up to this point are related to specificbody systems, but elderly people who increasetheir exercise seem to benefit in many other ways.These other benefits include helping to maintainnormal body weight by improving nutrition andusing more calories; increasing independence bygenerally slowing the onset of disability andphysical limitations; and helping to enhance psy-chological health by improving mood and senseof well-being while reducing boredom, anxiety,and stress.

The physical and psychological effects of exer-cising also increase older individuals' ability to

remain productive and economically self-suffi-cient. Their social situation is bolstered by theadditional social interactions obtained throughexercise programs and through an enhanced abil-ity to participate in other activities in the com-munity. Therefore, while increasing exercise hasnot been shown to lengthen life, it dramaticallyimproves the quality of life for older individuals.

EXERCISE RECOMMENDATIONS

Having reviewed how exercise benefits the el-derly, we will now consider information and sug-gestions that have been found important inachieving these benefits.

First, as with other mechanisms that maintainhomeostasis, adjustments made in body systemsto each form of exercise represent attempts tominimize or prevent disturbances in internal con-ditions. Furthermore, adjustments and improve-ments made by the body are specific to the de-mands placed on it. For example, if conditions inleg muscles are significantly disturbed by liftingheavy loads, those muscles will become strongerand therefore will be less affected when the loadsare lifted a few days later. By contrast, if condi-tions in leg muscles are disturbed by an activityinvolving many repeated actions that do not re-quire much strength, such as walking briskly fora long distance, the adjustments in the body willincrease stamina for walking but will have littleeffect on muscle strength.

Set Goals

The first step in preparing to increase exercise isto establish specific goals. Then activities can beselected that will cause the body to make the ad-justments needed to achieve those goals. For ex-ample, if an increase in the range of motion of thearms is desired, activities requiring movement of t:)the arms over wide angles can be selected. If in-creasing the grip strength of the hands is a goal,activities using strong grasping should be under-taken. As more goals are identified, a greater num-ber and variety of exercises or activities must be !employed. i

In a more general way, if exercise is being used jto improve the functioning of the circulatory and Irespiratory systems, a number of activities de-manding faster blood flow and increased respi-


ration can produce the desired results. Examplesinclude walking or riding a bicycle at a fast pace,running, and swimming.

Evaluate and Individualize

When one is deciding on exercises, attention mustbe paid to the condition of the person who is par-ticipating in the exercises. Careful attention to anelderly person's physical condition is particularlyimportant because of the higher incidence of dis-ease and the increased heterogeneity among olderpeople. At this point, a qualified professionalshould perform a physical examination andevaluation of the participant and the informationobtained should be used to determine the appro-priateness of the anticipated activities. At leastone follow-up examination and evaluation shouldbe performed several days or a few weeks afterthe individual has taken up the new level of ac-tivity. Data from the first and later examinationsand evaluations should be used to determine

what changes are occurring because of the in-creased exercise and to suggest improvements inthe activities.

Plan a Program

Once appropriate activities have been selectedand the condition of the participant has been as-certained, decisions about the intensity and thelength of exercise can be made. The time allottedfor exercise should include time for warming upand cooling down. The frequency with which theactivity will be performed can also be established.A healthy person should exercise at least 30 min-utes each session, with sessions occurring at leastevery three days.

Generally, starting in with a fairly low level ofintensity and a short duration of activity is best,especially for very sedentary or frail individuals,who can achieve substantial benefits from rela-

tively low levels of exercise. Also, such individu-als are more likely to sustain injuries or other ad-verse effects from a sudden increase in physicalactivity. Beginning with low levels of exercise alsohelps prevent negative attitudes by minimizingthe discomfort caused by an increase in exercise.

Consideration might also be given to the num-ber of weeks or months during which the partici-pant expects to perform the activity. Exercise pro-grams lasting only a few weeks produce little ben-

efit, while those lasting several months or longeryield significantly better results. Longer-lastingprograms are especially important for olderpeople since the rate of improvement caused byexercise decreases with age. Sustained participa-tion in the exercise program is aided by usingpositive feedback and other motivational strate-gies, such as combining exercise sessions withsocial activities. Since the exercise program shouldlast for an extended time, it is important to pro-vide for proper nutrition.

As the exercise program continues, the inten-sity or duration of each exercise or the frequencywith which it is performed each week should beincreased gradually so that the participant con-tinues to improve. If the exercise is not increased,the participant's level of physical fitness will soonstabilize, and boredom may become a problem ifthere is no variation. Furthermore, the psychologi-cal benefits of exercise become most apparent.once a high intensity and greater frequency havebeen achieved.

Minimize Problems

Although the benefits are directly proportional tothe intensity, duration, and frequency of exercise,care must be taken not to exceed reasonable lim-its. As levels of exercise increase, so do the risksof overheating, being physically injured, and de-veloping complications from existing diseases.

Very strenuous activities present a special dan-ger to those with atherosclerosis because peopletend to hold their breath while pulling or push-ing with great force. Blood pressure rises to a veryhigh level during such maneuvers, placing a greatburden on the heart and arteries. A heart attack,a stroke, or damage to the retina or vitreous hu-mor can result. When the straining ends, there isa sudden drop in blood pressure, placing additionalburdens on the heart and sometimes causing dizzi-ness, fainting, and falling. These problems can belargely avoided by minimizing exercises requiringgreat strength and maximizing activities involvingfree movement of parts of the body.

Consider Alternatives

When one is discussing exercise, focusing in onactivities whose primary purpose is exercise iseasy (e.g., aerobics, weight lifting, jogging). Us-ing such activities and the many available exer-


cise machines and devices provide means of care-fully regulating the amount of exercise obtainedand measuring physical status and improvementsin physical fitness.

While some individuals enjoy such purpose-ful exercise, others find it unpleasant, expensive,or unavailable. These individuals can still obtain

plenty of beneficial exercise through activitieswith other primary purposes. Examples includerecreational activities such as dancing, sports, andhiking and activities related to occupations requir-ing physical work. Activities performed in caringfor one's home and family, such as gardening andmowing a lawn, shopping, and doing laundry, canprovide opportunities to get healthful exercise.Choosing to walk or climb stairs rather thanriding can add substantially to the amount of ben-eficial exercise obtained.

Achieving the benefits of increasing exerciseoften involves nothing more than substitutingmuscle power for motor power. What may beneeded first, however, is replacement of the no-tion that using minimal physical effort means liv-ing well with the realization that only throughregular physical exercise can an older personachieve "the good life."

DRIVING MOTOR VEHICLES

Driving accidents increase in numbers and in ratesas the age of the drivers increase. For example,elderly drivers have twice as many accidents permile compared with younger drivers. Elder driv-ers who have accidents are more likely to sustainserious or fatal injury than are younger drivers.At the same time, the number and proportion ofelder drivers are increasing, and they will con-tinue to increase at faster rates for the next few

decades. By 2024, drivers over age 64 will makeup 25 percent of all drivers.

Potential accident situations often require mak-ing quick and coordinated responses in new situ-ations. Therefore, elder drivers can be very safedrivers until they meet surprising or complicatedsituations that demand quick reactions in unfa-miliar situations. Examples of problematic situa-tions for elders are intersections. In such demand-

ing situations, age changes in muscle strength,speed, reaction time, and coordination contributeto an age-related decrease in driving ability. Agechanges and age-related abnormalities in otherbody systems also contribute to reduced drivingability. These changes and abnormalities are de-scribed in other chapters.

Neurological parameters that change very littlewith normal aging include implicit memory ofdriving skills and making automatic coordinatedresponses. Other age-related changes that do nothave a major impact on driving ability in eldersinclude modest decrements in vision and in hear-

ing. The small effects on driving safety amongelders from decrements in vision and in hearingmay be due to elders avoiding driving at timesand conditions where these decrements are im-

portant (e.g., nighttime, bad weather, heavy traf-fic, high speeds, time pressures). In general, over-all cognitive ability has little to do with drivingskills until cognitive abilities become severely re-duced. Therefore, people in early stages of demen-tia can still be good drivers.

Important neurological factors that limit driv-ing ability for elders include age-related decreasesin avoiding distraction; changing attentionquickly; responding quickly in unfamiliar situa-tions; noticing, accurately identifying and re-sponding to sudden changes in the visual field;noticing and responding to a novel change in theenvironment; and distinguishing between impor-tant and unimportant items in the visual field.

Driving is very important to elders for manyreasons including mobility; independence; a senseof self-efficacy; and a sense of self-worth. Loss ofdriving often causes major psychological, social,and economic problems for elders. Demands toprovide alternative means of transportation in-crease (e.g., family, friends, community groups,private companies, governmental agencies).

Recommendations that can accommodate these

diverse factors include providing reliable, practi-cal and regular tests for evaluating elder drivers;providing education and training to maintain orimprove elders' driving skills and safety; anddeveloping alternative transportation for eldersas they give up or lose their driving privileges.

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MUSCLE SYSTEM 8d.pdfSYSTEM There are three types of muscle in the human body. The most abundant type...

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