Reliability of Hand-Held Dynamometry and Its Relationship with Manual Muscle Testing in Patients with Osteoarthritis in the Knee

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1 Reliability of Hand-Held Dynamometry and Its Relationship with Manual Muscle Testing in Patients with Osteoarthritis in the Knee Karen W. Hayes, PhD, PT' ludith Falconer, PhD, 0TR/L2 Copyright All rights reserved. erformance of fundamental activities of daily living, such as rising from a chair, walking, and climbing -- steps,. is enhanced by adequate knee extensor strength (1.9, 20). The knee extensors of patients with osteoarthritis (OA) have been shown to be weak as a result of the disease (13, 20). Patients with OA, therefore, are routinely tested for muscle strength to aid in planning treatment and monitoring progress. The most frequently used reliable quantitative measure of muscle strength is isokinetic dynamometry, which is costly and requires heavy, stationary equipment. One advantage is that it serves as a medium for exercise in addition to testing. For clinical and research settings in which isokinetic exercise is not necessary, both manual muscle testing (MMT) and hand-held dynamometry (HHD) are desirable testing methods because they are inexpensive, highly portable, and easy to perform. The reliability of HHD has been studied with healthy subjects (1, 10, 17) and subjects with neuromuscular disorders (7, 8, 26, 30, 31). Healthy subjects do not provide adequate range of strength measurements to determine reliability of measurement in specific patient populations. Reliability and validity of muscle strength The reliability of data gathered with hand-held dynamometers (HHD) and the relationship between HHD measurements and manual muscle tests (MMT) have been assessed only with healthy subjects and patients with neuromuscular disorders, not with homogeneous groups of patients with orthopaedic problems. In this study, HHD and MMT were used to measure the strength of knee extensor muscles of 43 patients with osteoarthritis. Test-retest reliability of HHD was determined, and the two testing systems were compared. The HHD measurements indicated that the knee extensor muscles were weak; the MMT grades indicated good strength. The HHD intraclass correlation coefficient for knee extensor strength measurements was The HHD measurements increased as the MMT grades increased. The Kendall tau correlation coefficient between HHD measurements and MMT grades for the knee extensor muscles was Hand-held dynamometry is less subjective than MMT, especially at the stronger grades. For the purpose of documenting progress of patients, HHD provides a reliable, quantitative method. The use of HHD with weak examiners, weak patients, and inadequate trunk stabilization is questionable. Hand-held dynamometry is useful for clinical and research settings where isokinetic testing is either unfeasible (ie., home care) or costprohibitive. Key Words: osteoarthritis, knee, muscle strengthening ' Assistant professor, physical therapy, Northwestern University Medical School, Chicago, I1 'Assistant professor, physical therapy and medicine (arthritis), Northwestern University Medical School, Chicago, I1 Support for this study was provided by the Arthritis Health Professions Association, Arthritis Foundation, National Office, and NIH (NIAMS) Multipurpose Arthritis Center Grant No. AM data are limited to the population for which those characteristics were determined. Consequently, examination of the reliability of HHD measurements among specific patient populations in orthopaedic physical therapy is appropriate. The purposes of this study were: I) to determine the intratester reliability and standard error of measurement (SEM) of HHD measurements with patients with OA and 2) to compare HHD measurements with grades from a MMT. The hy- potheses for the study were: I) reliability would be adequate (greater than 0.80) for use in patient care and clinical research, and there would be no difference in measurements on repeated tests and 2) there would be a positive, linear relationship between the two measures. METHOD Subjects Subjects were 43 ambulatory adults with chronic OA who partici-

2 Copyright All rights reserved. pated in a clinical trial of ultrasound and exercise. Subjects had OA diagnosed by a physician on clinical criteria (2), at least a 10" limitation of either knee flexion or extension (based on " (1 l)), and each provided a signature of informed consent. Procedure Knee extensor strength was measured by MMT and HHD. All subjects were evaluated by one of two experienced physical therapists or an experienced physical therapist assistant who was a physical therapy student at the time of the study. Each examiner tested different patients. All measurements were performed in the sitting position with the subjects' legs dangling over the end of an examination table. Subjects were instructed to remain erect and to hold the edge of the table at their sides to keep their thighs in contact with the table and their hips at a constant angle. The clinic had no provision for trunk stabilization. The absolute values of knee extensor torque may decrease without stabilization (16), but both MMT and HHD would be similarly biased. Manual muscle testing was always performed before HHD testing and was separated from HHD testing by other passive tests of joint condition. For MMT measurements, the subject extended the knee against gravity, and the examiner a p plied a break test just proximal to the ankle with the leg near the subject's full extension. Traditional MMT grades were used (1 2). The dynamometer used for HHD measurements (SPARK Instruments and Academics, Inc., PO Box 5 123, Coralville, IA ) recorded force in pounds or kilograms. The scales ranged from 0-60 Ibs in 8-oz increments or from 0-27 kg in 250-g increments. The scale was calibrated against known weights and was accurate to 200 g. No examiner had previous expe- rience with HHD, and each practiced with the instrument using the instruction manual (28) as a guide prior to subject measurement. For HHD measurements, lever arm length was held constant by placing the dynamometer near the ankle at a point 80% of the distance between the lateral joint line of the knee and the lateral malleolus. Examiners positioned the knee in 65" of flexion. Examiners held the dynamometer perpendicular to the limb surface and performed a make test; that is, the patient was instructed to exert as much force as possible against the dynamometer, but the examiner did not attempt to overcome the sub- Healthy subjects do not provide adequate range of strength measurements to determine reliability measurement in specific patient populations. ject's effort (5). Subjects held the contractions for 4 seconds, and examiners repeated the test after seconds. The examiner recorded the reading on the dynamometer in kg. The HHD measurements were repeated immediately after the first test; there was no second trial of MMT. Examiners knew the results of the first HHD test when repeating the procedure, but they could not see the scale during testing. Data Analysis For subjects with bilateral disease, data from the knee most limited in passive range of motion were used. Descriptive statistics were used to profile subjects on age, chronicity. MMT grades, and HHD measurements. To determine reliability of HHD, the intraclass correlation coefficient (ICC) was calculated from repeated measures analysis of variance. The SEM was com uted from the ICC (SEM = sd d-1-1cc). Reliability was computed for each examiner using ICC(3,l) and for all subjects pooled across examiners using ICC(1,l) (27). To compare the two testing systems, MMT grades and HHD measurements were tabulated to see if the HHD measurement increased as the MMT grades increased. In addition, the mean of the two HHD measurements was correlated with the MMT grades using the Kendall rank correlation coefficient (tau). Data were analyzed using SPSS (SPSS Inc., 444 N. Michigan Avenue, Chicago, IL ) and a Macintosh personal computer (Apple Computer, Inc Mariani Avenue, Cupertino, CA 95104). RESULTS The majority of the subjects were women (71.7%), with a mean age of 65.6 years (SD=12.7) and knee range limitations that had been present for an average of 6-7 years (Table 1). Compared with published studies of people with healthy knees (7, 8), HHD strength measurements for the knee extensors were low. Examiners assigned MMT grades indicating that the group had fair minus to normal grades for knee extension with a median grade of good. Only two subjects had grades less than fair plus. The three examiners demonstrated little variability in their testretest reliabilities. They ranged from for knee extension (Table 2). Test-retest reliability for all subjects and all examiners combined was good at The SEM was slightly less than 1 kg. When the mean HHD strength measurements were tabulated by Volume 16 Number 3 September 1992 JOSPT

3 Copyright All rights reserved.!i SD Range Age (years) Chronicity (months) Mean of two dynamometer readings for knee extension (kg) TABLE 1. Descriptive characteristics of 43 subjects with osteoarthritis. Fint Measurement - Second Measurement X SD 1 SD ICC' SEM (kg) Examiner 1 (N = 25) Examiner 2 (N = 10) Examiner 3 (N = 8) b All subjects (N = 43) All ICC, p <.01. Occasion effect, p <.05. TABLE 2. Knee extension dynamometer readings (kg), test-retest reliabilities (KC), and standard errors of measurement (SEM) for three examiners using hand-held dynamometry with patients having osteoarthritis. MMT grade, the HHD measurements increased as MMT grades increased (Table 3). There was overlap in the range of HHD measurements; both the highest and lowest HHD measurements were awarded to patients graded with fair plus MMT grades. The Kendall tau coefficient between the two methods of testing knee extension muscle strength was 0.24 (pc.05). DISCUSSION Based on the HHD measurements, knee extensor muscles a p peared weak. Weakness is expected in a sample of older people with OA (13, 15, 20, 23, 24). In healthy joints, the coefficient of friction is MMT Grade Si. SD Range N Fair minus Fair Fairplus Good Normal TABLE 3. Comparison of hand-held dynamometer strength measurements (kg) with manual muscle test (MMT) grades. low, but if joint surfaces become eroded, as they do in OA, this coefficient increases and can lead to decreased strength (29). Strength differences among the subjects measured by the three evaluators may have occurred, in part, because the first two examiners evaluated subjects referred from a rheumatology clinic, and the third examiner evaluated subjects referred from a geriatrics practice. Reliability of Hand-Held Dynamometry Measurements A test-retest reliability coefficient of 0.92 was comparable to those obtained by other investigators. Data from the instrument have shown test-retest reliabilities of for knee extension with patients with various neurological involvements (7, 26). Intertester reliability has ranged from (8). Similar variability has been reported for people without disease or injury (1, 17) and in patients with muscular dystrophy (30). The variability among individual examiners in this study is similar to that found in these previous studies. The small number of subjects for two of the examiners may also have affected their reliability coefficients. Wadsworth et al noted a learning effect in their reliability study of the dynamometer (3 1). Their retest values were significantly higher than the first test values. That pattern was not apparent in this study. Of the three comparisons made among the individual examiners in this study, the second measurement was higher than the first twice and lower once (Table 2). Although the learning or practice effect was uncontrolled in this study, this response pattern suggests that the effect was not present. One limitation of HHD that may affect both validity and reliability of Manual muscle testing uses a limited ordinal scale with 6-74 categories. measurements is the strength of the tester (1 0, 3 1, 32). If the tester is weak, the measurement will be lower than the patient's actual capability. Subjects may not have been exhibiting full strength if they perceived that the tester could not match their efforts. In this study, no examiner reported any difficulty in matching the subjects' efforts. In a study that examined the effect of tester weakness on reliability of HHD measurements, intrarater reliability was low for the weaker testers, especially when testing stronger muscles such as the knee extensors (32). The high intrarater reliability coefficient for a large lower extremity muscle group in this study suggests that tester strength was not a problem. The HHD measurements could have been affected by gravity. Gravity would tend to decrease the knee

4 Copyright All rights reserved. extensor force measurements. Methods of correcting measurements for the effects of gravity have been proposed for isokinetic dynamometry but were not used for the HHD measurements in this study. While the actual force measurements might be affected, rendering the validity of HHD measurements questionable, the reliability or consistency of measurements would not be affected. Relationship between Dynamometry and Manual Muscle Tests The reliability, validity, and sensitivity to change of MMT have been challenged (3, 4, 18, 21). Manual muscle testing uses a limited ordinal scale with 6-14 categories. While more categories may serve to increase the sensitivity to change, they may also increase the subjectivity and decrease the reliability of the scale (1 4, 18). The lower grades of MMT are relatively easy to judge because they have precise definitions based on body position and the effect of gravity. Above the fair grade, in which a combination of gravity and manual force provides resistance, the grading becomes inaccurate and very subjective (9, 18, 21) depending on the amount of manual resistance the subject can withstand. Beasley found that therapists could not distinguish among knee extensor forces that varied by as much as 25% (3) and that they graded as normal patients who had lost as much as 50% of the muscle's tension production (4). Good and normal grades are assigned based on the tester's personal expectations of the subject's ability to provide resistance and are influenced by the subject's age and general health. In this study, personal expectations related to age and the presence of a disease often associated with weakness in this muscle group may contribute to the relatively higher MMT grades compared with HHD measurements. The length of time the contraction is held may also influence the grade. Nicho- las et al demonstrated that the characteristic that determined a muscle test grade was the impulse of the motion, that is, the product of the average force generated and the time over which the effort is expended (25). The two different methods of testing muscle strength are related based on the comparison of grades; the HHD scores declined as the MMT grades declined. The results of this study support Bohannon's (6) conclusion that MMT and HHD measure the same construct but that MMT overestimates strength in comparison with HHD. Lilienfeld also found a tendency for MMT grades to be higher than quantitative measures of strength in polio patients (2 l). The correlation between The three examiners demonstrated little variabiliy in their testref est relia bilities. HHD and MMT for the knee extensors, however, was considerably lower than the 0.74 coefficient found by Bohannon for the same muscle group (6). The low correlation in this study can be attributed in part to restriction of range of the MMT scores. Nearly all subjects in this study were in the top three categories of MMT grades, which was expected because all subjects were ambulatory. The MMT grades might be expected to exceed the HHD scores because the MMT was performed with a break test and the HHD scores were obtained with a make test. Bohannon demonstrated that break force exceeded make force (5). so a higher grade on the MMT is reasonable. However, the number of subjects having fair plus to normal MMT grades when the HHD read- ings were low also underscores the subjective nature of the MMT grading system in the good to normal range and the tendency of MMT to overestimate strength. The difference in grades might also be attributable to the different angles of measurement used in the two methods of testing. The MMT of knee extension was performed with the knee near the subject's terminal extension but with the knee at 65" of flexion for HHD measurements. In the sitting position, maximal torque for knee extension is produced at 60" of knee flexion (22). The optimal knee angle was used for HHD but not for MMT. This difference put the muscle at a disadvantage for MMT and could account for differences in grading. However, the grades were higher for MMT, so the difference in angles does not explain overestimation of strength with MMT. CONCLUSIONS Hand-held dynamometry measurements of knee extensor strength of patients with OA are adequately reliable both for clinical research and in clinical decision-making. There is some variability in the ability of individual examiners to use the instrument consistently. Hand-held dynamometry of patients with OA identified measureable weakness in the knee extensors that was not detected by MMT. Hand-held dynamometry measurements are less subjective than MMT grades, especially in stronger subjects. The validity of HHD measurements for specific purposes must be considered, however. Inadequate trunk stabilization, weakness of the examiner, extreme weakness of the patient, and the effects of gravity may render data from HHD less useful under some circumstances. Bearing these limitations in mind, in settings where isokinetic testing is either unfeasible (ie., home care) or cost-prohibitive, HHD is a Volume 16 Number 3 September 1992 JOSPT

5 Copyright All rights reserved. useful method of testing knee strength. ACKNOWLEDGMENTS JOSPT The authors thank the Biostatistical and Data Management Core staff, especially Ahn Chung and Xochitl Salvatore, of the Northwestern University Multipurpose Arthritis Center for their assistance in data processing, data management, and manuscript review. We also thank Katie Sirianni, PT, Linda Tieman Roherty, PT, and Babette Sanders, MS, PT, for serving as the evaluators in this study. REFERENCES Agre lc, Magness ll, Wright KC, Baxter TL, Patterson R, Stradel L: Strength testing with a portable dynamometer: Reliability for upper and lower extremities. 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