Biomechanical Basis for Stability - An Explanation to Enhance Clinical Utility

8/11/2019 Biomechanical Basis for Stability - An Explanation to Enhance Clinical Utility

1/5

Journa l of Orthopaedic Sports Physical Therapy2001;31(2) :96-100

Biomechanical Basis for Stability: nExplanation to Enhance Clinical UtilityStuart M. McGill Ph Dlacek Cholewicki PhD2

Th e term stability, as used in the field of biome-chanics, remains undefined in many clinical cases.This fact can impede the design of therapies intend-ed to enhanc e joint stability. In fact, Fritz et a1,6 in areview on lumbar instability, conc luded th at At pres-ent, much controversy exists regarding the prop erdefinition of the condition, the best diagnostic meth-ods, and the most efficacious treatment approaches.Some progress has been made in the biomechanicsfield toward the formulation and implementation ofstability in musculoskeletal linkages an d joints.

The purpose of this review is to synthesize and in-terpret the biomechanical foundation for stabilitywhile avoiding mathematical complexity, to demon-strate th e notion of stability using specific musculo-skeletal examples, and to propose the next logicalsteps to full utilization of th e stability concept for o p

timal rehabilitation. This review is not intended as ascholarly treatise bu t rathe r as a sho rt commentar yaimed at providing clinicians with a vantage point formaking clinical decisions. Finally, because we arespine biomechanists, and because the original workdefining the mechanics of stability of musculoskeletalsystems used the spine as an example, this article em-phasizes the spine in its examples.

In the late 1980's. Anders Bergmark, a professor ofsolid mechanics at the University of Lund in Sweden,elegantly formalized stability in a muscular system.Using a spine model' with joint stiffness an d 40 mus-cles, he formalized mathematically the concepts of

energy wells, stiffness, stability, and instability. For themost par t, this classic work went unrecognized, large-ly because the engineers who understood its mechan-

Faculty of Applied Health Sciences Department of Kinesiology Univer-sity of Waterloo Waterloo Ontario Canada.

Department of Orthopaedics and Rehabilitation Yale University Schoolof Medicine New Haven Conn.Send correspondence to Stuart McCill Faculty ofApplied Health SciencesDepartment of Kinesiology University of Waterloo 200 University AvenueWaterloo Ontario N2L 3G 1 Canada. E-mail: mcgill@healthy waterloo.ca

ics lacked the biological/clinical perspective, and therelatively complex mathematics thwarted its wide-spread use in clinics. Subsequent to Bergmark's ef-forts, several groups have added sophistication to theformalization of joint stability. An explanation of thetheory requires 4 steps.

Potential nergy

The foundation of stability begins with the conceptof potential energy which, for our purposes, takes 2basic forms. In the first form, objects (with mass)have potential energy by virtue of their height abovea datum. An apple has potential energy while still onthe tree because this energy would be transformedinto kinetic energy if it were to fall.

potential energy mass*gravitational con-stant*height

In the second form, elastic bodies may possess po-tential energy by virtue of their elastic deformationunder load, storing potential energy which is recov-ered when the load is removed (such as what h a ppens when an elastic band is loaded an d unl oaded ).The first form of potential energy describes the no-tions of energy wells a nd minimum potentialenergy. If a ball is placed into a bowl it is stable. Thisis true because if one were to apply a small force tothe ball ( or a perturbation) the ball would roll upthe side of the bowl but then come to rest again in

the position of least potential energy-the bottom ofthe bowl. This system can be made more stable by ei-ther deepe ning th e bowl or increasing the steepnessof its sides (Figure 1 ). Conversely, a ball placed on aflat surface or at th e t op of a hill (an upside downbowl) is unstable since any perturbation would causethe ball to roll away. Thus, the objective in creatingstability with this analogy is to create a bowl-shaped potential energy surface, o r an energy well.The ball will seek the position of minimum potential


2/5

FIGURE1 The ball in the bowl seeks the energy well or position of minimum potential energy (mgh). Deepening he bowl or increasing he steepnessof the sides increases stability.

energy (or mass multiplied by gravity and height-height being the variable to minimize). This corre-spon ds to a stable situation, but just how stable is it?The quantification of stability requires specificationof an unpertu rbed state or the amo unt of residualpotential energy and a study of the system perturbedby a force (which does work).

Th e previous analogy is a 2dimension al example(allowing motion in a single plane ). This would be

analogous to a hing ed skeletal joint that only has thecapacity for flexion/extension. Some ball an d socketjoints can rotate in 3 planes (3 degrees of freedom),requiring a 4-dimensional bowl. T he lumbar spine,for example, has 5 segmental joints, each able to r etate in 3 planes, for a total of 15 degrees of freedomand a bowl with more than 15 dimensions. Unstablespinal joints may also shear o r translate, addi ng an-othe r 3 degrees of freedom per joint and resultingin more than 30 dimensions to the bowl. Obviously,this is a theoretical bowl since a real bowl has only 3dimensions; mathematics allows us to examine a 30

dimensional bowl. If the height of the bowl was de-

creased in any o ne of the 30 dimensions, the ballwould roll out. In clinical terms, there are many ana-tomical structures responsible for m aintaining th e

height of the bowl for each dimension .

Whole ody Stability

Whole body stability can be briefly addressed to il-lustrate the breadth of the concept. Consider themass within the base of support (Figure 2). The sys-tem is considered stable if it would survive a minorperturbation applied to the top of the cone. A largerperturbation is required to cause the cone to rotateover 1 support, raising the potential energy to the

point where the cent er of mass falls outside th e baseof support into a region of instability (or 8 de-grees in Figure 2 . A larger 0 deepens the poten-tial energy well. Thus , stability in the rigid wholebody example is enhanc ed only by increasing themass, widening the base of support, o r lowering thecenter of mass.

lastic nergy and Stiffness

Having considered the analogies incorporating p etential energy by virtue of height, potential energycan be considered as a function of stiffness and stor-

age of elastic energy, which is much more useful formusculoskeletal application. Elastic potential energycan be calculated from the formula:

potential energy 1/2*k*xa2

where an elastic structure having stiffness (k) storespotential energy by virtue of it being stretched a dis-tance (x) . In o the r words the greater th e stiffness,the g reater the steepness of the sides of the bowl(from the previous analogy), and the more stable thestructure. Thus stiffness creates stability (Figure 3).Active muscle creates tendo n force but also producesa stiff member and, in fact, the greater the activation

of th e muscle, the greater this stiffness. Furthermore,joints possess in here nt joint stiffness, as ligamentsand oth er capsular structures contribute stiffness thatincreases towards the bou ndary of joint motion. Inthis way, the motor control system can control stabili-ty of th e join ts through active muscle activation and ,

FIGURE2. Wholebdy stability is increased by increasing the mass, low- to a lesser degree, by placing in positions thatering the height of the mass, or widening the base of support. A largervalue of 0 , which is modulated by the base width and mass position, passive stiffnessincreases the ability to survive a pertubation P) nd therefore is more tor con trol system leads to in appropr iate magnitudesstable. of muscle force and stiffness, allowing a valley for

J Orthop Sports Phys Ther .Volume 31 Number .February 2 1 97


3/5


4/5

FIGURE4. Spine stiffness is achieved y a complex interaction of stiffening stnpanel).

stiffness (the walls are creat ed not by single musclesbut by a system of muscles that effect, or act, in thatparticular direction of joint moti on). Th e resultingrotation or translation at th e joint overloads a passivetissue to the point of damage. In fact, such an insta-bility was observed in an experiment that collectedvideoflouroscope records from power lifters liftingheavy loads where a single spine motion segmentbuckling occu rred that resulted in injury. he dam-aged joint subsequently loses passive stiffness, possi-bly causing buckling at even lower loads in the fu-ture. This conceptualization of stability is beginningto provide clinicians with specific target levels ofmuscle activation necessary to achieve sufficient sta-bility. The levels of activation observed by Cholewickian d McGil14 an d Cholewicki e t a15 have d emon stra tedthat sufficient stability of the lumbar spine isachieved (with the spine in a neutral curvature) inmost people with modest levels of coactivation of themuscles form ing th e abdominal wall (at least below10% of maximum isometric contraction). While largemuscular forces are rarely required, it appears thatsuccess in daily activities requires relatively low levels

lctures along the spine left panel) and those forming the torso right

of activation maintained for long periods of time. Pa-tient management requires further thought as tissueinjury alters the a mou nt of stability required. An in-jury causing inherent losses in passive tissue stiffness(specifically disc o r ligament) results in join t laxity1sthat necessitates higher levels of muscular activation/stiffness to ens ure sufficient ~tability.~ . xamples ofspecific stabilization exercises have been quantifiedand ranked for muscle activation magnitudes togeth-er with the resultant spine load as summarized byMcGill. Functionally, a patien t must be able tomaintain sufficient stability dur ing necessary daily ac-

tivities: getting on and off the toilet, in a nd out ofthe car, up an d downstairs, etc. Sufficient stabilityin these examples results from light cocontraction ofthe stabilizing musculature. This proposition suggeststhat the margin of safety when performing tasks, par-ticularly the tasks of daily living, is not compromisedby insufficient strength but rather points to the im-portance of en dura nce. Epidemiological evidencesuggests that strong abdominals may n ot provide theprophylactic effect that had been hoped for. Recent

J Orthop ports Phys Ther Volume 3 Number 2. Februa~y 001


5/5

work suggests that endu rable muscles reduce the riskof futur e back trouble^.^.^

While basic science has underscored the need foran anatomical system to be stable before it is present-ed with a physical challenge, the clinical techniquefor doing so continues to be developed.15 Clinicalquestions to b e addressed include identifying howthe various compone nts of t he anatomy at particular

joints contribute to stability and the ideal ways to en-hance their contribution; what magnitudes of muscle

activation are required to achieve sufficient stabilityin a wide variety of t sks and type of patients; whatare the best methods to reedu cate faulty motor con-trol systems to both achieve sufficient stability and re-duce the risk of inappropriate motor patters occur-ring in the future? Addressing mobility problemsfrom the perspective of sufficient stability requiresa team ap proach. It is a unifying concept that canenable clinicians, biom echanistseng ineers, physiolo-gists, and anatomists t o collectively tackle th e mobili-ty problems that are so important for quality of life.

REFEREN ES

1. Bergmark A. Stability of the lumbar spine: a study in me-chanical engineering. Acta Orth op Scand. 1989;60:3-53.

2. Biering-Sorensen F Physical measurements as risk indi-cators for low-back trouble over a one-year period. Spine.

3. Cholewicki J , McGill S. Lumbar posterior ligament in

volvement during extremely heavy lifts estimated fromflouroscopic measurements. ] Biomech. 1992;25:17-28.Cholewicki J, McGill S. Mechanical stability of the in vivolumbar spine: implications for injury and chronic lowback pain. Clin Biomech. 1996;ll:l-15.Cholewicki J, Panjabi M, Khachatryan A. Stabilizing func-tion of the t runk flexor-extensor muscles around the neu-tral spine posture. Spine. 1997;22:2207-22 12.Fritz JM, Erhard RE Hagen BF Segmental instability ofthe lumbar spine. Phys Ther. 1998;78:889-896.Gertzbein SD, Seligman J, Holtby R et al. Centrode pat-terns and segmental instability in degenerative disc dis-

ease. Spine. 1985;10:257-261.Hoffer J, Andreassen S. Regulation of soleus muscle stiff-ness in premamillary cats. ] Neurophysiol. 1981;45:267-285.Luoto S, Heliovaara M, Hurri H, Alaranta M. Static backendurance and the risk of low back pain. Clin Biomech.1995;10:323-324.McGill S. Biomechanics of low back injury: implicationson current practice and the clinic. Biomech. 1997;30:465475.McGill S. Low back exercises: evidence for improving ex-ercise regimens. Phys Ther. 1998;78:754-765.Panjabi MM. The stabilizing system of the spine. Part IIneutral zone and instability hypothesis. ] Spinal Disord.1992;5:390-396.Panjabi MM, Dancereau JS, Oxland TR, Bowen CE. Multidirectional instabilities of traumatic cervical spine injuries in a porcine model. Spine. 1989;l4: i l l l - l l l5 .Richardson C, Jull G, Hodges P Hides J. Therapeutic Exercise for Spinal Segmental Stabilization in Low Backh i n . New York, NY: Churchill-Livingstone; 1999.Stevans J, Hall K Motor still acquisition strategies for re-habilitation of low back pain. ] Orthop Sports Phys Ther.1998;28:165-167.

Orthop Sports Phys Ther.Volume 31 Number 2eFebruary 2001

Biomechanical Basis for Stability - An Explanation to Enhance Clinical Utility

Documents

Transcript of Biomechanical Basis for Stability - An Explanation to Enhance Clinical Utility