ARTHRITIS & RHEUMATISMVol. 42, No. 1, January 1999, pp 25–32© 1999, American College of Rheumatology

DOES LAXITY ALTER THE RELATIONSHIP BETWEEN STRENGTH ANDPHYSICAL FUNCTION IN KNEE OSTEOARTHRITIS?

LEENA SHARMA, KAREN W. HAYES, DAVID T. FELSON, THOMAS S. BUCHANAN,GRETCHEN KIRWAN-MELLIS, CONGRONG LOU, YI-CHUNG PAI, and DOROTHY D. DUNLOP

Objective. Since strengthening interventions havehad a lower-than-expected impact on patient function instudies of knee osteoarthritis (OA) and it is known thatlaxity influences muscle activity, this study examinedwhether the relationship between strength and functionis weaker in the presence of laxity.

Methods. One hundred sixty-four patients withknee OA were studied. Knee OA was defined by thepresence of definite osteophytes, and patients had tohave at least a little difficulty with knee-requiringactivities. Tests were performed to determine quadri-ceps and hamstring strength, varus-valgus laxity, func-tional status (Western Ontario and McMaster Univer-sities Osteoarthritis Index Physical Functioning subscale[WOMAC-PF] and chair-stand performance), body massindex, and pain. High and low laxity groups were definedas above and below the sample median, respectively.

Results. Strength and chair-stand rates corre-lated (r 5 0.44 to 0.52), as did strength and theWOMAC-PF score (r 5 20.21 to 20.36). In multivari-ate analyses, greater laxity was consistently associatedwith a weaker relationship between strength (quadri-ceps or hamstring) and physical functioning (chair-stand rate or WOMAC-PF score).

Conclusion. Varus-valgus laxity is associated witha decrease in the magnitude of the relationship betweenstrength and physical function in knee OA. In studiesexamining the functional and structural consequencesof resistance exercise in knee OA, stratification of

analyses by varus-valgus laxity should be considered.The effect of strengthening interventions in knee OAmay be enhanced by consideration of the status of thepassive restraint system.

Patients with knee osteoarthritis (OA) haveworse quadriceps strength than do subjects without OA(1–4), and quadriceps strength has been shown to be adeterminant of physical functional status in patients withknee OA (5–9). Although recent studies of musclestrengthening have reported efficacy, the impact onpatient function has been surprisingly modest (9,10)given the role that muscle plays in joint function. Thisprobably relates to the presence of other factors inter-fering in the relationship between strength and function.Knee laxity may represent such a factor. Knee laxity, orinstability, may be broadly defined as the displacementor rotation of the tibia with respect to the femur (11).Muscle activity is influenced and redirected by thepresence of soft tissue laxity (11,12). Soft tissue laxitynecessitates that greater muscular work be directedtoward the goal of achieving joint stabilization underdynamic conditions.

Clinically, static knee stability is assessed in thefrontal (varus-valgus) and sagittal (anterior-posterior)planes. Varus-valgus rotation is greater in patients withknee OA than in subjects with healthy knees (13,14).Varus-valgus laxity in any one patient with knee OA ismultifactorial, possibly resulting from primary laxity ofthe capsule or ligaments, pseudolaxity due to loss ofcartilage and/or bone height, chronic capsuloligamen-tous stretch due to malalignment, or combinations ofligamentous, meniscal, muscular, and capsular pathol-ogy (15). Anterior-posterior translation has been shownto decline with increasing severity of knee OA, eventhough cruciate ligament pathology was common atadvanced stages, suggesting that joint stiffness due tocapsular changes or osteophytic growth overrides thecruciate ligament insufficiency (13).

Supported by NIH grant AR-30692 and by the GreaterChicago Chapter of the Arthritis Foundation.

Leena Sharma, MD, Karen W. Hayes, PhD, PT, GretchenKirwan-Mellis, BS, Congrong Lou, MS, Yi-Chung Pai, PhD, MPT,Dorothy D. Dunlop, PhD: Northwestern University, Chicago, Illinois;David T. Felson, MD, MPH: Boston University, Boston, Massachu-setts; Thomas S. Buchanan, PhD: University of Delaware, Newark.

Address reprint requests to Leena Sharma, MD, Division ofRheumatology, Northwestern University, Ward Building 3-315, 303East Chicago Avenue, Chicago, IL 60611.

Submitted for publication April 22, 1998; accepted in revisedform July 9, 1998.

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In the unloaded state, varus-valgus stability isprovided by the ligaments, capsule, and other soft tis-sues, and in the loaded state, by interactions betweenligaments, other soft tissues, condylar geometry, andtibiofemoral contact forces at the joint interface gener-ated by muscle activity and gravitational forces (11,16).Under dynamic conditions, normal mechanics and sta-bility of the knee depend on proprioceptive systems andcortical awareness of the tone or tension in the jointstabilizers. These systems enable muscle to interactdynamically with the passive restraints to provide stabil-ity, while allowing motion and attenuating load (15–17).Stabilizing forces are continuously adapting to jointorientation and activity (11).

Impairment of the passive restraint system ad-versely affects joint mechanics (18,19). In the loadedstate, joint line opening or femoral condylar lift-off isresisted by the ligaments, capsule, and tibiofemoralcontact forces (11). Muscle forces contribute greatly tothese contact forces (11). Coordinated muscle activitymay or may not be able to compensate for diseasedand/or malfunctioning passive restraints (12,17). Oncethe condylar lift-off moment overcomes the tibiofemoralcontact forces, dynamic varus or valgus angulation of thetibia occurs (11). It is likely that, at some level of laxity,muscle activity can no longer prevent condylar lift-off,and that strength makes a smaller contribution to func-tion. How an impaired passive restraint system affectsthe relationship between strength and patient-centeredfunction has not been previously explored.

Varus-valgus laxity at the knee is a manifestationof an impairment in the passive restraint system. Thisimpairment may be a result of individual or combinedpathologies. For the distinction between impairmentand pathology, we have relied on the report by Ver-brugge and Jette that describes the disablement process(20). They defined “pathology” as encompassing struc-tural abnormality and “impairment” as a dysfunctionthat may occur in the primary locale of the pathology,but may also occur in secondary locales either immedi-ately or after some delay.

Although strength is very likely to contribute tofunctional status to some degree in all patients with kneeOA, including those with lax knees, knee laxity may limitthe contribution made by muscle strength. If knee laxityalters the relationship between strength and function inpatients with knee OA, this would have implications forthe expectations regarding the outcome of muscle-strengthening intervention, a major component of theusual treatment program for knee OA (21). Also, a

revised therapeutic approach for those patients with OAand lax knees may result in better outcomes.

The purpose of this study was to evaluatewhether, in patients with knee OA, the association ofstrength and functional status is weaker in the presenceof greater varus-valgus laxity.

PATIENTS AND METHODS

Patients. One hundred sixty-four patients were en-rolled in the baseline phase of a longitudinal study examiningthe contribution of specific mechanical, neural, and muscularfactors to radiographic progression and functional decline inknee OA. Patients were recruited from the community throughadvertisements in 67 neighborhood organizations and seniorcenters, local newspapers and magazines, press releases, lettersto subjects in the Aging Research Registry of the BuehlerCenter on Aging of Northwestern University, and referralsfrom local physicians.

Inclusion and exclusion criteria were based on consen-sus recommendations (22,23). Inclusion criteria were definiteosteophyte presence in the medial and/or lateral tibiofemoralcompartment and at least “a little” difficulty with knee-requiring activities. Exclusion criteria were intraarticular cor-ticosteroid injection into either knee within the previous 3months, uncomplicated knee surgery within the previous 6months, complicated knee surgery within the previous year,bilateral total knee replacement, history of avascular necrosis,rheumatoid arthritis or any other systemic inflammatory ar-thropathy, periarticular fracture, Paget’s disease, villonodularsynovitis, joint infection, ochronosis, neuropathic arthropathy,acromegaly, hemochromatosis, Wilson’s disease, osteochon-dromatosis, gout or recurrent pseudogout, or osteopetrosis.

Measurement of strength. Strength was tested using acomputer-driven isokinetic dynamometer (Cybex 340 system;Cybex Medical, Hauppauge, NY). Quadriceps and hamstringstrength were measured isometrically to assess maximumtorque-generating capacity, and isokinetically at 120°/second toassess maximal torque during movement. Because previousstudies on knee OA have emphasized the relationship withquadriceps strength, less is known about the relationshipbetween hamstring strength and function. The hamstrings arebelieved to play an important role in joint stabilization (24).Both isometric and isokinetic strength were measured becauseit remains unclear which is more predictive of physical func-tioning in knee OA. Since most daily knee-requiring activitiesare dynamic and not static, dynamic muscle performance maybe more closely linked to lower extremity function.

A single tester assessed all patients according to adetailed protocol. Patients were seated on a Cybex bench withthe hip in neutral rotation, neutral abduction/adduction, and80° of flexion. Patients were stabilized through the use of chest,pelvis, and thigh straps. The ankle pad of the dynamometerwas placed 2 cm proximal to the medial malleolus to allowankle dorsiflexion during the tests (25). The mechanical axis ofthe dynamometer was aligned with the approximate axis of theknee through the lateral epicondyle of the femur. Subjectsplaced their hands on the sides of the bench. Right-left orderof testing was alternated between patients. The tester verbally

26 SHARMA ET AL

encouraged the patients to achieve maximal torques (26). Thecomputer recorded the data in ft-lbs and corrected the data forgravity effects. The mean value of test trials was used foranalysis (25).

For the isometric measurements, the knee was posi-tioned in 60° of flexion for extension tests and 45° of flexion forflexion tests (27), and measurements were determined bygravity goniometry. Flexor-extensor order of testing was ran-domized. Patients were instructed regarding the procedure andallowed to undergo 1 submaximal and 1 maximal trial for 5seconds each. Subjects then rested for 30 seconds. The actual testconsisted of 2 sets of 2 maximal contractions for 5 seconds each.

During isokinetic testing at 120°/second, range of mo-tion was limited to 0–90° for joint protection. If contracturesprevented this much motion, range of motion limits were set tostop motion 5° before the end of available extension andflexion ranges. Following instruction, patients were given 4warmup repetitions, beginning with submaximal contractionsand building to maximal contractions. Following a 30-secondrest, patients performed 3 maximal test repetitions.

The equipment was calibrated weekly. To determinethe reliability of the strength data, the actual test repetitionsfor each type of test, each muscle group, and each leg wereanalyzed using analysis of variance (ANOVA) with repeatedmeasures and the intraclass correlation coefficient (3,k)(ICC[3,k]) because the mean of measurements was used inanalyses. Reliability coefficients were high, all exceeding 0.98.

Measurement of varus-valgus rotation. Devices tomeasure knee laxity in the frontal plane are not available.Computerized systems (such as the Genucom) are no longer inproduction. Varus-valgus rotation is most commonly assessedby physical examination. However, the reliability of physicalexamination estimates of varus-valgus laxity is poor in kneeOA (28,29); within-observer agreement has been found to be0.55 (28). We therefore designed a simple measurementsystem that addresses the major sources of variation during thephysical examination, i.e., inadequate immobilization of thedistal thigh and ankle, incomplete muscle relaxation, variationof the knee flexion angle, variation of load applied during thetest, and crude means of measuring varus-valgus rotation withload application (28–30).

The system consisted of a bench on which the subjectsat and an attached arc-shaped, low-friction track. The trackwas 30 cm in radius measured from the center of the knee andran medially or laterally. The foot or distal shank (30 cm fromthe knee) was firmly attached to a sled, which traveled withinthe track. A hand-held dynamometer was fitted into either sideof the sled and was used to apply a fixed load. The subject wasin a seated position, with the thigh and ankle immobilized andthe study knee maintained at 20° flexion (11). Load wasapplied by the examiner and an auditory signal indicated whena load of 40N (12 Nm) was reached (Figure 1). Laxity wasmeasured as the angular deviation at the sled after applicationof varus and valgus load. Because accurate identification of theneutral point is typically not possible, total varus-valgus rota-tion was examined, defined as the sum of varus and valgusrotation for each knee, as previously described (13,14,31).

All measurements of varus-valgus rotation were per-formed by the same examiner (LS) (applying load) and thesame assistant (GK-M) (measuring angular deviation at thedefined load), both of whom adhered to a protocol that

included use of anatomic landmarks for patient positioning,patient instructions, and examiners’ positions. To assess reli-ability, 12 subjects with knee OA underwent repeat measure-ment 2 weeks after their study evaluation. Reliability of totalvarus-valgus rotation measurement was determined for eachknee both within the single 4-trial session and between ses-sions. Reliability was calculated using ANOVA with repeatedmeasurements and ICCs. For within-session reliability, theICC(3,4) was used, because the mean of sessions was to be usedin future analyses. Between-session reliability was determinedusing the mean of the 4 measurements each day and the ICC(3,1).The within-session coefficients ranged from 0.85 to 0.96. Thebetween-session coefficients ranged from 0.84 to 0.90.

Measurement of functional status. Functional statuswas assessed in 2 ways, by using the Likert version of the WesternOntario and McMaster Universities OA Index Physical Functionsubscale (WOMAC-PF) (higher score indicates worse function,possible range 0–68) and the chair-stand performance (timerequired for 5 repetitions of rising from a chair and sitting down,using the protocol of Guralnik et al [32,33]). The WOMAC-PF(34,35) is a disease-specific (i.e., for knee or hip OA) self-administered questionnaire with 17 questions comprising a phys-ical function scale, and is widely recommended for use in studiesinvolving patients with knee OA (22,23). The sit-to-stand transferis closely linked to knee status (36). Among the hip, knee, andankle, the knee often exhibits the greatest peak torques duringthis task (37–39). The chair-stand test in conjunction with 2 othermeasures has been used to predict subsequent disability insubjects over age 70 (32).

Evaluators of each category (strength, laxity, and func-tional status) were blinded to the results from the other 2categories.

Radiographic measurements. A detailed protocol (40)for radiographic evaluation of the knees was followed, includ-ing joint position, criteria for x-ray beam alignment relative tothe center of the knee, use of radiopaque markers to account

Figure 1. Direction of force application and the resulting motion atthe knee in the determination of varus-valgus laxity in the right lowerextremity.

LAXITY, STRENGTH, AND FUNCTION IN KNEE OA 27

for radiographic magnification, and definition of anatomiclandmarks for measurement. The anteroposterior view wasobtained from knees in a weight-bearing, semiflexed positionto achieve superimposition of the anterior and posterior jointmargins. The heel was fixed and the foot rotated until the tibialspines were central relative to the femoral notch. Knee posi-tion was confirmed by fluoroscopy. All radiographs wereobtained in the same unit by 1 of 2 trained technicians. Oneexperienced reader (LS) made radiographic measurementsand assessments using an atlas for radiographic OA (41).Global tibiofemoral radiographic severity was scored using theKellgren/Lawrence (K/L) grading system (42).

Other measurements. Leg dominance was ascertainedby asking the patients the following: “In order to kick a ball,which leg would you use?” Pain was recorded as average painin the last week, on separate 0–100-mm visual analog scales foreach knee. Body mass index was measured as the weight (in kg)divided by the height (in m2).

Statistical analysis. Chair-stand performance datawere analyzed as a rate, expressed as the number of completedtests per minute, based on the time required to complete asingle test of 5 repetitions. A rate of 0 reflected inability toperform this test. Since functional status (i.e., WOMAC-PFscore and chair-stand performance) was specific to person, knee-or limb-specific data (i.e., strength and laxity) were summedacross right and left knees for analyses involving functional status.

Univariate correlations between measures of quadri-ceps strength (isokinetic and isometric) and function (chair-stand performance and WOMAC-PF score), correlations be-tween hamstring strength (isokinetic and isometric) andfunction, and correlations between varus-valgus laxity andfunction were calculated with Pearson correlation coefficients.The correlation between strength and function was thenexamined in the high laxity (those with values above thesample median) and low laxity (values less than or equal to thesample median) subsets of patients. Multiple regression wasused to calculate the partial correlation (partial r) betweenstrength and function in laxity subsets after controlling for age,sex, body mass index, and pain.

RESULTS

Characteristics of the study sample are listed inTable 1, including descriptive statistics for age, bodymass index, pain, isometric and isokinetic quadricepsand hamstring strength, varus-valgus rotation,WOMAC-PF score, and chair-stand performance. Thesample included 118 women and 46 men. Only 39patients had unilateral knee OA. In 76 patients, thehighest K/L grade between the 2 knees was 2, in 53 thehighest grade was 3, and in 35 the highest grade was 4. Itwas determined that 91% of the sample was right-handed and 89% were right-leg dominant. Applyingeither definition of dominance, we found that strength(isometric or isokinetic quadriceps and isometric orisokinetic hamstring) did not differ between the domi-nant and nondominant limbs, even after controlling forseverity of knee OA (by K/L grade).

Moderately strong correlations were found be-tween chair-stand rate and isokinetic quadriceps strength(r 5 0.52, 95% confidence interval [95% CI] 0.39, 0.62) andisometric quadriceps strength (r 5 0.50, 95% CI 0.37, 0.60).Weaker correlations were found between theWOMAC-PF score and quadriceps strength measuredisokinetically (r 5 20.32, 95% CI 20.45, 20.18) andisometrically (r 5 20.25, 95% CI 20.39, 20.10).

Similarly, chair-stand performance correlatedwith hamstring strength measured isokinetically (r 50.52, 95% CI 0.40, 0.63) and isometrically (r 5 0.44, 95%

Table 1. Characteristics of patients with knee osteoarthritis (n 5 164)*

Mean 6 SDor no. Range

Age, years 62.6 6 11.5 33–91Body mass index, kg/m2 31.9 6 7.2 19.3–61.9WOMAC-PF score 23.5 6 13.8 0–58Chair-stand, seconds 15.7 6 5.5 7.7–41.3Pain

Right 36.8 6 26.7 0–95Left 35.5 6 30.4 0–100

Isometric quadriceps strength, ft-lbsRight 67.3 6 36.6 4.5–251.3Left 67.8 6 40.6 10.8–277.0

Isokinetic quadriceps strength, ft-lbsRight 51.8 6 31.7 0–171.7Left 52.2 6 31.1 0–184.0

Isometric hamstring strength, ft-lbsRight 53.2 6 24.9 11.0–167.5Left 55.7 6 24.8 13.5–163.3

Isokinetic hamstring strength, ft-lbsRight 30.7 6 19.7 0–109.3Left 30.9 6 19.7 0–101.7

Varus-valgus rotation, degreesRight 5.1 6 1.9 1.8–11.0Left 4.6 6 1.8 1.5–10.3

K/L grade, no. of kneesRight

Grade 0 7Grade 1 12Grade 2 78Grade 3 42Grade 4 25

LeftGrade 0 1Grade 1 19Grade 2 76Grade 3 42Grade 4 26

* Except where otherwise indicated, values are the mean 6 SD. TheWestern Ontario and McMaster Universities Osteoarthritis IndexPhysical Functioning scale (WOMAC-PF) has a possible range of0–68, with a higher score indicating worse function. Chair-stand is thetime required for 5 repetitions of rising from a chair and sitting down(19 patients were unable to perform this test). Pain was measured asaverage pain in the last week, on separate 0–100-mm visual analogscales for each knee. Strength was measured using an isokineticdynamometer as described in Patients and Methods. Varus-valgusrotation is the sum of varus and valgus rotation as described in Patientsand Methods. The Kellgren/Lawrence (K/L) radiographic gradingsystem is described in ref. 42.

28 SHARMA ET AL

CI 0.31, 0.56). The WOMAC-PF score also correlatedwith isokinetic hamstring strength (r 5 20.36, 95% CI20.49, 20.23), but weakly with isometric hamstringstrength (r 5 20.21, 95% CI 20.36, 20.06).

The median of the summed right and left kneevarus-valgus rotation was 9.25°. This median value wasused to define the cutoff value for patients in the highlaxity group (summed right and left knee varus-valgusrotation .9.25°, n 5 80 subjects) and those in the lowlaxity group (summed right and left knee varus-valgusrotation #9.25°, n 5 84 subjects).

The low laxity group had a better WOMAC-PFscore than did the high laxity group (mean 6 SD 20.8 613.8 versus 26.5 6 13.3; 95% CI for difference 29.9,21.5). The chair-stand rate was slower in the high laxitygroup compared with the low laxity group, but thedifference was not significant (3.50 6 1.8 versus 3.97 61.8). As a continuous variable, greater varus-valgus laxity

was modestly associated with a worse WOMAC-PFscore (r 5 0.20, 95% CI 0.05, 0.34), but not with chair-standperformance (r 5 20.15, 95% CI 20.30, 0.003).

The correlation between strength and physicalfunctioning was examined within laxity subsets, as sum-marized in Table 2. In those patients with greatervarus-valgus laxity, the correlation between quadricepsor hamstring strength and either WOMAC-PF score orchair-stand rate was weaker than the correlation de-tected in those with more stable knees.

We then analyzed the relationship betweenstrength and physical functioning in laxity subsets aftercontrolling for age, sex, body mass index, and pain. Asshown in Table 3, the partial correlation between strengthand physical functioning measured by either WOMAC-PFscore or chair-stand rate was consistently smaller in thosewith greater laxity. In these models, pain, strength, andbody mass index predicted functional status.

Table 2. Correlation between strength and functional status in laxity subsets*

Patients in low laxity group (n 5 84) Patients in high laxity group (n 5 80)

r 95% CI r 95% CI

Isometric quadriceps strength/WOMAC-PF† 20.22 20.41, 20.006 20.17 20.38, 0.05Isokinetic quadriceps strength/WOMAC-PF† 20.31 20.49, 20.10 20.23 20.43, 20.01Isometric quadriceps strength/chair-stand rate‡ 0.55 0.38, 0.68 0.42 0.22, 0.59Isokinetic quadriceps strength/chair-stand rate‡ 0.58 0.42, 0.71 0.42 0.22, 0.59Isometric hamstring strength/WOMAC-PF† 20.19 20.39, 0.03 20.09 20.30, 0.13Isokinetic hamstring strength/WOMAC-PF† 20.38 20.55, 20.18 20.25 20.45, 20.03Isometric hamstring strength/chair-stand rate‡ 0.50 0.32, 0.64 0.34 0.13, 0.52Isokinetic hamstring strength/chair-stand rate‡ 0.58 0.42, 0.71 0.43 0.23, 0.59

* Laxity subsets were defined by the sample median of 9.25°. “Low laxity” patients had summed right and left knee varus-valgus rotation #9.25°.“High laxity” patients had summed right and left knee varus-valgus rotation .9.25°. 95% CI 5 95% confidence interval.† Correlation is between strength (higher value indicates stronger) and Western Ontario and McMaster Universities Osteoarthritis Index PhysicalFunctioning scale (WOMAC-PF) (higher score indicates worse function).‡ Correlation is between strength (higher value indicates stronger) and chair-stand rate (higher value indicates better performance).

Table 3. Correlation between strength and functional status in laxity subsets, after controlling for age, sex, body mass index, and pain*

Patients in low laxity group (n 5 84) Patients in high laxity group (n 5 80)

r 95% CI r 95% CI

Isometric quadriceps strength/WOMAC-PF† 20.15 20.36, 0.07 20.03 20.25, 0.20Isokinetic quadriceps strength/WOMAC-PF† 20.27 20.46, 20.05 20.19 20.40, 0.04Isometric quadriceps strength/chair-stand rate‡ 0.39 0.19, 0.56 0.22 20.006, 0.42Isokinetic quadriceps strength/chair-stand rate‡ 0.42 0.22, 0.59 0.27 0.05, 0.47Isometric hamstring strength/WOMAC-PF† 20.19 20.39, 0.03 20.05 20.27, 0.18Isokinetic hamstring strength/WOMAC-PF† 20.30 20.49, 20.09 20.21 20.42, 0.02Isometric hamstring strength/chair-stand rate‡ 0.35 0.14, 0.53 0.16 20.07, 0.37Isokinetic hamstring strength/chair-stand rate‡ 0.42 0.22, 0.59 0.31 0.09, 0.50

* Laxity subsets were defined by the sample median of 9.25°. “Low laxity” patients had summed right and left knee varus-valgus rotation #9.25°.“High laxity” patients had summed right and left knee varus-valgus rotation .9.25°. 95% CI 5 95% confidence interval.† Partial correlation is between strength (higher value indicates stronger) and Western Ontario and McMaster Universities Osteoarthritis IndexPhysical Functioning scale (WOMAC-PF) (higher score indicates worse function).‡ Partial correlation is between strength (higher value indicates stronger) and chair-stand rate (higher value indicates better performance).

LAXITY, STRENGTH, AND FUNCTION IN KNEE OA 29

DISCUSSION

Our results suggest that the presence of increasedvarus-valgus rotation in patients with knee OA is asso-ciated with a diminution in the magnitude of the rela-tionship between strength and physical functioning.Strength was more closely associated with chair-standperformance than with the WOMAC-PF score.

Greater laxity was consistently associated with aweaker relationship between strength and physical func-tioning, regardless of whether the performance-based orthe self-report approach was used, in multiple regressionanalyses that controlled for age, sex, body mass index,and pain. In the multivariate analyses involving the highlaxity subset, 6 of 8 confidence intervals included 0,indicating that a partial correlation of 0 is plausible formost of these relationships. In analyses involving thelower laxity subset, only 2 of 8 confidence intervalsincluded 0. These findings support the presence of aphenomenon that has important implications.

The results suggest that, in the presence of agiven level of varus-valgus laxity, muscle status makes asmaller contribution to ease of task performance than itdoes in patients with more stable knees. This may relateto an impairment in the passive restraint system that issufficiently severe that muscle activity (even by strongmuscle) cannot dynamically prevent joint-line separation(11,12). These results raise the possibility that musclestrengthening may have a smaller impact in lax knees,and that attending to muscle without considering thestatus of passive restraints may result in less-than-optimal outcomes. Alternatively, attending to the statusof the passive restraints (using, for example, orthotics)may improve the results of strengthening programs.Little is known about how to treat varus-valgus laxity inknee OA. The results of the present study suggest theneed for future studies to develop specific interventions.From a more general perspective, muscle activity andeffort may or may not compensate for joint pathology.To enhance the results of muscle-strengthening inter-ventions, the impact of the structural abnormality shouldbe considered. These hypotheses should be tested intherapeutic trials.

Lax knees may warrant a tailored approach, notonly because of the impact of laxity on the functionalstatus outcome of strengthening interventions, but alsobecause, in the setting of laxity, muscle strengtheningmay accelerate disease progression. Higher muscleforces during gait have been demonstrated in patientswith knee OA with varus malalignment and laxity (12).These forces are believed to be compensatory, but mayalso result in higher compressive forces across the joint

(12). Strengthening the lax knee is not without risk;localized areas of articular cartilage may be repetitivelyloaded, resulting in disease progression. This possibilityhas implications for ongoing studies examining the struc-tural effects of strengthening interventions in knee OA.

Stratification of analyses by disease subsets hasbeen encouraged in studies of patients with OA (23).However, the factors that might be used to determinesubsets are often not clear. The results reported hereinsuggest that the impact of strengthening interventionson functional status in knee OA should be examined inlaxity subsets. Stratification by mechanical factor subsetsmay also be useful in studies of disease progression.Theoretically, muscle strengthening may well slow therate of progression in some mechanical factor subsets bypromoting a load attenuation mechanism, and acceler-ate it in other subsets by increasing load focally. Bothfunctional and structural outcomes may be most im-proved by rehabilitative approaches that are designedspecifically to address the mechanical impairmentspresent in an individual patient. It seems unlikely thatthe same approach will have the same benefit in allpatients with knee OA.

A limitation of this study is that it was notpopulation-based. These data were obtained during thebaseline evaluation phase of a longitudinal study involv-ing a cohort of diseased patients. Subjects were recruitedfrom several sources in the community to maximize thegeneralizability of the results and efficiently target asufficient number of patients with knee OA to allowanalyses in laxity subsets and to be sure of reasonabledistributions of strength, laxity, pain, and functionalstatus. It is also important to acknowledge that this studywas cross-sectional and hypothesis-generating. The im-pact of laxity upon the strength–function relationship inknee OA should be examined in longitudinal studies.

This is the first report of the relationship betweenlaxity and physical functioning in knee OA. Previousinvestigation into laxity in knee OA has emphasizedcomparisons of laxity measurements between OA pa-tients and controls (13,14). The paucity of informationrelating to varus-valgus laxity in knee OA relates, inpart, to the difficulty in obtaining reliable measure-ments. A system was described herein that addressedmajor sources of variation in the physical examinationmaneuver, resulting in substantial improvements in thequality of the measurements.

The purpose of this study was to examine theimpact of an impaired passive restraint system on thestrength–function relationship in knee OA, a questionthat has not been examined previously. Neither thepresent nor previous investigations involving laxity in

30 SHARMA ET AL

knee OA have subcategorized patients by possible causeof laxity. At present, no systems exist on which to basesubject categorization in this regard. Development of aclassification scheme is worthy of future study. Sincedevelopment of varus-valgus laxity in an individual withknee OA is likely to be multifactorial, such a schememust consider that many patients with knee OA may nothave a single, dominant cause.

The findings reported in the present study areconsistent with those of other studies in which quadri-ceps strength has been correlated with physical function-ing in knee OA. McAlindon et al (5) found that isomet-ric quadriceps strength contributed to the HealthAssessment Questionnaire score in patients with kneeOA, in analyses adjusted for other factors includingpain. Rejeski et al (6) reported a correlation betweenisokinetic quadriceps strength and stair-climb time (r 520.53) and between quadriceps strength and lift-carrytime (r 5 20.54). Consistent with our results, thecorrelation between strength and self-reported physicalability was weaker than the correlation between strengthand timed performance. Fisher et al (9) did, however,find a strong correlation (r 5 0.65) between quadricepsstrength and self-reported functional status in patientswith knee OA; collectively, quadriceps strength, endur-ance, and speed accounted for 75% of the variance ofthe Jette Functional Status Index score. Lankhorst et al(7) found that isometric and isokinetic torque measuresexplained between 23% and 35% of the variation offunctional capacity assessed using a nonvalidated nu-merical rating system. Van Baar et al (8) found thatstrength correlated with task performance and self-reported disability, in univariate and multivariate analy-ses. The study reported herein utilized both chair-standperformance and a disease-specific self-report instru-ment to assess functional status, controlled for pain andother factors, and examined the impact of varus-valguslaxity.

Given the results of these previous studies andthe critical role played by muscle in joint function, it issurprising that muscle-strengthening interventions donot have more dramatic effects on functional status, andappear to provide little more benefit than aerobic pro-grams. Ettinger et al (10) found that older patients withknee OA who underwent a resistance exercise programhad only 8% lower scores on a physical disability ques-tionnaire, 8% lower pain scores, 4% improvement in6-minute walk distance, 8% improvement in time for alifting and carrying task, and 18% improvement in timefor getting in and out of a car compared with a groupreceiving only health education. Improvements after anaerobic exercise program were slightly better. The mod-

est benefits of the resistance program may have beenrelated to its intentionally gentle nature. Nonetheless, inan uncontrolled, smaller study utilizing a more aggres-sive and progressive strengthening program, dependencywas reduced by only 10% and walk time by 12%,although a 30% reduction in difficulty with activities ofdaily living was detected (9).

The apparent lack of difference between theoutcomes of aerobic and resistance exercise (10) sug-gests that aerobic capacity is at least as important adeterminant of disability in knee OA as muscle strength,and/or that other factors influence the relationshipbetween strength and function. The findings in thepresent study support the notion that varus-valgus laxitymay represent one such factor. Given the paucity ofpublished information on modifiers of the strength–function relationship in knee OA, it is difficult to drawconclusions about the relative importance of laxity as amodifier in comparison with other factors. The searchfor factors moderating the relationship between strengthand function should not stop with laxity. Other aspectsof pathologic anatomy or physiology that define themechanical environment of the knee as well as psycho-social factors (key determinants of physical functioningin cross-sectional studies) are likely to modify the effectof strengthening interventions and should be investi-gated.

In conclusion, the presence of varus-valgus laxityis associated with a decrease in the magnitude of therelationship between quadriceps or hamstring strengthand physical functioning, both self-reported and ob-served. This has implications regarding expectations formuscle-strengthening interventions. Studies examiningthe long-term functional and structural consequences ofexercise programs in knee OA should consider stratify-ing analyses by specific mechanical factor subsets. Elu-cidation of the mechanisms by which this disease leads todisability will require exploration of the relationshipsbetween specific elements of disease and functionalstatus.

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