© 2000 Tufts University, 1096-6781/00/$15.00/0 Nutrition in Clinical Care,Volume 3, Number 4, 2000 216–224

An Update on Exercise Therapy for

Knee Osteoarthritis

Kristin Baker, PhD

The Boston University Multipurpose Arthritis and Musculoskeletal Disease Center, Boston, Massachusetts

j

A

BSTRACT

Knee osteoarthritis (OA) is the most common joint disor-der in the United States. In the elderly population,symptomatic knee OA accounts for more dependencyin lower-extremity tasks than any other disease. Treat-ments have generally targeted pain, assuming that dis-ability would be lessened as a direct result of improve-ments in pain control. However, there is evidence thatpain and disability have different determinants. Exer-cise has the potential to mitigate many of the factorsthat lead to disability. In 1995, on the basis of severalsmall, positive studies, the American College of Rheu-matology recommended strength training and aerobicexercise for the treatment of knee OA. In the past sev-eral years, a number of intervention studies on exercisehave been published. It is notable that all of the recentexercise studies report positive effects on pain and/ordisability, although there is considerable variability inthe magnitude of the effect. This variability may be dueto differences in study design, exercise protocols, andstudy participants. Further studies are needed to betterdefine the mechanisms responsible for the positive ef-fects of exercise and to formulate a clear, concise ex-ercise protocol that may have the potential to be-come a public health intervention for knee OA.

NutrClin Care.

2000;3:216–224

j

K

EY

W

ORDS

:

knee osteoarthritis, exercise

Osteoarthritis (OA) is the most common joint disor-der in the United States.

1

The highest rate of symp-

tomatic OA occurs in the knee.

2

Data from theFramingham Osteoarthritis Study suggest that symp-tomatic knee OA (defined as pain on most days pluspositive findings on a radiograph of the symptom-atic knee) occurs in 6.1% of adults over the age of30 and in 11% of those over the age of 65.

3

Themanagement of OA has considerable direct and in-direct health care costs and imposes a major socio-economic burden.

4

Few satisfactory treatments are available forknee OA, and although pain, loss of function, anddisability are all important adverse outcomes ofknee OA, treatments have largely targeted onlypain, with the implicit assumption that functionwould improve with better pain control. However,the literature suggests that separate pathways af-fect pain and physical function. Non-steroidal anti-inflammatory drugs (NSAIDs) and acetaminophenrelieve pain by about 20% but have a very small ef-fect on physical function (4%–9%).

5

Joint replace-ment improves pain and physical function by asmuch as 50% to 100%, with less improvement inthose with lower preoperative physical function.

6–9

In addition, Walsh et al

10

showed that marked func-tional limitations still existed in individuals oneyear post knee-joint replacement, with 30% to 40%weaker knee extensor muscles and significantlyless work capacity than age- and gender-matchedcontrols. It is therefore important to develop treat-ments for knee OA that have a more formidable im-pact on physical function in order to complementcurrent pharmacological and surgical interventions.The following review will discuss determinants of

Reprint requests to Kristin Baker, PhD, Arthritis Center, RM A203, BostonUniversity School of Medicine, 715 Albany Street, Boston, MA 02118.E-mail: Krbaker@bu.edu

Nutr Clin Care, July/Aug 2000–Vol 3, No. 4

Exercise for Knee Osteoarthritis

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physical function (disability) and the potential forexercise as a therapy to improve physical functionand pain in knee OA. Based on the evidence, spe-cific exercise recommendations will be made.

DETERMINANTS OF PHYSICALFUNCTION (DISABILITY)

Knee OA accounts for more dependency in lowerextremity tasks such as walking and stair climbingthan any other disease, especially in the elderly.

11

Figure 1 illustrates the factors associated with theloss of physical function and disability in knee OA.It has been well established that individuals withknee OA have weaker quadriceps muscles thanthose without knee OA.

12–14

Slemenda et al

13,15

showed a 20% deficit in knee extensor strength inwomen with knee OA, even when pain was absent,and demonstrated that this loss of strength mayprecede the development of knee OA. Several stud-ies have shown quadriceps weakness to be an inde-pendent predictor of self-reported disability.

16–18

Ithas further been suggested that there may be a

threshold for strength that increases the risk for dis-ability.

16,19,20

Neurological deficits have also been reported inpersons with knee OA. Hurley and Newman

21

re-ported a 19% muscle reflex inhibition in the af-fected knee vs the unaffected knee of patients withunilateral knee OA. They suggested that damage inthe joint may affect joint receptors that provide af-ferent information to muscles and muscle spindlesabout the joint position. Sharma et al

22

have shownimpaired proprioception in patients with knee OAcompared to controls without knee OA. Studieshave shown a relationship between knee proprio-ception and physical function.

23–26

The ability to maintain full range of motion inthe knee joint has been shown to be an indepen-dent predictor of physical function.

18

The knee-flexion threshold for many lower-extremity tasks,such as rising from a chair, climbing stairs, andwalking, is 110 degrees. In a study by Badley et al

27

subjects who could not flex their knees to at least70 degrees had difficulty with walking and transfertasks. Range of motion may be restricted by lack ofelasticity and flexibility in the muscles surrounding

Figure 1. Model of physical function/disability in knee OA.

218

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BAKER Nutr Clin Care, July/Aug 2000–Vol 3, No. 4

the joint and in the periarticular soft tissues, suchas the joint capsule and ligaments.

Philbin et al

28

reported individuals with end-stageknee OA to be severely deconditioned, with reducedpeak oxygen consumption and a trend toward morefrequent coronary artery disease compared to age-and sex-matched controls. Aerobic deconditioningmay also increase disability, especially in lower-extremity tasks such as walking and climbing stairs.

17

Studies have also shown psychosocial factors, suchas self-efficacy, depression, anxiety, and pain-copingskills, to be associated with, or independent predic-tors of, pain and disability.

17,18,29

The interrelation-ship of these physiological and psychosocial vari-ables is undoubtedly complex, but a treatment thathas the potential to affect multiple variables mayhave a better chance of success.

EXERCISE AS A TREATMENT FOR KNEE OA

Many of the factors that lead to disability can be im-proved with exercise, and few therapies are as mul-tifaceted. For example, resistance training in pa-tients with knee OA has been shown to improvemuscle strength, reflex inhibition, and propriocep-tion, as well as having a positive effect on psycho-logical factors such as depression and self-effi-cacy.

26,30–35

Exercise may also slow the progression of jointdamage. It has been hypothesized that the cumula-tive effects of repeated impulse loading of thelower limb may be a factor contributing to the on-set and progression of knee OA.

36,37

Strong kneeextensors may decrease the impulse loading of thelower limb by slowing the deceleration phase be-fore heel strike. It has also been suggested thatjoint loading via weight-bearing exercise is re-quired to maintain the health and integrity of carti-lage.

38,39

In 1995, the American College of Rheumatology(ACR) recommended exercise in the treatment ofknee OA, specifically quadriceps strengthening andaerobic conditioning.

40

These recommendationswere based on the results of several then-currentsmall studies that demonstrated benefits of exer-cise in knee OA but, nonetheless, left many ques-tions unanswered.

35,41–46

Left unanswered werequestions regarding the most appropriate exerciseto reduce disability (ie, specific components of ex-

ercise programs responsible for observed improve-ments in physical function—cardiovascular fitness,strength, and flexibility—and/or socialization) andincrease long-term compliance and long-term ef-fects of exercise.

Since that time, a number of exercise interven-tion studies, including several large, randomized,controlled clinical trials, have been reported in theliterature. Therefore, it is now appropriate to re-evaluate the evidence for the benefits of exercise inknee OA and provide more specific exercise guide-lines that physicians and health care workers canprescribe to their patients.

EXERCISE STUDIES

In the past several years, there have been 7 ran-domized, controlled trials of the effects of variousexercise protocols on knee OA.

32,33,47–51

Details ofthese studies are described in Table 1.

Unfortunately, interpretation and comparison ofthese exercise trials are somewhat complicated bydifferences in the mode, volume, and intensity oftheir exercise protocols, time of follow-up, and out-come measure methodologies. The exercise proto-cols of all studies included a muscle-strengtheningcomponent, while many also included physicaltherapy regimens, flexibility training, and/or aero-bic exercise. Although all studies had some positiveoutcomes, the response in the magnitude of effecton pain and disability varied between studies.

The largest of these studies was an 18-month,single-blind trial in which participants were ran-domly assigned to one of three interventions—aero-bic walking, strength training, or a health educationprogram that served as the control condition.

48

Thefirst 3 months of the study utilized supervised groupexercise sessions, and the remaining 15 months re-lied on home-based exercise programs with limitedsupervision. The primary outcome measure wasself-reported disability. Participants in both exercisegroups reported modest but significant improve-ments in disability, pain, and physical performancemeasures when compared to controls. The moremodest effect of exercise in this study compared tosome of the other trials may have been due to thelength of the follow-up period and greater lack ofadherence to exercise protocols at 18 months (50%)compared to shorter, more supervised interventionstudies.

Nutr Clin Care, July/Aug 2000–Vol 3, No. 4

Exercise for Knee Osteoarthritis

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219

Tab

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.

Ra

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220

j

BAKER Nutr Clin Care, July/Aug 2000–Vol 3, No. 4

Varying treatment protocols in these studies maycontribute to the variable findings regarding the ef-fect of exercise. Three of the 8 studies, for exam-ple, included physical therapy regimens (eg, manualtherapy, venous therapy, or joint-mobility exer-cises).

47,50,51

In studies such as these—where multi-ple interventions are utilized—it is impossible todetermine the relative effect of each one.

In addition, different studies often used varyingtypes of strengthening exercises, which may resultin different physiologic effects. For example, func-tional benefits from isometric contraction exer-cises might be limited to a small range around thejoint angle of training.

52

In isotonic and isokineticexercises, on the other hand, the joint moves througha specified range against a nonvarying and vary-ing external resistance, respectively. Strengthen-ing programs may also vary according to whetherthey are designed to isolate specific muscle groups(open-chain), or exercise multiple muscle groupssynergistically (closed-chain). In an open-chain ex-ercise such as leg extension, knee extensors are tar-geted, such that motion of the knee is independentof motion at the hip or ankle. However, in closed-chain exercises such as squatting and stair-step-ping, multiple muscle groups (ie, knee extensors,hip extensors, and trunk muscles) work synergisti-cally, motion of the knee is accompanied by mo-tion in the ankle and hip, and the foot is in contactwith the floor. Specific activities that become diffi-cult for individuals with knee OA, such as risingfrom a chair, may benefit from targeted open-chainexercises such as knee extension; however, closed-chain exercises more closely mimic the movementsrequired for many activities of daily living.

Differences in study participants also may havecontributed to the variability in outcomes of thesestudies. For example, it is not clear that individualswith more severe knee OA or with joint laxity re-spond as well to exercise. There is evidence that therelationship between strength and physical functionis diminished in individuals with greater joint laxity.

53

And, in fact, authors of two studies

33,47

reported thatthe participants who dropped out had more severesymptoms than those who completed the study. Onestudy that specifically recruited participants withmore severe knee OA, found lesser effects on painand disability than those observed in other studies.

50

Finally, differences in measurement tools used forpain and disability outcomes may have accounted

for some of the variability in the magnitude of theeffect of exercise between studies. The WesternOntario and McMaster Universities’ Osteoarthritis In-dex (WOMAC) has been shown to have greater sta-tistical efficiency than other indices, including theArthritis Impact Measurement Scales (AIMS) andLequesne Index.

54

Future studies should use similaroutcome measures to improve comparability.

The mechanism for the beneficial effect ofstrengthening remains unclear. Muscle strength wouldbe a logical explanation, but only 3 of the 7 studies inTable 1 reported quadriceps strength improvement.In 2 of these, the strength improvement was minimal(5%–10%); the other study reported greater improve-ments, but only in the least affected leg.

50

There areseveral possible reasons for this. One possibility isthat the exercise dose may have been insufficientto be an effective intervention for strengthening.Furthermore, the mode of testing was not always con-sistent with the mode of training, which can limit theability to see strength improvements.

52

For example,in the Ettinger et al study,

48

participants were trainedwith isotonic contractions but tested with isokineticcontractions. In addition, if there is a threshold ofstrength for disability, depending on the baselinestrength of individuals, small or large strength im-provements may be required to lessen disability. Fur-ther studies are needed to better define the effects ofvarious types of strength training—isometric, iso-tonic, and/or functional—on physical function andpain in knee OA.

Other physiological factors aside from strength,such as proprioception, knee-joint range of motion,and aerobic conditioning, as well as psychologicalfactors, may have improved with the exercise butrarely were measured. In a modified cross-over study,Hurley and Scott

26

reported that 5 weeks of strengthtraining reduced disability and improved strength andproprioception in individuals with knee OA. It hasalso been shown that strengthening exercises can im-prove cardiovascular endurance in these patients,which suggests that decreased cardiovascular fitnessmay be secondary to decreased muscle function.

46

Education and socialization interventions have beeneffective in reducing pain in knee OA, but not as effec-tive in reducing disability.

55–59

Therefore, we cannotrule out the possibility that the attention provided tothe subjects during exercise interventions had someimpact on the observed improvements. In fact, manyof the studies detailed in Table 1 reported improve-

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Exercise for Knee Osteoarthritis

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221

ments in control groups that received a similaramount of attention as treatment groups—sometimesenough improvement that between-group differencesin some outcome measures were diminished.

Evidence from 2 of the recent exercise trials indi-cates that improvements may be maintained for ayear to 18 months.

47

,

48

Long-term compliance to ex-ercise programs was measured in only one study

48

and was found to be similar to that observed in otherexercise studies involving healthy older men andwomen (85% at 3 months, 70% at 9 months, and 50%at 18 months).

60

These recent studies also provideevidence that a home-based exercise program maybe as effective as an exercise program with a physicaltherapist or in a clinical setting.

32,48

This is an impor-tant finding, because supervised exercise is resource-intensive, which may limit generalizability.

CONCLUSIONS

Recent research indicates that exercise in the formof a weight-bearing activity, such as walking or lightresistance training, can have a positive affect on pain

and physical function in individuals with knee OA.Specific recommendations are detailed in Table 2.Resistance training should include or progress fromisometric exercises to some isotonic and functionalexercises. Flexibility exercises should be incorpo-rated to maintain elasticity in the muscles and periar-ticular soft tissues. Physical therapy, predominantlymanual therapy, may provide additional benefit, butmore research is needed to better define the effectof this type of therapy.

Recommendations have not changed much sincethe 1995 ACR guidelines, but there is a better un-derstanding of possible mechanisms and targets forfuture exercise studies, such as muscle strength,neurological deficits, disease severity, biomechani-cal deformities, and psychosocial factors. In addi-tion, more studies should be conducted to assessthe long-term effects of exercise on knee OA and todevelop better strategies for enhancing long-termcompliance with exercise programs.

Guidelines for appropriate exercises can be ob-tained in materials published by the Arthritis Foun-dation, including the

PACE I

and

PACE II

videosand

Pathways to Better Living with Arthritis

video.

Table 2.

Exercise Recommendations for Knee OA Patients*

Strength Training Aerobic Training Flexibility

Goal Improve quadricep strength;balance quadricep strengthgains with strength gains in otherlower extremity muscle groups

Improve cardiovascular fitness Balance strength gainswith joint range ofmotion

Exercises Isotonic, closed- andopen-chain exercises:• Knee extension• Hamstring curl• Squats• Stair stepping

If isotonic exercise limited by pain,substitute isometric exercise andproceed to isotonic exercise as tolerated:• Straight leg raises• Isometric hamstring curl

Walking, if tolerated.If not tolerated secondaryto pain, substitute pool exercise

Stretch each major musclegroup utilized in strengthand aerobic training

Frequency Twice weekly (may be combinedwith aerobic exercise or doneon alternate days)

Twice weekly (may be combinedwith strength training or done onalternate days)

To follow strength trainingand aerobic exercise

Intensity 2 sets of 12–15 repetitions; aiming for an8 on the modified Borg scale

†

at the lastrepetition; alternate with lower-intensitytraining (5 on Borg scale

†

) every 4–6 weeksto avoid overstressing the joints and boredom

40%–60% of predicted maximumheart rate (220

2

age)as tolerated

*Exercise contraindications include recent MI, uncontrolled angina, hypertension and/or arrhythmia, aortic aneurysm, and severe valvular heart dis-ease. For more complete list see Kenney, WL, Humphrey RG, Bryant CX, eds.

ACSM’s Guidelines for Exercise Testing and Prescription.

5th ed. Balti-more: Williams and Wilkins; 1995.

†

Borg, G. and Linderholm H. Perceived exertion and pulse rate during graded exercise in various age groups.

Acta Med. Scand.

1967;472:194–206.

222

j

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Physical therapists can also help patients developappropriate, individualized exercise programs.

Kristin Baker, PhD, has indicated no significantrelationships with commercial supporters.

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