Isometric Muscle Force Measurement for Clinicians Treating Patients With Osteoarthritis of the Knee

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1 Arthritis & Rheumatism (Arthritis Care & Research) Vol. 49, No. 1, February 15, 2003, pp DOI /art , American College of Rheumatology ORIGINAL ARTICLE Isometric Muscle Force Measurement for Clinicians Treating Patients With Osteoarthritis of the Knee MARLENE FRANSEN, 1 JACK CROSBIE, 2 AND JOHN EDMONDS 1 Objective. To evaluate the discriminant validity and, for clinicians, the test retest reliability of isometric force in patients with knee osteoarthritis (). Methods. Mean isometric muscle strength data collected for 113 patients with knee were compared with published normative data for 131 asymptomatic subjects. Results. Patients with knee, ages years, could attain only 40 53% of the knee extensor force and 35 46% of the knee flexor force generated by their age- and sex-matched peers. One-week test retest intraclass correlations ranged from 0.79 to Random measurement error demonstrated a favorable signal-to-noise ratio. Conclusion. The similar decrease in knee extensor and knee flexor strength demonstrated by patients with knee compared with their age- and sex-matched asymptomatic peers suggests that strategies should be directed at both muscle groups to optimally limit joint damage. Isometric muscle force measurement can be a useful assessment tool for clinicians. KEY WORD. Knee osteoarthritis isometric force reliability. INTRODUCTION Muscle strengthening, particularly for the knee extensors, has been consistently advocated as a treatment for individuals with symptomatic osteoarthritis () of the knee. Adequate knee extensor strength is essential during ambulation to attenuate the heel strike transient, thus reducing potentially damaging impulsive loading of the knee joint (1). A recent prospective study demonstrated that reduced knee extensor strength, adjusted for body weight, significantly increased the risk of incident knee (2). However, poor load distribution across the knee joint, as evidenced by increased adduction moments during stance, has also demonstrated an association with increased radiologic and symptomatic disease progression (3). Apart from general weight loss or high Dr. Fransen s work was supported by a National Health and Medical Research Council of Australia postgraduate scholarship. 1 Marlene Fransen, PhD, MPH, PT (current address: University of Sydney, New South Wales, Australia), John Edmonds, MB, BS, FRACP: St. George Hospital, Kogarah, New South Wales, Australia; 2 Jack Crosbie, PhD, PT: University of Sydney, New South Wales, Australia. Address correspondence to Marlene Fransen, PhD, MPH, PT, Senior Research Fellow, Institute for International Health, PO Box 576, Newtown, New South Wales 2042, Australia. mfransen@iih.usyd.edu.au. Submitted for publication October 17, 2001; accepted in revised form April 21, tibial osteotomy, only adequate knee flexor strength has the potential to oppose both destructive adduction moments (4) and anterior shear forces operating on the knee joint during weight-bearing activities (5). Lower limb muscle strength decreases with age, the decline generally beginning in the fifth decade (6 9). Reduced lower limb muscle strength is particularly marked in people with symptomatic knee (10,11). More importantly, several cross-sectional studies have established that reduced knee extensor strength in people with knee is strongly associated with increased pain and poor physical function (10 12). In fact, threshold values below which knee extensor strength is critical for independent walking, standing balance, and sit stand function have been demonstrated (13 16). Knee flexor strength has received considerably less attention in the literature. Although it appears that knee extensor strength losses precede those of the knee flexors (10), several studies have suggested that both knee flexor and knee extensor strength are lost with established symptomatic disease (17,18). The general assumption seems to be that knee flexor force losses are attributable to diseaserelated disuse atrophy and are reversed once the knee extensors are strengthened and the patient becomes more mobile. The clinician s access to fairly simple electronic devices measuring isometric muscle force is becoming easier. Isometric muscle testing potentially provides the clinician 29

2 30 Fransen et al with an objective assessment tool that will be able to register change in people with knee who are not yet reporting, for various reasons, physical function limitations (19). Measurement reliability is often reported only in terms of relative association or correlation. Although correlation coefficients provide some indication of relative association between 2 measurements and is useful for large research trials, a clinician needs an absolute measure to confidently demonstrate that a change has occurred that could not be explained by measurement error. Although the intrasession reliability of isometric muscle force measurement has been reported as mostly adequate (20), test retest assessment over more clinically relevant periods has been reported as involving too much random measurement error to be useful for clinicians evaluating progression in individual patients (21,22). However, the significance of any absolute measure of random measurement error is relative to the magnitude of change possible (signal-to-noise ratio). While mostly small studies have frequently documented loss of lower limb muscle force in people with knee, there is a lack of normative data, meaningfully stratified by age and sex, to evaluate the magnitude of this difference. Random measurement error needs to be considerably smaller than the magnitude of real change possible if isometric muscle force testing is to be a useful assessment tool for the clinician. The first aim of the current study was to establish discriminant validity by quantifying the difference in isometric knee extensor and knee flexor force between patients with knee and age- and sex-matched asymptomatic individuals. The second aim of this study was to quantify, in absolute terms for the clinician, 1-week test retest reliability for isometric knee extensor and knee flexor force in patients with knee. METHODS The patient sample consisted of community-dwelling individuals who were referred for outpatient physical therapy and had given informed consent to participate in a randomized clinical intervention study (23). Patients were invited to participate if they were age 50 years and older, had knee pain on most days of the past month, and had evidence of radiographic tibiofemoral joint disease. Patients were excluded if they had received intraarticular cortisone injections within the past 2 months, or had lower limb joint arthroplasty, unstable cardiac comorbidity precluding exercise at 50 60% maximum heart rate, or comorbidity affecting gait. To help define the sample by radiographic disease status, patients with knee were required to obtain a weight-bearing, magnification-controlled, semiflexed radiograph of their most painful knee using a validated protocol for assessment of medial joint space width (JSW) (24). This study was approved by the South-Eastern Sydney Area Health Service Ethics Committee. Knee extensor and knee flexor isometric muscle force were evaluated on 2 occasions separated by 1 week, at approximately the same time of day and with the same postanalgesia interval, if applicable. At each testing occasion, patients also evaluated current pain and physical function using the Western Ontario and McMaster Universities Arthritis Index (WOMAC) questionnaire with 100-mm visual analog scales (25). Bilateral isometric knee extensor and knee flexor muscle force were assessed using a load cell (XTran Model S1W; Applied Measurement, Victoria, Australia) fixed onto the metal framework of a chair and connected to a simple software program sampling at 80 Hz. The XTran load cell can generate a maximum of 450N ( 30% permitted overload) and has a repeatability of 99.98%. The unit s calibration stability, before and after the 18 months of data collection, was confirmed by production of identical regression equations. During assessment patients were seated on the chair with the back and thigh well supported, hands resting on the thighs, the foot free, and the knee passively drawn into 90 o flexion by gravity. A soft cuff, attached via an adjustable nonelastic metal cord to the load cell, was fitted with Velcro just above the ankle. Isometric make tests were used as patients were asked to build up their force and then to push or pull as hard as they could for 5 seconds. Both muscle groups were tested 3 times on each limb in a standardized sequence as follows: left extensors, left flexors, right extensors, right flexors. data on asymptomatic age- and sex-matched peers were extrapolated from data published by Andrews et al (6). This reference data set was chosen as the assessment procedure: an isometric 5-second make test carried out with the participant seated and the knee drawn passively into 90 o flexion was identical to that used in the current study with patients with knee. Furthermore, the scores compared were the mean peak force of the first 2 trials obtained at an initial assessment occasion. One-week test retest reliability was assessed using intraclass correlation coefficients (ICC 2,1) with their corresponding 95% confidence intervals. To assess the absolute amount of random measurement error for which a clinician would need to account within a clinically relevant 68% confidence interval, the standard error of the mean (SEM) was calculated from the standard deviation of the change scores and the intersession ICC (SD. [1-ICC]) (26). To assess the smallest detectable difference needed by the clinician to be confident that the change detected was real and beyond possible measurement error, this SEM was increased by a factor 2 to account for the error involved in each of the 2 scores producing the change score (27). Data were analyzed using SAS Proprietary Software Release 6.12 (SAS Institute, Cary, NC) and ICC.EXE (public domain software available from Dr. Roger Adams, Faculty of Health Sciences, University of Sydney, PO Box 170, Lidcombe 1825 Australia). RESULTS Data were collected on 113 patients. Because reported symptom duration and medial JSW were not normally distributed, these data were dichotomized. A cutoff of 2 mm was chosen because 2 mm medial JSW is considered to indicate advanced disease (28). The sample and the normative reference group were comparable for age, but

3 Muscle Force Measurement in Knee Patients 31 the sample had a higher body weight in each stratified group (Table 1). A large proportion of this sample of referred patients with knee reported symptom duration of 5 years or longer and advanced radiographic disease. The most painful limb of the patients with knee (75% had bilateral symptoms) is compared with the mean of the dominant and nondominant limbs of the reference values (Table 2). Mean knee extensor and flexor isometric force are presented both in absolute terms (Newtons) and as a percentage of body weight. For both the patient and the normative sample, absolute isometric knee extensor and flexor muscle force decreased with advancing age. Interestingly, muscle force expressed as a percentage of body weight did not show this consistent trend, being offset by decreasing body weight with advancing age. Within each age decade and for both males and females, patients with knee could attain only a significantly lower 46 54% of the knee extensor or knee flexor force demonstrated by the reference sample (all P 0.01). Because patients with knee had a higher mean body weight, muscle force was also expressed as a percentage of body weight to provide a possibly more comparable indicator of reduced potential capacity to absorb usual knee joint loading. Patients with knee in the age range years could attain only 40 54% of the knee extensor force generated by age- and sex-matched control subjects (Table 2). Similarly, this same group of patients could attain only 35 46% of the knee flexor force generated by age- and sex-matched controls. Among the patients ages years with knee, 62% were more than 2 standard deviations below reference data for knee extensor force and 74% were more than 2 standard deviations below the reference data for knee flexor force. Generally among patients with knee, females reported higher level of knee pain and physical disability compared with males (Table 3), although a larger proportion of males demonstrated advanced radiographic disease (medial JSW 2 mm) (Table 1). For patients with knee, paired t-tests could not identify any significant systematic changes in self-reported pain or physical function over 1 week (P 0.05) (Table 3). For isometric muscle force among patients with knee, mean 1-week test retest ICC 1,2 values were mostly high and ranged from 0.79 to 0.95 (Tables 4 and 5). The amount of random measurement error, as defined by the smallest detectable difference at the clinically acceptable 68% confidence level, was small to moderate when considered in relation to either the baseline muscle force or to the range realistically available as defined by the matched reference values. The smallest detectable differences needed ranged from 6% to 17% above or below baseline values and represented 3 20% of the range available according to the matched reference data. For example, a female (age years) with knee would need to show a change of at least 16N in knee extensor force, using the outlined assessment protocol, for a clinician to be moderately confident that an actual change had occurred. A change of 16.2N probably represents 11% of mean baseline value (142.3N; Table 2) and 10% of the probable range available (296.0N 142.3N; Table 2). Table 1. Baseline characteristics of patients* Men Women years years years years years (n 25) (n 28) (n 29) (n 33) (n 25) (n 24) (n 26) (n 16) (n 26) (n 12) Characteristic Age, years Weight, Newtons % with symptoms for 5 years % with medial JSW 2 mm * Unless otherwise indicated, values are the mean SD. values are derived from Ref. 6. JSW joint space width.

4 32 Fransen et al Table 2. Baseline knee extensor and knee flexor isometric force* Knee extensors Knee flexors Ages (21) (48) (13) (30) Ages (23) (48) (13) (28) Ages (18) (43) (12) (25) Ages (20) (39) (11) (24) Ages (15) (38) (8) (23) * Values are the mean SD Newtons (% body weight). values are derived from Ref. 6. DISCUSSION This study shows that advancing age is associated with decreasing lower limb muscle force in persons with symptomatic. Furthermore, this study also demonstrates clearly that community-dwelling individuals with chronic symptomatic knee have lost considerable muscle force in both knee extension and knee flexion compared with age- and sex-matched normative reference values. In addition, this study shows that when assessing lower limb isometric muscle force in patients with knee, using a fixed load cell and obtaining a mean of 2 measurements on each testing occasion can provide the clinician with a useful objective assessment tool. Interestingly, when isometric muscle force is expressed as a percentage of body weight, the losses with advancing age appear to be minimal (Table 2). The loss of force with aging has been attributed to both a loss of functioning motor neurons as well as a reduced number of fast-twitch muscle fibers (29). Both animal and human studies have demonstrated that loading rate, rather than load magnitude, is the critical factor for cartilage damage in (30). Fast-twitch muscle fibers function to rapidly develop high tension for short periods of time, and, arguably, isometric testing is a poor indicator of the capacity of a muscle to reduce potentially destructive high loading rates. Isometric testing was chosen to allow the patients with knee optimum opportunity to control the loading rate on the knee joint and minimize the influence of pain inhibition during testing. However, it was beyond the stated scope of the current study to attempt to estimate the relative contribution of pain inhibition or loss of muscle fiber function to the reduced force generated. Higher loading rates at heel strike have been demonstrated among persons with reduced isokinetic knee extensor force (31) and those with mild, activity-related, knee pain (32) compared with controls. It is hypothesized that with reduced lower limb muscle force and impaired proprioception for the ordinary activities of daily living (walking, sitting, stair climbing), relatively minor insults may become major ones (33). A limitation of the current study is that although both testing procedures were 5-second make tests obtained at the same knee joint angle, the reference values were obtained with hand-held dynamometers, while the knee patients muscle force was assessed with a fixed load cell. It could, however, be easily argued that this difference in testing apparatus is most likely to result in a conservative estimate of discriminant validity. The muscle force measurable with a hand-held dynamometer is dependent on the upper force limit of the assessor (22). Although it was reported that testers were able to hold steadily against forces up to a maximum of 512N (6), the subjects themselves may have felt inhibited from producing their peak force during the testing procedure. The reduced ratio of knee extensor-to-knee flexor strength (Table 2) evident in the reference data (range ) compared with the knee patient sample (range ) may be a product of this inhibition and indicate that the reported reduction in knee extensor strength in patients with symptomatic knee compared with their age- and sex-matched peers is possibly an underestimate. This major physical Table 3. WOMAC pain and physical function scores in patients with knee * Pain Function n Test Retest Test Retest Ages Ages Ages Ages Ages * Values are the mean SD. WOMAC Western Ontario and McMaster Universities Arthritis Index; osteoarthritis. Mean score of 5 questions; range 0 (no pain) to 100 (extreme pain). Mean score of 17 questions; range 0 (no difficulty) to 100 (extreme difficulty).

5 Muscle Force Measurement in Knee Patients 33 Table 4. Knee extensor force in Newtons* ICC 2,1 (95% CI) Mean SD difference SEM Smallest detectable difference Ages ( ) Ages ( ) Ages ( ) Ages ( ) Ages ( ) * Test-retest reliability (ICC 2,1) and absolute measurement error (68% confidence level [CI]). limitation inherent in hand-held dynamometer testing has been reported by others (20,22) and would support preference to the acquisition of a fixed load cell when frequent use of lower limb muscle testing is anticipated. Greater pain and disability were reported by women with symptomatic knee (Table 3), even though a larger proportion of men had lost more medial JSW and therefore would be considered to demonstrate more severe radiographic tibiofemoral disease (Table 1). A possible explanation may be that a greater proportion of patellofemoral disease occurs in women compared with men (34), or in not incorporating osteophytes in the radiographic grading system (35). However, pain and disability may be more closely associated with the potential of the lower limb muscles to reduce the heel strike transient and to stabilize the knee on weight-bearing compared with actual radiographic damage. The potential of lower limb muscle force to protect the joint is probably best evaluated when force is expressed as a percentage of body weight rather than an absolute value. In fact, for males with knee, although there were expected decreases in absolute muscle force from the sixth to the seventh decade, there were no discernible changes in muscle force expressed as a percentage of body weight, self-reported knee pain, or physical function (Tables 2 and 3). In contrast, women in the seventh decade of life who had knee demonstrated decreased muscle force expressed as a percentage of body weight and reported increased knee pain and more difficulty with physical function tasks when compared with women in the sixth decade (Tables 2 and 3). Examining our data using extensor force adjusted for body height and weight (Newtons/kg.m) as described by Rantanen et al (14) suggests that 50% of this sample would not be able to mount a step 20 cm in height using the affected limb. For men in the sixth and seventh decades of life, the mean extensor force was 1.38 and 1.31 Newtons/kg.m, respectively, while corresponding values for women were 1.24 and 0.96 Newtons/kg.m. Rantanen et al demonstrated that only 20% of people with an isometric extensor force 1.24 Newtons/kg.m are able to ascend a step of 20 cm. In the current sample, 70 people had an extensor force 1.24 Newtons/kg.m, translating to 50% of patients (62 of 113) being unable to ascend a step of 20 cm in height. In fact, 37% of the current patient sample reported extreme difficulty ascending stairs (score 70 on the WOMAC 100-mm VAS). A second study by Rantanen et al (15) demonstrated the existence of a threshold minimum knee extensor force needed to be able to safely cross the street at signaled intersections. The clinical significance of the marked deterioration in knee extensor strength in these patients with knee is therefore, primarily a restriction in independent outdoor mobility with resultant increased vulnerability to loss of independence and institutionalization. This, in consequence, is likely to produce further deconditioning and strength losses. The smallest detectable difference needed over 1 week to account for the random measurement error found in the current study appears to be small to moderate in comparison with the mean baseline values or the amount of change that can realistically be expected. However, previous randomized controlled clinical trials have shown that individual changes in muscle force of this magnitude are rare in this population even with interventions demonstrating clinically significant effects in terms of reduced pain and improved physical function (23,36). Test retest random measurement error may be partly attributed to the typically fluctuating symptomatology of, but the results of the current study are in general agreement with those obtained in 30 younger adults with mostly soft tissue orthopedic knee disorders (22) and 25 healthy women Table 5. Knee flexor force in Newtons* ICC 2,1 (95% CI) Mean SD difference SEM Smallest detectable difference Ages ( ) Ages ( ) Ages ( ) Ages ( ) Ages ( ) * Test-retest reliability (intraclass correlation coefficient [ICC] 2,1) and absolute measurement error (68% confidence level [CI]).

6 34 Fransen et al (21). However, longer periods between assessment sessions and using different assessors at each occasion would involve even greater levels of random measurement error (27). Without knowledge of these caveats to isometric muscle testing and limitations of the presented data, clinicians may convey either unrealistically optimistic or pessimistic treatment results to the patient and possibly hamper therapeutic potential. It has been demonstrated, however, that taking the mean of more than 2 measurements at any one session, disregarding the initial assessment occasion, or taking a series of measurements and monitoring any trend can reduce random measurement error in measures of physical performance (22,37). Incorporating procedures known to reduce random measurement error within a standardized protocol for patients with of the knee will allow lower limb isometric muscle force testing using a fixed load cell to become a useful clinical assessment tool. Community-dwelling individuals with chronic knee have markedly reduced isometric knee extensor and knee flexor force compared with their age- and sex-matched asymptomatic peers. The similar difference in knee extensor and knee flexor muscle force suggests that muscle strengthening strategies should be directed at both muscle groups to optimally limit destructive increased loading rates, adduction moments, and shear stresses on the articular cartilage. The measurement of lower limb isometric muscle force using a fixed load cell can provide a useful objective assessment tool for the clinic. Clinicians, however, need to be aware of the influence of differing isometric muscle force assessment protocols on random measurement error in order to avoid poor clinical decision-making. REFERENCES 1. Jefferson RJ, Collins JJ, Whittle MW, Radin EL, O Connor JJ. The role of the quadriceps in controlling impulsive forces around the heel: proceedings of the Institution of Mechanical Engineers. Part H-J. Engineer Med 1990;204: Slemenda C, Heilman DK, Brandt KD, Katz BP, Mazzuca SA, Braunstein EM, et al. Reduced quadriceps strength relative to body weight: a risk factor for knee osteoarthritis in women? Arthritis Rheum 1998;41: Sharma L, Hurwitz DE, Thonar EJ-M, Sum JA, Lenz ME, Dunlop DD, et al. Knee adduction moment, serum hyaluronan level, and disease severity in medial tibiofemoral osteoarthritis. Arthritis Rheum 1998;41: Schnitzer TJ, Popovich JM, Andersson GB, Andriacchi TP. Effect of piroxicam on gait in patients with osteoarthritis of the knee. Arthritis Rheum 1993;36: Kannus P. Ratio of hamstring to quadriceps femoris muscles strength in the anterior cruciate ligament insufficient knee. Phys Ther 1988;68: Andrews AW, Thomas MW, Bohannon RW. Normative values for isometric muscle force measurements obtained with handheld dynamometers. Phys Ther 1996;76: Skelton DA, Grieg CA, Davies JM, Young A. Strength, power and related functional ability of health people aged years. Age Ageing 1994;23: Murray MP, Duthie EH, Gambert SR, Sepic SB, Mollinger LA. Age-related differences in knee muscle strength in normal women. J Gerontol 1985;40: Murray MP, Gardner GM, Mollinger LA, Sepic SB. Strength of isometric and isokinetic contractions: knee muscles of men aged 20 to 86. Phys Ther 1980;60: Slemenda C, Brandt KD, Heilman DK, Mazzuca S, Braunstein EM, Katz BP, et al. Quadriceps weakness and osteoarthritis of the knee. Ann Intern Med 1997;127: O Reilly SC, Jones A, Muir KR, Doherty M. Quadriceps weakness in knee osteoarthritis: the effect on pain and disability. Ann Rheum Dis 1998;57: Van Baar ME, Dekker J, Lemmens JA, Oostendorp RA, Bijlsma JW. Pain and disability in patients with osteoarthritis of hip or knee: the relationship with articular, kinesiological, and psychological characteristics. J Rheumatol 1998;25: Ferrucci L, Guralnik JM, Buchner D, Kasper J, Lamb SE, Simonsick EM, et al. Departures from linearity in the relationship between measures of muscular strength and physical performance of the lower extremities: the Women s Health and Aging Study. J Gerontol 1997;52A:M Rantanen T, Era P, Heikkinen E. Maximal isometric knee extension strength and stair-mounting ability in 75- and 80- year-old men and women. Scand J Rehabil Med 1996;28: Rantanen T, Guralnik JM, Izmirlian G, Williamson JD, Simonsick EM, Ferrucci L, et al. Association of muscle strength with maximum walking speed in disabled older women. Am J Phys Med Rehabil 1998;77: Buchner DM, Larson EB, Wagner EH, Koepsell TD, de Lateur BJ. Evidence for a non-linear relationship between leg strength and gait speed. Age Ageing 1996;25: Hall KD, Hayes KW, Falconer J. Differential strength decline in patients with osteoarthritis of the knee: revision of a hypothesis. Arthritis Care Res 1993;6: Tan J, Balci N, Sepici V, Gener FA. Isokinetic and isometric strength in osteoarthrosis of the knee. A comparative study with healthy women. Am J Phys Med Rehabil 1995;74: Guralnik JM, Fried LP, Salive ME. Disability as a public health outcome in the aging population. Annu Rev Public Health 1996;17: Hayes KW, Falconer J. Reliability of hand-held dynamometry and its relationship with manual mucle testing in patients with osteoarthritis of the knee. J Orthop Sports Phys Ther 1992; Madsen OR. Torque, total work, power, torque accelertion energy and acceleration time assessed on a dynamometer: reliability of knee and elbow extensor and flexor strength measurements. Eur J Appl Physiol 1996;74: Roebroeck ME, Harlaar J, Lankhorst GJ. Reliability assessment of isometric knee extension measurements with a computerassisted hand-held dynamometer. Arch Phys Med Rehabil 1998;79: Fransen M, Crosbie J, Edmonds J. Physical Therapy is effective for patients with osteoarthritis of the knee: a randomized controlled clinical trial. J Rheumatol 2001;28: Buckland-Wright J, Macfarlane D, Williams S, Ward R. Accuracy and precision of joint space width measurements in standard and macroradiographs of osteoarthritic knees. Ann Rheum Dis 1995;54: Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt L. Validation study of WOMAC: a health status instrument for measuring clinically-important patient-relevant outcomes following total hip or knee arthroplasty in osteoarthritis. J Orthop Rheumatol 1988;1: Hayes KW. The effect of awareness of measurement error on physical therapists confidence in their decisions. Phys Ther 1992;72: Roebroeck ME, Harlaar J, Lankhorst GJ. The application of generalizability theory to reliability assessment: an illustration using isometric force measurements. Phys Ther 1993;73: Buckland-Wright JC, Wolfe F, Ward RJ, Flowers N, Hayne C. Substantial superiority of semiflexed (MTP) views in knee osteoarthritis: a comparative radiographic study, without fluoroscopy, of standing extended, semiflexed (MTP), and Schuss views. J Rheumatol 1999;26: Larsson L, Grimby G, Karlsson J. Muscle strength and speed of movement in relation to age and muscle morphology. J Appl Physiol 1979;46: Radin EL, Burr DB, Caterson B, Fyhrie D, Brown TD, Boyd RD. Mechanical determinants of osteoarthrosis. Semin Arthritis Rheum 1991;21:12 21.

7 Muscle Force Measurement in Knee Patients Mikesky AE, Meyer A, Thompson KL. Relationship between quadriceps strength and rate of loading during gait in women. J Orthop Res 2000;18: Radin EL, Yang KH, Riegger C, Kish VL, O Connor JJ. Relationship between lower limb dynamics and knee joint pain. J Orthop Res 1991;9: Bland JH, Cooper SM. Osteoarthritis: a review of the cell biology involved and evidence for reversibility. Management rationally related to known genesis and pathophysiology. Semin Arthritis Rheum 1984;14: McAlindon TE, Snow S, Cooper C, Dieppe PA. Radiographic patterns of osteoarthritis of the knee joint in the community: the importance of the patellofemoral joint. Ann Rheum Dis 1992;51: Lanyon P, O Reilly S, Jones A, Doherty M. Radiographic assessment of symptomatic knee osteoarthritis in the community: definitions and normal joint space. Ann Rheum Dis 1998;57: O Reilly SC, Muir KR, Doherty M. Effectiveness of home exercise on pain and disability from osteoarthritis of the knee: a randomised controlled trial. Ann Rheum Dis 1999;58: Fransen M, Crosbie J, Edmonds J. Reliability of gait measurements in people with osteoarthritis of the knee. Phys Ther 1997;77: