Physical Stress Theory

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Tissue Adaptation to Physical Stress:

A Proposed Physical Stress Theoryto Guide Physical Therapist Practice,

Education, and Research

The purpose of this perspective is to present a general theorythe

Physical Stress Theory (PST). The basic premise of the PST is that

changes in the relative level of physical stress cause a predictable

adaptive response in all biological tissue. Specific thresholds define the

upper and lower stress levels for each characteristic tissue response.Qualitatively, the 5 tissue responses to physical stress are decreased

stress tolerance (eg, atrophy), maintenance, increased stress tolerance

(eg, hypertrophy), injury, and death. Fundamental principles of tissue

adaptation to physical stress are described that, in the authors

opinion, can be used to help guide physical therapy practice, educa-

tion, and research. The description of fundamental principles is

followed by a review of selected literature describing adaptation to

physical stress for each of the 4 main organ systems described in the

Guide to Physical Therapist Practice (ie, cardiovascular/pulmonary, integ-

umentary, musculoskeletal, neuromuscular). Limitations and implica-tions of the PST for practice, research, and education are presented.

[Mueller MJ, Maluf KS. Tissue adaptation to physical stress: a proposed

physical stress theory to guide physical therapist practice, education,

and research. Phys Ther. 2002;82:383403.]

Key Words: Adaptation, Biomechanics, Force, Stress.

Michael J Mueller, Katrina S Maluf

Physical Therapy . Volume 82 . Number 4 . April 2002 383

P

erspective


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Physical therapists have expertise in the applica-tion of interventionssuch as exercise, pos-tural instruction, orthotic devices, and modali-tiesthat allow them to modify the physical

stresses applied to tissues of the body.15 Physical stress isdefined as the force applied to a given area of biologicaltissue.6 Exercise interventions that modify physical stress

have been shown to decrease impairments, functionallimitations, disability, and pain in a variety of patientpopulations.79 These same interventions can help peo-ple with and without disease increase muscle perfor-mance,10 bone mineral density,11 and fitness levels.9 Anincreasing amount of evidence indicates that exercisecan have positive effects on disease processes such asdiabetes,12 arthritis,79 and coronary artery disease.13

We believe that the many different theories andapproaches currently used in physical therapy sharefundamental principles that can be organized into ageneral theory to guide prevention and treatment of abroad range of patient problems. The general theory wepresent is based on fundamental principles that appearto govern the adaptive response of biological tissues tophysical stress. Therefore, we refer to this theory as thePhysical Stress Theory (PST). The PST integrates exist-ing knowledge into a deliberately broad theory withpotential application to physical therapist practice, edu-cation, and research.

Physical therapists have emphasized the evaluation andtreatment of movement dysfunction.1416 Movement canbe defined by the basic physical components (mass

acceleration) of a segment or body. The mass accelerationof a body segment is defined as force (forcemass acceleration, Newtons second law of motion). The appli-cation of force over a given area of tissue duringmovement results in stress to the tissue (stressforce/area, where force may be applied in any direction [tension,shear, compression]).6 Although movement is a majorsource of physical stress on tissues, other forces gener-ated both inside the body (eg, isometric muscle contrac-tions) and outside of the body (eg, gravity) also may

contribute to tissue stress. The PST was developed toaddress how tissues, organs, and organ systems adapt tovarying levels of physical stress (Tab. 1). In addition, thistheory describes how other factors (Tab. 2) can modifyboth the level of physical stress and the response ofbiological tissues to a given stress level.

The PST focuses on the physical stresses that influenceall biological tissues. Tissues are formed from groups ofsimilarly specialized cells that cooperate to perform oneor more functions within the body.17,18 The 4 fundamen-tal types of tissue are: (1) epithelial tissue, which coversinternal and external surfaces of the body and formsglands, (2) connective tissue, which provides structuraland functional support to other tissues of the body,(3) muscular tissue, which has specialized contractileproperties for producing movement, and (4) nervoustissue, which collects, transmits, and integrates stimuli tocontrol the functions of the body.17,18 These 4 basictissues combine to form organs. Groups of anatomicallyor functionally related organs are referred to as organ

systems.17,18 Our discussion of the PST will be limited totissues that form the organ systems identified as mostrelevant to physical therapists in the Guide to PhysicalTherapist Practice (ie, cardiovascular/pulmonary, integu-mentary, musculoskeletal, and neuromuscular sys-tems).14 The PST does not address molecular or cellularmechanisms of adaptation. Rather, the PST identifiescommon principles from the literature that we suggestmay be used to predict adaptive tissue changes thatoccur in response to physical stress.

MJ Mueller, PT, PhD, is Associate Professor and Director of the Movement Science Laboratory, Program in Physical Therapy, WashingtonUniversity School of Medicine, 4444 Forest Park Blvd, Campus Box 8502, St Louis, MO 63110-2212 (USA) ([email protected]).

Address all correspondence to Dr Mueller.

KS Maluf, PT, MSPT, is a graduate student in the Movement Science Program, Program in Physical Therapy, Washington University School of

Medicine.

Both authors provided concept/project design, writing, and fund procurement. Dr Mueller provided project management. The authors

acknowledge Amy J Bastian, Marybeth Brown, David Brown, Scott D Minor, Barbara J Norton, Shirley A Sahrmann, and Linda VanDillen for

discussions and their review of a previous draft of the manuscript.

This work was supported by grants RO1 HD36802 and RO1 HD36895 to Dr Mueller from the National Center for Medical

Rehabilitation Research, National Institutes of Health, and a Promotion of Doctoral Studies Award from the Foundation for

Physical Therapy to Ms Maluf.

Many different theories and

approaches currently used in physical

therapy share fundamental principles

that can be organized into a general

theory to guide prevention and

treatment of a broad range of patientproblems.

384 . Mueller and Maluf Physical Therapy . Volume 82 . Number 4 . April 2002


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In its present form, we believe the PST can be appliedmost easily to the care of patients with primary problemsinvolving the musculoskeletal and integumentary sys-tems. However, we believe this theory also has potentialapplications for patients with primary problems involv-ing the cardiovascular/pulmonary and neuromuscularsystems.14 We contend that the PST relates to a contin-uum of care and, therefore, also has direct applicationsfor issues related to wellness and the prevention ofphysical disabilities.

The purpose of this perspective is to present a generaltheory, based on principles of adaptation to physicalstress, that we believe can be used to help guide physicaltherapy practice, education, and research. First, we will

describe the fundamental principles of the PST, fol-lowed by a review of selected literature describing adap-tation to physical stress for each of the 4 main organsystems described in the Guide to Physical Therapist Prac-tice.14 The fundamental principles are based on ananalysis of the literature and our clinical experiences.Because the principles are general and represent whatwe contend are the common denominators of a varietyof research findings and clinical approaches, referencesare not provided in the description of the theory or in

the listing of Fundamental Principles in Table 1. Sup-porting literature is provided in the subsequent sectionand is summarized in Table 3. The literature review also will address how we believe the PST relates to otherphysical therapy theories and approaches. Limitationsand implications of the PST for practice, research, andeducation will then be discussed.

Description of Physical Stress TheoryThe basic premise of the PST is that changes in therelative level of physical stress cause a predictable adap-tive response in all biological tissue (Tab. 1). Figure 1illustrates primary adaptive responses and thresholds fortissue adaptation in response to physical stress. Factorsthat may influence either the level of physical stress ontissues or the adaptive response of tissues to a given stresslevel are outlined in Table 2. The fundamental princi-ples of the PST are summarized in Table 1 and aredescribed in further detail below.

Fundamental Principle AChanges in the relative levelof physical stress cause a predictable response in allbiological tissues. Physical stress is the force, or load,acting on a given area of tissue.6 In the PST, we proposethat tissues accommodate to physical stresses by altering

Table 1.Summary of Fundamental Principles for Physical Stress Theory

Basic Premise: Changes in the relative level of physical stresscause a predictable adaptive response in all biological tissue.

Fundamental Principles:

A. Changes in the relative level of physical stress cause apredictable response in all biological tissues.

B. Biological tissues exhibit 5 characteristic responses to physicalstress (Fig. 1). Each response is predicted to occur within adefined range along a continuum of stress levels. Specificthresholds define the upper and lower stress levels for eachcharacteristic tissue response. Qualitatively, the 5 tissueresponses to physical stress are decreased stress tolerance (eg,atrophy), maintenance, increased stress tolerance (eg,hypertrophy), injury, and death.

C. Physical stress levels that are lower than the maintenancerange result in decreased tolerance of tissues to subsequentstresses (eg, atrophy).

D. Physical stress levels that are in the maintenance range resultin no apparent tissue change.

E. Physical stress levels that exceed the maintenance range (ie,overload) result in increased tolerance of tissues to subsequentstresses (eg, hypertrophy).

F. Excessively high levels of physical stress result in tissue injury.

G. Extreme deviations from the maintenance stress range thatexceed the adaptive capacity of tissues result in tissue death.

H. The level of exposure to physical stress is a composite value,defined by the magnitude, time, and direction of stressapplication.

I. Individual stresses combine in complex ways to contribute to theoverall level of stress exposure. Tissues are affected by thehistory of recent stresses.

J. Excessive physical stress that causes injury can occur from 1 ormore of the following 3 mechanisms: (1) a high-magnitudestress applied for a brief period, (2) a low-magnitude stressapplied for a long duration, and (3) a moderate-magnitudestress applied to the tissue many times.

K. Inflammation occurs immediately following tissue injury andrenders the injured tissue less tolerant of stress than it was priorto injury. Injured and inflamed tissues must be protected fromsubsequent excessive stress until acute inflammation subsides.

L. The stress thresholds required to achieve a given tissueresponse may vary among individuals depending on thepresence or absence of several modulating variables. Factorsthat can influence thresholds for tissue adaptation and injuryare summarized in Table 2 and include movement andalignment, extrinsic, behavioral, and physiological factors.

Table 2.Factors Affecting the Level of Physical Stress on Tissues or theAdaptive Response of Tissues to Physical Stress

Movement and Alignment FactorsMuscle performance (force generation, length)Motor controlPosture and alignment

Physical activityOccupational, leisure, and self-care activities

Extrinsic FactorsOrthotic devices, taping, assistive devicesFootwearErgonomic environmentModalitiesGravity

Psychosocial Factors

Physiological FactorsMedicationAgeSystemic pathologyObesity

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their structure and composition to best meet themechanical demands of routine loading. Deviationsfrom routine or steady-state loading provide a stimulusfor tissue adaptation that allows tissues to meet themechanical demands of a novel environment.

Fundamental Principle BBiological tissues exhibit 5characteristic responses to physical stress. Each response

is predicted to occur within a defined range along acontinuum of stress levels. Specific thresholds define theupper and lower stress levels for each characteristictissue response. These thresholds can be viewed asboundaries for the effective dose-response to physicalstress. The 5 qualitative responses to physical stress aredecreased stress tolerance (eg, atrophy), maintenance,increased stress tolerance (eg, hypertrophy), injury, anddeath.

Fundamental Principle CPhysical stress levels that arelower than the maintenance range result in decreasedtolerance of tissues to subsequent stresses. Atrophy isone common mechanism by which tissues become lesstolerant of subsequent physical stresses (Fig. 2). Atrophyoccurs when tissue degeneration exceeds tissue produc-tion and has been observed in all 4 major organ systemsin response to reduced stress levels (Tab. 3). Otherexamples of adaptations that may reduce stress toleranceinclude hormonal changes, altered cell membrane excit-ability, and changes in the material properties of tissues(Tab. 3).

Fundamental Principle DPhysical stress levels that arein the maintenance range result in no apparent tissue

change. Tissue homeostasis occurs when tissue degener-ation is equal to tissue production, resulting in tissueturnover without net gain or loss. The range of stresslevels that promote tissue homeostasis is defined as themaintenance stress rangeand may be different for differentpeople. This steady-state or equilibrium response occurswhen tissues are exposed to the same levels of stress towhich they have become accustomed.

Fundamental Principle EPhysical stress levels thatexceed the maintenance range (ie, overload) result inincreased tolerance of tissues to subsequent stresses.Hypertrophy is one common mechanism by which tis-sues become more tolerant of subsequent physicalstresses (Fig. 3). Hypertrophy occurs when tissue pro-duction exceeds tissue degeneration, and can be definedas a general increase in bulk of a tissue.19 In general,biological tissues adapt to increased levels of stress byincreasing cross-sectional area, density, or volume(Tab. 3). Other examples of adaptations that mayincrease tissue stress tolerance include hormonalchanges, altered cell membrane excitability, andchanges in the material properties of tissues (Tab. 3). Although stress overload can promote tissue hypertro-phy and improve stress tolerance, adequate recoverybetween bouts of increased stress is needed for thisadaptive response to occur (see below).20

Fundamental Principle FExcessively high levels ofphysical stress result in tissue injury. For the purposes ofthis theory, injury is defined as tissue damage caused byexcessive stress resulting in pain or discomfort, impairedfunction of the tissue, or both. Damages that are not felt

Figure 1.Effect of physical stress on tissue adaptation. Biological tissues exhibit 5adaptive responses to physical stress. Each response is predicted tooccur within a defined range along a continuum of stress levels. Specific

thresholds define the upper and lower stress levels for each character-istic tissue response. The relative relationship between these thresholds isfairly consistent between people, whereas the absolute values forthresholds vary greatly.

Figure 2.Effect of prolonged low stress lowers thresholds for subsequent adapta-tion and injury. Prolonged physical stress levels that are lower than themaintenance range result in decreased tolerance of tissues to subse-quent stresses (eg, atrophy). Although relative thresholds remain the

same, the absolute magnitude of physical stress is lower for eachthreshold. Injury (and all other adaptations) occurs at a lower level ofphysical stress than required previously.



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or are not causing noticeable dysfunction are not con-sidered clinically significant injuries. The maximum stressthresholdis defined as the amount of stress the tissue canbear just before it fails, if the tissue is fully rested andrecovered from previous stresses. Stress levels thatexceed the maximum stress threshold are consideredexcessive and result in tissue injury.

Fundamental Principle GExtreme deviations from themaintenance stress range that exceed the adaptivecapacity of tissue result in tissue death. The PST repre-sents the potential range of responses of viable tissues toa given level of physical stress. To the extent that thestress level can be altered, viable tissues exhibit a variablestress response. For example, healthy tissues that areinjured as a result of exposure to high stress levels mayreturn to the maintenance range if stress levels areappropriately reduced. In cases where stress levels devi-ate substantially from maintenance conditions, tissuesmay no longer be viable and tissue death can result.Thus, tissue death can occur when tissues are exposed toeither extremely high or extremely low stress levels andare unable to adapt or recover.

Fundamental Principle HThe level of exposure tophysical stress is a composite value, defined by themagnitude, time, and directionof stress application (Fig. 4).Stress magnitude refers to the amount of stress (forceper unit area) on a tissue at any given moment in time.Time factors include the duration, the number of repe-titions, and the rate at which stress is applied to tissues ofthe body. Each of these factors has a direct relationshipwith the level of stress exposure. For example, a longer

duration of stress application results in a greater level ofstress on the tissue. The direction of stress applicationalso influences tissue adaptation. Stress will have adifferent effect depending on whether it is applied intension, compression, shear, or torsion. Use of a com-posite measure to quantify the level of stress exposureimplies that stress levels may be altered by changing one

or more of the stress-related variables that comprise thismeasure.

Fundamental Principle IIndividual stresses combinein complex ways to contribute to the overall level ofstress exposure. Tissues are affected by the history ofrecent stresses. In the PST, the effect of any given stress will depend on the previous stress experience of thetissue. For example, 1 repetition of a biceps muscle curlagainst a resistance of 13.6 kg (30 lb) may have littleeffect on subsequent muscle performance. However, 3sets of 10 repetitions, 3 times a week, for 2 weeks can leadto muscle hypertrophy and increase the muscles abilityto generate force. One consequence of the cumulativeeffect of repeated stresses is that tissues require periodsof rest in which the level of stress exposure is substan-tially reduced so that they can adapt and recover fromprevious stresses.

Fundamental Principle JExcessive physical stress thatcauses injury can occur through 1 or more of thefollowing 3 mechanisms: (1) a high-magnitude stressapplied for a brief duration, (2) a low-magnitude stressapplied for a long duration, and (3) a moderate-magnitude stress applied to the tissue many times. This

Figure 3.Effect of overload stress raises thresholds for subsequent adaptationand injury. Prolonged physical stress levels that are higher than themaintenance range result in increased tolerance of tissues to subsequentstresses (eg, hypertrophy). Although relative thresholds remain the same,the absolute magnitude of physical stress is higher for each threshold.Injury (and all other adaptations) occurs at a higher level of physicalstress than required previously.

Figure 4.Physical stress level is a composite value. Stress magnituderefers to the

amount of stress (force per unit area) on a tissue. Time factors include theduration, number of repetitions, and the rate at which stress is appliedto tissues of the body. Stress will have a different effect depending onwhether it is applied in tension, compression, shear, or torsion.



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principle is closely related to Fundamental Principle H(Tab. 1), which identifies stress magnitude and timefactors (eg, duration and number of repetitions of stressapplication) as important variables used to determinethe composite level of stress to which tissues are exposed.

Fundamental Principle KRegardless of the mechanism

of injury, inflammation occurs immediately followingtissue injury and renders the injured tissue less tolerantof stress than it was prior to injury. In the context of thePST, the threshold for subsequent tissue injury is low-ered as a result of previous injury and the presence ofinflammation. Stress levels that did not cause painbefore the tissue was injured would then have thepotential to cause pain and further tissue damage.Injured and inflamed tissues must be protected fromsubsequent excessive stress until acute inflammationsubsides. Continued stress, even at a level considerednormal for uninjured tissue, can prevent tissue regener-ation following injury.

Fundamental Principle LThe stress thresholdsrequired to achieve a given tissue response may varyamong individuals depending on the presence orabsence of several modulating variables. In developingthe PST, we identified 4 categories of variables (Tab. 2)that can modulate either: (1) the level of stress ontissues, or (2) the response of tissues to a given level ofphysical stress (ie, threshold values for tissue adaptationand injury). Because of the complex interaction amongvariables, it is not yet possible to determine the absolute value of the thresholds for tissue adaptation proposed

within this theory. However, we propose that the PSTcan be used to construct models that predict the relativeinfluence of both stress-related and nonmechanical vari-ables on tissue adaptation and injury. With additionalresearch, it should be possible to estimate the relativethreshold value, based on a percentage of maximumstress tolerance, that is required to promote a desiredtissue response given the individual circumstances ofeach patient. Rehabilitation programs then could betailored to enhance factors contributing to tissue repairand to minimize factors contributing to tissue failure.

Factors that can change the level of stress on tissues orthe threshold values for tissue adaptation and injury aresummarized in Table 2 and include movement andalignment, extrinsic, psychosocial, and physiological fac-tors. Each of these factors is described below, and thosefactors that can be modified with physical therapy inter-ventions are emphasized.

Movement and Alignment FactorsAccording to the PST, movement is the most importantfactor that physical therapists can use to influence tissueadaptation. As noted, movement occurs because of

forces. Forces are applied to tissues over a given area thatresult in stresses, which contribute to tissue adaptation asoutlined in this theory. Physical therapists often focus onthe treatment of movement dysfunction,15,16,21,22 andone theory has designated movement as its core princi-ple.15 By comparison, the core principle of the PST istissue adaptation in response to physical stress. As sum-

marized in Table 3, movement can have beneficialeffects (eg, exercise-related tissue hypertrophy) as well asdetrimental effects (eg, overuse injury) on tissues of thebody. According to the PST, movement and alignmentfactors are considered primarily for their ability to placestress on biological tissue. The components of move-ment and alignment considered by this theory aremuscle performance, motor control, posture and align-ment, and physical activity.

Muscle PerformanceMuscle performance (force generation, muscle length)is a critical aspect of movement that can influence tissuestress. Muscles are highly adaptable. They generatemovement and, hence, forces that place stress on tissues.Muscle also is an important shock absorber, andmuscle contraction is well recognized for its ability toprotect bones, cartilage, and ligaments from excessivestress.23 As a general theory, the PST can be used incombination with other theories and approaches thatprovide a more detailed analysis of the mechanisms by which muscle performance contributes to stress ontissues of the body.2430

Motor Control

Motor controlhas been defined as the study of the natureand cause of movement,31 and it, therefore, represents amajor component of physical therapists expertise.21

Evaluation of the ways in which people control theirmovements to accomplish tasks provides physical thera-pists with insight into how stresses are applied to tissuesof the body during movement. For example, Malufet al32 have proposed that the daily repetition of similarmovements and postures may result in excessive stress ontissues of the low back. These authors suggest thatphysical therapists can identify and modify motorrecruitment patterns which potentially contribute to

patients low back pain during the performance of dailyactivities. Maluf et al32 contend that an important role ofphysical therapists is to identify patterns of movementthat contribute to excessive tissue stress and to teachpatient-appropriate movement strategies to prevent tis-sue injury and pain.

Posture and AlignmentKendall et al25 have emphasized the relationshipbetween posture, impairments, and pain. The Kendallsbasic premise, based on their clinical observations, isthat there is a standard or ideal posture and that



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deviations from this ideal posture lead to characteristicpatterns of musculoskeletal impairments and pain.25(p5)

For example, the Kendalls predict that a person withexcessive lumbar lordosis would have weak abdominaland hamstring muscles, with short, strong low-back andhip flexor muscles.25(p126)

Some studies33

37 have questioned, and even refuted, alarge relationship between these variables in people withand without back pain. Rather than emphasizing anideal standard of posture and hypothesizing that there isa large relationship among specific postures, impair-ments, and pain patterns, the PST proposes that pain iscaused by excessive tissue stress and that postural devia-tions are one ofmanypotential variables that contributeto the excessive stress levels that result in pain. Wecommonly observe people with poor posture who arepain-free and other people with good posture whohave pain. The types of activities performed by people varies widely, resulting in different stress demands ontissues of the body.

The PST predicts that no one ideal posture exists for allpeople because tissues will adapt to meet the uniquestress demands of each person. Injury occurs whentissues are unable to adapt to meet the demands of agiven posture or task. Therefore, rather than comparinga persons posture to an ideal standard, the therapistsexamination should focus on the postures or movementsthat cause pain.26,32,38,39 Within this context, posturaldeviations become one of many potential factors thatmay place stress on injured tissues. In some people, the

postural deviation may be the primary factor contribut-ing to excessive tissue stress (see Implications for Phys-ical Therapist Practice and Implications for Researchsections). In our view, the PST expands upon theKendalls theory by proposing that postural deviationsare one important component of musculoskeletal pain;however, pain patterns should be evaluated in a broadercontext that considers other potential sources of tissuestress.

Physical ActivityPhysical activity is another component of movement that

results in tissue stress. The US Department of Healthand Human Services and the American College of SportsMedicine have adopted the definition of physical activityas bodily movement that is produced by the contractionof skeletal muscle and that substantially increases energyexpenditure.40(p4) Physical activity may be divided intothe specific subcategories of occupational, leisure, andself-care activities. The PST predicts that physical activityimproves health because it increases stress on a broadrange of tissues, making the tissues more tolerant ofsubsequent physical activity. Because the tissues aremore tolerant of physical stress, they are less likely to be

injured. This reduction in the likelihood of injury occursregardless of whether the tissue is part of the cardio-vascular/pulmonary, integumentary, musculoskeletal, orneuromuscular system. Increased physical activity hasbeen linked to many positive health benefits, includinglower risk for noninsulin-dependent diabetes melli-tus,41 stroke,42 and obesity.43 The federal government, in

Healthy People 2010, has set a number of goals toincrease physical activity (Objective 22; Physical Activityand Fitness) in people who are otherwise healthy. Webelieve that physical therapists should use their expertiseto provide instruction on how people can increaseoverall physical activity without injuring specific struc-tures (ie, back or knee).

Extrinsic FactorsExtrinsic factors are factors outside of the body that caninfluence either the level of stress on tissues or thethresholds for tissue adaptation and injury. For thepurpose of this perspective, those extrinsic factors thatcan be modified or utilized by physical therapists areemphasized.

Orthotic devices can be used to modify physical stress onbiological tissues and can be used as an adjunct to otherinterventions in several phases of tissue adaptation. Anorthotic device can be used to relieve stress from injuredtissue (eg, a resting hand splint for patients with carpaltunnel syndrome, a lumbosacral corset for a person withlow back pain). An orthotic device also can be used toapply stress to tissue to cause a change in the tissue(eg, orthotic devices that apply low loads for prolonged

periods to increase muscle length and joint range ofmotion at the elbow).44 According to the PST, orthoticdevices are an appropriate adjunct to treatment whenother means of movement can not adequately controlstress on the tissue to meet desired guidelines. Wecontend that components of the orthotic device shouldbe chosen for their ability to achieve a desired stress levelon the tissue. Likewise, taping45,46 and assistive devices(eg, crutches, walkers, canes) may be effective adjunctsto help modify stress on injured tissues.

Footwear is another extrinsic factor that can influence

stresses applied to the foot. Narrow toe boxes arethought to apply damaging stresses to the forefoot thatlead to the formation of bunions and hammer-toedeformities.47 However, footwear also can be used fortherapeutic purposes such as helping to decrease stresson the foot and ankle at heel strike (ie, shock absorp-tion) or helping to control excessive movement of therear foot.48

The ergonomic environment is another extrinsic factorthat can be modified to influence tissue stress. Changesin the ergonomic environment can be achieved by



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redesigning the job site or work tools to reduce thedemands of jobs that require high force, high repetition,and awkward postures.49 Primary principles of ergo-nomic design include reduction of extreme jointmotion, excessive force magnitudes, and highly repeti-tive movements. Appropriate size and placement of aworkstation can reduce extreme joint motion. Tools or

machines can be designed to reduce excessive forces orrepetitive motions (eg, using an electric drill rather thana manual one).49

Modalities (eg, heat, cold, electrical stimulation) areextrinsic factors that can be used to modify the level oftissue stress or the response of biological tissues to stressapplication. As described in the PST, modalities have asecondary role in treatment but may be indicated toaugment the bodys own adaptive capabilities. Forinstance, electrical stimulation can be used to augmentshort- and long-term muscle force production, especiallyin the presence of pathology that limits the normal forcegenerating capacity of muscle (eg, after ligament injury50

or spinal cord injury51). Heat is an example of a modalitythat can be used to modify the response of tissues tophysical stress. For example, elevation of muscle temper-ature is thought to help prevent strain injuries (ie, raisethe threshold for muscle injury) by allowing muscles tostretch more and tolerate higher loads before failure.52

Gravity is an extrinsic factor that physical therapists canuse to modify the external load on the body. By modi-fying the orientation of the body or limbs with respect tothe ground, physical therapists can increase or decrease

forces exerted on the body during movement and,consequently, alter the forces that must be generatedinternally to produce movement.53 Modalities or devices,such as aquatic therapy54 or weight-supported walkingdevices,55 also may be used to modify tissue stress byaltering the effects of gravity.

Psychosocial FactorsPsychosocial factors represent those factors that areunique to the lifestyle of each person and that poten-tially can be modified. Psychosocial factors can have aprofound influence on tissue adaptation, especially as

related to tissue injury.56

We believe psychosocial factorsprimarily influence individual threshold values for tissueadaptation and injury. For example, researchers investi-gated the role of mechanical and psychosocial factors inthe onset of forearm pain in a 2-year-long prospectivestudy (N1,953).56 Besides linking injury to repetitivemovements of the arm or wrist (relative risk of 4.1 and3.4, respectively), the investigators reported that peoplewho were only occasionally or never satisfied withsupport from supervisors and colleagues had a relativerisk of 4.7 (95% confidence interval2.2, 10) for fore-arm injury. These data indicate that, in some circum-

stances, psychosocial factors can be as important asmechanical factors for tissue injury. Although we recog-nize that psychosocial factors can play an important rolein a persons response to physical stress, this role is notdeveloped extensively in this perspective. We encourageothers to develop the psychosocial component of thePST.

Physiological FactorsPhysiological factors influence the ability of tissue torespond to physical stress, but these factors often aredifficult, if not impossible, to modify. Physiological fac-tors assume a less important role in physical therapytreatment compared with the factors described previ-ously because physical therapists usually can not modifyor treat physiological factors directly. Physical therapistsshould be aware of the influence of physiological factors,however, because these factors will affect the prognosisof tissue adaptation and recovery from injury.

Medications can influence tissue physiology and, conse-quently, the ability of tissues to adapt to physical stress.For example, corticosteroid use has a complex effect ontissue. Corticosteroids can simultaneously cause adecrease in inflammation in one tissue (positive effecton stress tolerance) and cause atrophy in other tissuessuch as skin, bone, and muscle (negative effect on stresstolerance).57 Physical therapists do not prescribe medi-cations. However, the impact of medications on theadaptive capacity of tissue is important, and physicaltherapists should be aware of these effects on theirpatients. In addition, exercise may help to offset muscle

atrophy from medications such as glucocorticoids.58

Age is another important physiological factor. Althoughit is beyond the scope of this perspective to discuss theeffects of age on tissue adaptation, it is important todiscuss several general points. Aging has a negative effecton tissue adaptation. The magnitude of this negativeeffect in humans, however, is not clear. Kohrt59 believesthat a considerable portion of the negative effects thathave been attributed to aging may be due to age-associated decreases in activity. Two predictions regard-ing the effects of age can be derived from the PST. The

first prediction is that aging lowers the ability of tissues totolerate stress and has a general tendency to lower thethreshold for injury, similar to the effect illustrated inFigure 2. The level of stress required to promote tissuehypertrophy and increase stress tolerance generally isgreater for young subjects who are healthy comparedwith older subjects. Young people, therefore, can exer-cise and stress their tissues more aggressively with lessfear of injury than older people. The second predictionis that the negative effects of aging can be modifiedthrough increased stress on the tissue. Increased stress,applied through progressive increases in activity or exer-



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cise, theoretically can help prevent or reverse some ofthe negative effects of aging (Fig. 3). Increasing evi-dence suggests that tissues remain responsive to physicalstress well into old age and that positive adaptations toincreased activity and exercise can result in decreases infunctional limitations and disability.7,8,10

Systemic pathology includes the many diseases that canaffect the ability of tissues to adapt to physical stress.Examples of systemic pathology that physical therapistsoften encounter are diabetes mellitus and rheumatoidarthritis. Similar to the effects of aging, many forms ofsystemic pathology lower the ability of tissue to toleratestress, and they have a general tendency to lower thethreshold for injury. One example of this principle is theeffect of peripheral neuropathy on cortical bone mass inthe feet and hands of patients with diabetes. Patients with diabetes and peripheral neuropathy have beenshown to have lower cortical bone mass, with a higherincidence of metatarsal fractures, compared with amatched group of patients with diabetes alone.60 Asinterpreted by the PST, pathology associated withperipheral neuropathy reduces the injury threshold forbone to a level experienced during normal walking.

Although the PST recognizes the negative impact ofsystemic pathology on tissue adaptation, it also proposesthat carefully applied physical stresses can have positiveeffects for people with chronic disease. Growing evi-dence supports this prediction.9,13,6163 Traditionally,people with rheumatoid arthritis have been excludedfrom vigorous activities that might exacerbate joint

inflammation.9 However, studies have shown that pro-gressive resistive exercises and aerobic exercise pro-grams can increase muscle performance, fitness levels,and bone mineral density with no exacerbation of dis-ease activity in patients with rheumatoid arthritis.9,61,62

The positive effects of aerobic exercise for patients withcardiovascular disease have been well documented.13

Likewise, evidence indicates that increased activity, evenin small increments, can have positive effects on reduc-ing the burden of hyperinsulinemia and diabetes.63

William H Herman, an associate editor for ClinicalDiabetes, summarized the effect of exercise on diabetes:

An ideal treatment for type 2 diabetes would lowerblood glucose concentrations acutely, improve long-term glycemic control, and enhance insulin sensitiv-ity. It would improve mild to moderate hypertension,reduce low-density lipoprotein cholesterol and triglyc-erides, increase high-density lipoprotein cholesterol,and serve as an adjunct to caloric restriction forweight reduction. Finally, it would increase patientssense of well-being and improve their quality of life.As a drug, such an agent would be a blockbuster, withgreater than $1 billion per year sales potential. Yet

such a treatment exists and has been recognized forcenturies. It is regular physical activitythat isexercise.12

Although systemic pathology may lower the thresholdfor tissue injury, many people with systemic pathologyrespond positively to exercise, both in terms of increased

stress tolerance and reduction of disease complications.

We also consider obesity a physiological factor within thecontext of the PST that can be modified, but not easily.Obesity is defined as an increase in body weight beyondthe limitation of skeletal and physical requirements, asthe result of an excessive accumulation of fat.64 Bodyweight provides an indication of the stresses that must beborne by tissues of the body during physical activity. ThePST predicts that obesity is a risk factor for certain typesof injury and a negative factor for recovery from injury.In addition, obesity has been linked with low activitylevels.43 People who are obese, therefore, may be atgreater risk of injury than people who are not obesebecause high-stress, low-repetition activities are thoughtto be more damaging than low-stress, high-repetitionactivities.49,6568 Evidence exists to support this predic-tion. For example, a population-based (n350, 95 menand 255 women, 55 years of age or older) longitudinalstudy (mean follow-up duration5.1 years) reportedthat the risk of incident radiographic knee osteoarthritis was significantly increased among subjects with higherbaseline body mass index (odds ratio18.3, 95% confi-dence interval5.1 65.1).69

Literature ReviewThe PST focuses on how biological tissues respond tophysical stress. Existing evidence and approaches thatshare common principles with the PST will be describednext for each of the 4 major systems described in theGuide to Physical Therapist Practice.14 Because physicaltherapists often work with patients with injuries, eachsection will begin by discussing how physical stress cancontribute to injury. Discussion of the effects of physicalstress on injury will then be followed by a discussion ofthe effects of physical stress on tissue atrophy, mainte-nance, and hypertrophy. These effects are summarized

in Table 3.

Musculoskeletal SystemSome authors29,70 have described mechanical principlesinvolved in the management of patients with low backpain. McGill70 proposed that injury, or failure of atissue, occurs when the applied load exceeds the failuretolerance of the tissue. In addition, McGill70 contendedthat the structures of the back are influenced by thehistory of recent physical stresses, so that the accumula-tion of individual stresses can cause injury. He arguedthat the characteristics of the load (load rate, mode of



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Evidence also supports the idea that tissues within themusculoskeletal system atrophy and become less tolerantof physical stress if stress on the tissue diminishes belowa baseline level.59,8588 Unfortunately, tissues typicallyatrophy at a faster rate than they hypertrophy.59,85 Mus-cle can lose 6% to 40% of its ability to generate forceover a 4- to 6-week period of bed rest or immobiliza-

tion.86 Bone mineral density also is lost in response todiminished physical stress. Leblanc et al87 found thatBMD is reduced 3% to 4% at the femoral neck andlumbar spine after 17 weeks of bed rest in young menwho are healthy. Holick88 summarized the current liter-ature by indicating that unloading of the skeleton, eitherdue to strict bed rest or in zero gravity, leads, on average,to a 1% to 2% reduction in BMD at selected skeletal siteseach month.

Likewise, ligaments respond to reduced mechanicalstress. Woo and colleagues85,89 have documented adecline in the mechanical properties of the rabbitmedial collateral ligament in response to 9 weeks ofimmobilization. These researchers reported that thestiffness, the ultimate load before failure, and theenergy-absorbing capacity of the immobilized medialcollateral ligament-bone complex were approximatelyone third of those variables in the contralateral, nonim-mobilized control limb.89

Integumentary SystemTissues in the integumentary system also demonstratepatterns of response to physical stress similar to thosedescribed for the musculoskeletal system. Mechanisms of

excessive stress resulting in injury to the skin have beendescribed by Brand24 regarding his work with patientswith Hansen disease and foot ulcers. Brand24 proposedthe same 3 basic mechanisms of injury to skin that wedescribe for musculoskeletal tissues and that are out-lined in this theory (Principle J, Tab. 1). According toBrand,24 direct mechanical damage to the skin can occur with high levels of pressure (100 kg/cm2 or 1,300 psi)applied for a brief duration, such as when a person stepson a tack. Ischemic skin lesions can occur when arelatively low magnitude of pressure (15 psi) is appliedto the skin for a long duration, such as when people wear

tight shoes or are confined to bed for prolonged periodsof time. Inflammatory autolysis, similar in concept tocumulative trauma injury, can occur when pressures of amoderate magnitude (20 psi) are applied to the skinhundreds or thousands of times each day, such asstresses applied to plantar tissues during normal walk-ing.24 Integument failure due to repetitive stress wasshown when repetitive stress (10,000 repetitions per day)was applied to denervated rat footpads in an attempt tosimulate stress on the foot during walking. Footpadulcers occurred within 7 to 10 days of commencing thesimulated walking procedures.90 Brand24 and others91,92

subsequently documented similar relationships amongtime, pressure, and skin breakdown in patients withdiabetes and peripheral neuropathy.

Skin also exhibits positive adaptations to controlledincreases in physical stress. Clinicians have observed thatskin adapts to progressive loading with improved stress

tolerance. A common practice for treating patients witha new orthotic or prosthetic device is to slowly progressthe magnitude and duration of weight-bearing to allowskin and underlying tissues to adapt to new weight-bearing forces.93 In addition, the incidence of blisters inthe feet of runners has been found to be highest in theearly stages of training, and it is much lower in peoplewho consistently run relatively long distances (eg, 48 km[30 miles] per week) as compared with people who arejust beginning to run.94

Sanders et al95 have provided a review of the growingbody of literature that suggests that skin exhibits anadaptive response to stress similar to the response ofother biological tissues. Skin under tension showschanges similar to ligaments and tendons under tension.Increases in collagen fibril diameter, collagen cross-linking, and sulfated proteoglycan content render skinmore resistant to tensile forces. Skin adapts to increasesin shear stress with an increase in the size and density ofcells at the basement membrane and an increase in thethickness of the stratum corneum (the outermost celllayer on the surface of the skin). Increased thickness ofthe stratum corneum results in a greater volume of skinthrough which shear loads can be distributed. Distribu-

tion of shear loads causes a reduction in the sheargradient and, consequently, decreases the potential forskin breakdown.

Cardiovasuclar/Pulmonary SystemSimilar to what can occur to tissues in the musculo-skeletal and integumentary systems, excessive physicalstress can injure tissues in the cardiovascular/pulmonarysystem. Mechanical stretch induced by high blood pres-sure is thought to be an initial event that leads to cardiachypertrophy and eventual cardiac failure.96 High bloodpressure is an established risk factor for conditions such

as stroke,97

myocardial infarction,98

and atheroscleroticdisease.99 In addition, vigorous and unaccustomed phys-ical activity has been associated with a higher incidenceof cardiovascular events.100

The positive effects of physical stress in the form ofexercise also are well documented for tissues in thecardiovascular/pulmonary system. McArdle et al13 out-lined fundamental principles of exercise training for thecardiopulmonary system. The overload principle statesthat exercise overload specific to an activity must beapplied to enhance physiologic improvement and bring



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about a training effect. As described for other systems,achieving overload without tissue injury requires anappropriate balance among training intensity (or mag-nitude), frequency, and duration (time factors). Whendeveloping training programs, physical therapists alsomust consider whether stress levels designed to improvecardiovascular performance have the potential to cause

injury to tissues in the musculoskeletal system.

Training can enhance cardiovascular function in ath-letes, people who are sedentary, people with disabilities,and people with previous cardiac impairments.13 Train-ing causes adaptations in tissues within the cardio-vascular and pulmonary systems, including an increasein the weight and volume of the heart, an increase in thesize of the left ventricular cavity and the thickening of its walls, an increase in plasma volume, an increase inmaximum cardiac output, and an increase in lungvolumes. Intensive training can increase aerobic capacity15% to 30% during the first 3 months, with as much asa 50% increase over a 2-year period (Tab. 3).101105

Similar to other systems described, the positive effects ofphysical stress (ie, aerobic exercise training) are quicklyreversible for tissues in the cardiopulmonary system.13

Reductions in metabolic and exercise capacity can occurafter only 1 to 2 weeks of detraining, and many otherimprovements are lost within several months of detrain-ing.13 McArdle et al,13 for instance, reported a 25%reduction in maximum oxygen consumption in 5 sub-jects who remained bedridden for 20 consecutive days.Maximal stroke volume and cardiac output also

decreased, resulting in a loss of aerobic capacity by anaverage of 1% per day.13(p396)

Neuromuscular SystemIn our opinion, adaptation of the nervous system inresponse to physical stress has not been studied asextensively as the adaptive response of tissues in themusculoskeletal, integumentary, and cardiovascular/pulmonary systems. Increasing evidence, however, sug-gests that physical stress and activity affect the peripheraland central nervous systems in a manner similar to thatof other tissues in the body.106108 Excessive stress can

cause injury to tissues within the nervous system via the3 mechanisms discussed for other systems. Examples ofnerve injuries caused by high-magnitude stress appliedover a brief duration include spinal cord injuries result-ing from gunshot wounds or motor vehicle accidents.Nerve injury also may be caused by lower-magnitudestresses applied repetitively or over a long duration.Examples of these mechanisms of nerve injury includecarpal tunnel syndrome, tarsal tunnel syndrome, tho-racic outlet syndrome, ulnar nerve palsy, spinal stenosis,and lumbar root lesions.

Novak and Mackinnon28,109,110 described excessive phys-ical stress from various postural alignments and repeti-tive motions that can contribute to nerve injury andpain. The incidence of cumulative trauma disorders hasrisen dramatically over the last 15 years and has beenlinked to jobs that require high-force and high-repetition activities, especially when performed in awk-

ward postures.49(pp3

7) The odds ratio for developingcarpal tunnel syndrome has been reported to be greaterthan 15 for high-force, high-repetition jobs comparedwith jobs requiring low-force, low-repetition activities.111

Pressures within the carpal tunnel have been shown tobe higher when the wrist is in extreme flexion orextension compared with a neutral position.112 Further-more, jobs requiring excessive wrist deviations are con-sidered a risk factor for injury to the median nerve.109

Lumbar spinal stenosis is another condition that canplace excessive physical stress on neural tissues.113 Move-ments and postures of lumbar extension can furtherreduce the diameter of the spinal canal and vertebralforamen in people with spinal stenosis, causing compres-sion of lumbar nerves and resulting in lower-extremitypain and parathesias.113 People with lumbar stenosisgenerally have reduced lower-extremity symptoms whentheir spine is flexed because this posture allows thevertebral foramen to widen, thereby reducing compres-sive and shear forces on neural tissues.114

Mechanical injury of nerve tissue can occur with low-magnitude stresses applied for a long duration (com-pression injury) or from repetitive bouts of moderate-

magnitude stresses (cumulative trauma disorders).Although some nerve injuries may require surgical cor-rection, we believe one role of the physical therapist is todetermine how excessive stress on the nerve can berelieved in a conservative fashion to facilitate healing ofthe nervous tissue.

We have limited our discussion of the neuromuscularsystem to nerve injuries caused by mechanical stress onthe nerve and surrounding structures. Although beyondthe scope of this perspective, a growing body of litera-ture suggests that neurons adapt to high and low levels

of physical activity by altering their electrical activi-ty106,108,115118 (Tab. 3; eg, rate of discharge, threshold ofrecruitment). The observation that physical activity canstimulate neural adaptation has important implicationsfor the rehabilitation of patients with primary neurolog-ical disorders (eg, stroke, traumatic brain injury). Wepredict that the fundamental principles outlined in thePST can be used to describe neural adaptations follow-ing central or peripheral nervous system injury, and weencourage others to investigate this prediction.



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Limitations of the Physical Stress TheoryThe PST represents an effort to integrate commonprinciples from the approaches and evidence describedabove. We also expand existing theories to place the roleof tissue injury within a continuum of tissue viability sothat the theory encompasses pathological conditionsand principles of wellness and prevention. Like any

theory, the PST has limitations. These limitations will bediscussed next, followed by a discussion of the implica-tions of the theory for physical therapist practice, edu-cation, and research.

In the PST, we assume that tissue exposure to physicalstress can be quantified, and we assume that knowledgeof the level of stress exposure and factors that influencethe adaptive capacity of tissues (eg, age, disease) can beused to predict tissue adaptation and injury. In itscurrent form, however, the PST does not define absolutethreshold values for tissue adaptation and injury. Forexample, we do not know whether a tissue is about to beinjured until it begins to show signs of inflammation(ie, pain, heat, swelling, or redness). Although attemptsare being made to identify biological markers that mayidentify thresholds of adaptation or injury (ie, integrinsin the muscle119 and serum keratan sulfate in interver-tebral disks120), much more research is needed to definethese thresholds in multiple tissue types.

We believe the PST provides an appropriate frameworkfor investigating the influence of multiple factors on thelevel of physical stress required for tissue adaptation andinjury avoidance. Despite the absence of specific values

for thresholds of change, the PST currently provides anoverall framework that outlines relative relationshipsalong a continuum of tissue responses to physical stress.Elaboration and refinement of this preliminary framework should eventually allow for a quantitative under-standing of the thresholds that indicate when atrophy,maintenance, hypertrophy, injury, and death of varioustissues begin.

Another limitation of this theory is that it describeschanges at the tissue level and does not indicate howtissue change is related to functional limitations or

disability. Therefore, we believe the PST must be used incombination with other theories or models of disable-ment121 to address the entire spectrum of physicaldisability. We believe that physical disablement modelsare critical for understanding the overall relationshipbetween disease or pathophysiology and resultingimpairments, functional limitations, and disability.121Wefurther believe that the PST can be used to complementthe physical disablement models by helping us to under-stand the mechanisms of injuries and repair at a tissuelevel.

Certain areas within physical therapy are not well devel-oped in the PST in its current form. For example, thePST does not address issues of adaptation in the centralnervous system. In addition, the PST does not identifythe specific psychosocial factors that most influencetissue adaptation and injury. We believe the currenttheory, however, does provide a useful framework a wide

range of physical therapy issues and has implications forpractice, research, and education.

Implications for Physical Therapist Practice We believe the PST provides a useful framework toapproach patient care. It emphasizes the role of physicalstress by addressing factors known to influence the levelof physical stress and the response of tissues to stressapplication. According to this theory, the primary role ofthe physical therapist is to modify physical stresses toachieve a desired goal. Two broad goals will be discussed.The first goal is to reduce pain and subsequent impair-ments, functional limitations, and disabilities that resultfrom injury. The second goal is to increase activitytolerance. Each of these goals will be discussed further insubsequent sections.

Implications for Treating Patients With Tissue InjuryThe PST provides a general model for evaluating andtreating people with injury. Chronic tissue injury, definedas injury that results in pain lasting greater than 8 weeks,often is caused by stresses of moderate magnitude thatare repeated hundreds or even thousands of times aday.24,49,63 Examples of injuries resulting from this mech-anism include many forms of back and cervical pain,70

patellofemoral pain,122 tendinitis injuries (eg, Achillestendon, posterior tibial tendon), impingement syn-drome of the shoulder, stress fractures,71,72 neuropathicplantar ulcers,24 and carpal tunnel syndrome.49 Theprimary questions asked in the evaluation and treatmentof these and other forms of tissue injury are: (1) Whatfactors appear to be contributing to excessive stress onthe injured tissue? and (2) How can these contributingfactors be modified to reduce stress on the tissue andallow the tissue to heal?

We believe proper identification of the injured tissue

usually can help determine the factors contributing toexcessive stress on the tissue. For example, suspectedtendinitis might lead the clinician to investigate sourcesof stress caused by movements that are controlled by themuscle and tendon. In many cases, however, physicaltherapists are not able to confirm injury of a specifictissueusing the tools currently available to them. The PSTdescribes general principles of tissue adaptation andinjury, applicable to all types of biological tissue, that canbe used to guide treatment regardless of whether theinjury can be localized to a specific structure. In ourview, appropriate treatment, therefore, does not always



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require accurate identification of the injured structure.For instance, if low back pain can be eliminated byteaching the patient to move at the hips rather thanmoving at the lumbar spine when movements of thelumbar spine cause pain, identifying the specific injuredtissue is of little benefit.

We believe general principles that govern the responseof all biological tissues to stress can be used to guidephysical therapy intervention, even when pathology of aspecific structure cannot be determined. In this context,we contend that the most important question to guidetreatment becomes: What factors appear to be contrib-uting to excessive stress on the injured tissue? Thisquestion relates to Fundamental Principles F, I, J, and Kin Table 1. In our opinion, the factors in Table 2 shouldbe systematically evaluated for their potential to contrib-ute to excessive tissue stress or for their potential tomodify the response of tissues to stress application. Inour experience, various types of injury tend to beassociated with a similar set of contributing factors. Forexample, we have observed that factors contributing tooveruse injuries of the foot and ankle often includemovement and alignment factors (ie, excessive prona-tion or supination,123 changes in activity) and extrinsicfactors (ie, footwear). Factors that contribute to exces-sive tissue stress or modify the tissue response can beadded to or subtracted from the model depending onindividual characteristics of the patient and the thera-pists current working set of hypotheses.

We suggest that a physical therapist develop a hypothesis

regarding the factors that are contributing to excessivetissue stress and causing injury. The next logical ques-tion is: How can I modify these contributing factors toreduce stress on the tissues and allow the tissues to heal?Successful outcomes, in our opinion, help to confirmthe initial hypothesis, whereas poor outcomes direct theclinician to investigate other potential sources of stressnot identified by the initial hypothesis. This approach isconsistent with the hypothesis-oriented algorithm forclinicians.124

The basic principles discussed can be illustrated with

several examples. Maluf et al32

described evaluation andtreatment of a patient with chronic low back pain. Theauthors speculated that repetition of rotation and exten-sion movements of the lumbar spine contributed toexcessive stress and injury to structures within the lowback region. Low back symptoms improved followinginstructions designed to restrict lumbar rotation andextension movements during the performance of dailyactivities.32 McPoil and Hunt27 described a similarapproach in the treatment of a patient with heel pain.The authors speculated that a recent change in activitylevel, poor footwear, and excessive pronation at the

subtalar joint contributed to excessive stress on theplantar fascia and resultant heel pain. The treatmentplan, therefore, was designed to modify these factors todecrease stress on the plantar fascia (eg, modifying thepatients work schedule to limit weight-bearing time,purchasing shoes with cushioned midsoles, obtainingtemporary orthotic devices to control excessive

pronation).27

Similar principles can be applied to the evaluation andtreatment of many types of wounds. Mueller and Dia-mond125 described an approach emphasizing reductionof stress on plantar tissues in a patient with diabetes,peripheral neuropathy, and a chronic plantar ulcer (21months). They speculated that a fixed equinus andrear-foot varus deformity contributed to excessive pres-sure on tissues in the forefoot during walking andprevented the chronic ulcer from healing. The woundhealed after a total contact cast, followed by protectivefootwear, was applied to reduce excessive stress at thesite of the wound.125An approach based on principles ofstress reduction also has been described for the treat-ment of peripheral nerve injuries such as carpal tunnelsyndrome and brachial plexus injuries.109 Novak andMackinnon109 have described how postures, movements,impairments, and physical activities can contribute tostress on peripheral nerve tissue. Furthermore, theseauthors have described how factors contributing toexcessive stress on peripheral nerves can be modified toallow the nerve tissue to heal and, consequently, reducethe symptoms associated with nerve injury.109

Even after sources of excessive stress have been identi-fied and removed, we contend that injured tissues will beless tolerant of stress than they were prior to injury dueto pain, inflammation, and disuse associated with theinjury (Principles C and K, Tab. 1). Therefore, after painand inflammation have subsided, previously injuredtissues, in our opinion, should be exposed to progres-sively higher levels of physical stress to gradually restorethe tissues ability to tolerate greater levels of stress(Principle E, Tab. 1). Stress, we contend, should beapplied progressively, with attention paid to the bodysneed for rest and recovery.

Implications for Improving Activity ToleranceOur examples were designed to illustrate how to pro-mote healing and progressive strengthening of injuredtissues. The goal of helping patients to improve activitytolerance will now be considered. Examples of this goalinclude increasing muscle peak force production,achieving independence in transfers, and improvingwalking tolerance. Activity can be considered a form ofphysical stress.40 Therefore, a systematic approach forevaluating and treating individuals with the goal ofimproving activity tolerance can be derived from the



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PST. We propose that the following questions are rele-vant to the goal of increasing activity tolerance: (1) Whatis the activity goal? (2) What are the current modifiablefactors limiting the activity goal? and (3) How shouldthese factors be modified to meet the activity goal? Forexample, the activity goal of an elderly person may be tostand independently from a sitting position. A physical

therapist may hypothesize that the primary modifiablefactors limiting this goal are: (1) lower-extremity muscleatrophy resulting in poor force production, (2) de-creased dorsiflexion range of motion, (3) poor motorcontrol (movement and alignment factors), and (4) alow seat surface (extrinsic factor). The persons age is animportant physiological factor that may limit the poten-tial for tissue adaptation and lower the threshold fortissue injury. A treatment plan designed to modify thesefactors based on the PST might include: (1) a progres-sive resistive exercise program for lower-extremity exten-sor muscles with at least 70% of maximum effort, 2 to 3times a week, for several weeks to increase muscle forceproduction, (2) stretching exercises to increase dorsi-flexion range of motion, (3) practice standing from aseated position using appropriate movement strategies,and (4) advising the patient to obtain a higher chair sothat lower muscle force production is needed to achievethe activity.

The overload principle (Principle E, Tab. 1),13,40 inwhich maintenance stress levels are raised to the rangeof stress that promotes tissue hypertrophy, has tremen-dous implications for physical therapy and forms thebasis for progressive resistance exercises to increase

muscle performance and activity tolerance. Physicalstress must be of sufficient magnitude and repetition tocause the desired change in performance. The role ofthe physical therapist, in our view, is to instruct people inthe appropriate magnitude and repetition of exercise oractivity to provide an adequate stimulus for hypertrophyof intended tissues without injuring other tissues. Although clinicians are accustomed to using the over-load principle to achieve hypertrophy of muscle, thesesame principles can be applied to other tissues such asligament, tendon,126 and skin95 (Tab. 3).

Implications for Physical Therapist EducationIn a time when scientific and medical knowledge isexpanding at an exponential rate, we believe the PSTcan help educators focus on appropriate content for asubstantial portion of the physical therapy curriculum. According to the PST, key elements of the curriculumshould include a clear understanding of how the bodyadapts to physical stress and how physical therapists canapply or modify stress on the body to achieve desiredoutcomes (Tabs. 1 and 3).

In the PST, we propose that several key content areasshould be emphasized. Kinesiology, we argue, should betaught with an emphasis on tissue mechanics and howdifferent types of movement contribute to stress onspecific tissues. Classes related to exercise physiology, inour view, should present guidelines for exercise andactivity prescription that are designed to fall within the

range of stress that promotes tissue hypertrophy whileavoiding injury. We contend that instruction aboutorthotic devices or modalities should focus on their usein modifying either the level of stress applied to the bodyor the bodys response to stress. The content of classesrelated to medicine and pathology should focus onproviding an understanding of how particular diseasesor pathologies modify the bodys response to variousforms of physical stress including exercise and activity.Classes related to clinical diagnosis and managementcould be organized using the approach described in thesection titled Implications for Physical Therapist Prac-tice. We contend that the list of factors in Table 2 couldform the focus of factors to consider for evaluation andintervention. Although there are other content areasthat must be addressed in a curriculum (eg, administra-tion), we contend that the factors listed above are themost important and could serve as the focus of contentunique to a physical therapy curriculum. Studentsshould be trained to develop a clinical hypothesis thatidentifies the most important factors contributing to apatients problem, to address each modifiable factor,and to revise the initial hypothesis based on evaluationof treatment outcomes.

Implications for ResearchThe PST provides a framework for generating researchhypotheses that focus attention on what we believe to bethe most important and basic treatment technique usedby physical therapists: modifying physical stresses tofacilitate tissue adaptation and prevent injury. Althoughemphasizing the role of physical stress (Tab. 1), the PSTalso recognizes the multifactorial nature of tissue adap-tation and injury (Tab. 2).

We believe this theory helps to clarify and expand ondocumented theoretical approaches currently used in

physical therapy. Disablement models have provided anexcellent theoretical framework for studying the rela-tionship among impairments, functional limitations, anddisability. We agree that a limitation of these models,however, is that they often do not address the underlyingcauseof the patients symptoms.127 For example, disable-ment models postulate that back pain leads to certainfunctional limitations, such as difficulty performinghousehold chores. However, we believe these models donot encourage identification of the specific mechanismsthat cause the pathology leading to low back pain. ThePST helps to fill this theoretical void by proposing that



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excessive physical stress causes the pathology, resultingin pain and subsequent disability.

We believe the PST also can be used to complement theAmerican Physical Therapy Associations current Clini-cal Research Agenda for Physical Therapy.128 Theagenda was designed to support, explain, and enhance

physical therapy clinical practice by facilitating researchthat is useful primarily to clinicians.128(p499) The ClinicalResearch Agenda consists of 72 questions, organizedaccording to the elements of the physical therapistpatient/client management model. In addition to gen-erating new research questions, we believe the PSTprovides a theoretical framework to help direct researchfor questions in the research agenda. For example,question 1.4.9. of the agenda asks, Do measures ofpostural alignment in people with spinal disorders influ-ence clinical decision making, and, if so, how?128(p505)

The PST directs the investigator to consider posture andalignment factors in the context of how these factorsmight contribute to excessive physical stress on thespine. Based on the PST, an investigator might hypoth-esize that, in the general population, posture and align-ment contribute a small but meaningful effect size tospinal dysfunction. The PST suggests that other factorssuch as movement patterns, age, ergonomic environ-ment, and psychosocial factors also must be consideredin clinical decision making when examining and man-aging patients with spinal dysfunction. Furthermore, webelieve the PST provides an organized framework foridentifying and studying the relative contribution ofeach of these factors to the development, persistence,

and recurrence of spinal disorders. This is one exampleof the many multifactorial problems encountered byphysical therapists. In the PST, we suggest that contrib-uting factors should not be investigated in isolationwhen studying the etiology of any given disorder.

Another example of how the theory could be used togenerate and test hypotheses put forth in the agenda isin the management of skin breakdown on the plantarsurface of the foot in patients with diabetes and periph-eral neuropathy. Question 2.2.1. asks, What are themodifiable risk factors for cumulative trauma

syndrome?128(p505)

Neuropathic plantar ulcers are oneof many conditions that can be considered cumulativetrauma syndrome.24 Investigators91,129 have determinedthat the location of plantar ulcers corresponds with areasof high physical stress, identified by peak plantar pres-sure. Identifying the magnitude of pressure that predictsinjury of plantar tissues, however, has been elusive.130

The PST predicts that there is no single threshold ofpeak pressure that predicts injury for all patients. Rather,the theory suggests that a combination of stress-related variables (eg, magnitude, duration, repetition, anddirection of pressures) must be considered when assess-

ing a patients risk for skin breakdown (FundamentalPrinciples H and J, Tab. 1).24 Furthermore, the thresh-old for injury is influenced by individual movement andalignment factors (eg, activity level, foot deformities),extrinsic factors (eg, type of shoes), psychosocial factors(eg, adherence), and physiological factors (eg, severityof peripheral vascular disease and peripheral neuropa-

thy) (Tab. 1). Researchers who are attempting to inves-tigate the complex issue of skin breakdown would needto account for these many factors. The PST provides aframework for connecting the factors that can be used toguide research questions for a broad range of problemsencountered by physical therapists.

ConclusionThe PST emphasizes the application and modification ofphysical stress on tissues of the human body to elicitpositive adaptations and avoid injury. Many of thethoughts presented in this article are not new. Thesummary and integration of these thoughts into adefined theory, however, does offer a comprehensiveapproach that we believe can help to direct physicaltherapist practice and research for a wide range ofpeople with and without impairments. We assert thatthere is considerable evidence to support current phys-ical therapist interventions directed at modifying physi-cal stress. We challenge readers to test, modify, anddevelop this theory through practice and publishedresearch. Our hope is that readers will see commonelements described in this perspective in their own areasof practice and that physical therapy can continue tomove forward in a more systematic and evidence-based

manner.

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