ASSESSMENT OF MUSCLE strength is an integral part of

Transcription

1 653 Muscle Force Measured Using Break Testing With a Hand-Held Myometer in Normal Subjects Aged to 69 Years Beverley A. Phillips, Ph, Sing K. Lo, Ph, Frank L. Mastaglia, M ABSTRACT. Phillips BA, Lo SK, Mastaglia FL. Muscle force measured using break testing with a hand-held myometer in normal subjects aged to 69 years. Arch Phys Med Rehabil 00;81: Objective: To measure the strength of 17 muscle groups in the upper and lower extremities in a large group of healthy subjects using break testing with a hand-held myometer, and to examine the intrasession and intersession reliability of the testing protocol. Subjects and Instrumentation: A convenience sample of men and women in each decade of age from to 69 years (n 0) was tested using a Penny & Giles hand-held myometer. Results: Reliability coefficients were.85 for both intrasession and intersession reliability, except for the ankle dorsiflexors. Men exerted a significantly greater force than women for all muscle groups. Age, weight, and side of testing were significant predictors of force in the majority of muscle groups. The fifth percentile values, as the lower limit of normal, are reported separately for gender and side of testing for each decade of age. Conclusion: Using the testing protocol specified in this study, data from patients with various neuromuscular diseases may be compared with the appropriate gender- and age-matched normal data to accurately identify the presence of weakness. Key Words: Myometry; Strength; Rehabilitation. 00 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation ASSESSMENT OF MUSCLE strength is an integral part of the management of patients with neuromuscular disorders, particularly in determining the response to treatment. For an assessment protocol to be useful in the clinical setting, it is important that it is specific to the clinical characteristics of the disease and is sufficiently sensitive to detect change in muscle performance using serial measurements. 1,2 The methods of assessment used also must be sufficiently sensitive to detect muscle weakness in comparison with the strength of normal From the Centre for Neuromuscular and Neurological isorders, University of Western Australia, Australian Neuromuscular Research Institute, Perth, Western Australia (rs. Phillips, Mastaglia); the epartment of Medicine, University of Western Australia, Perth, Western Australia (r. Mastaglia); and the Faculty of Health and Behavioural Sciences, eakin University, Melbourne, Victoria, Australia (r. Lo). r. Phillips is currently affiliated with the School of Physiotherapy, Faculty of Medicine, entistry and Health Sciences, the University of Melbourne, Melbourne, Victoria, Australia. r. Lo is currently affiliated with the epartment of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong, China. Submitted June 28, Accepted in revised form August 10, Supported by the Neuromuscular Foundation of Western Australia. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to r. Bev Phillips, School of Physiotherapy, Faculty of Medicine, entistry and Health Sciences, the University of Melbourne, Victoria, 3010, Australia. 00 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation /00/ $3.00/0 doi: /mr subjects, 3 particularly because there is often diffuse muscle weakness in patients with neuromuscular disorders, making comparison with an unaffected opposite limb impossible. Manual muscle testing scales, such as the Medical Research Council scale, 4 are not sufficiently reliable or responsive to accurately monitor changes in muscle strength over time. 5 Hand-held myometry provides a quantitative measurement of muscle strength and has been shown to be more sensitive to change than manual muscle testing. 3,6 There are 2 different methods of testing using a myometer. For a make test, the examiner holds the myometer stable and the subject exerts a maximal force against the myometer for a period of 3 to 5 seconds. For a break test, the subject exerts a maximal force against the myometer and the examiner applies sufficient resistance to just overcome the force exerted by the subject, thus causing the start of an eccentric contraction by the preloaded muscles, the resultant force measured being greater than is measured using make tests. 7 Comprehensive normal data have been reported in 2 studies using different myometers to perform make testing for a number of muscle groups 8,9 ; however, issues such as testing muscle force against gravity, lack of reliability studies, and incomplete reporting limit the usefulness of the results of the studies using break testing. 10,11 The primary aim of this study was to use the break testing method of hand-held myometry to measure the strength of 17 muscle groups in the upper and lower extremity in a large group of normal subjects using a testing protocol that we perform to measure muscle strength in patients with inflammatory myopathy. The intrasession and intersession reliability of the testing protocol and the effect of gender, age, weight, height, side of testing, and limb dominance on muscle strength in normal subjects were also examined. METHOS Subjects A convenience sample of 0 normal subjects were tested after obtaining written informed consent, including men and women from each decade of age from to 69 years. Exclusion criteria included performance of heavy manual labor or strenuous resisted strength training, presence of residual pain or disability from previous injury to any of the joints or muscles of the upper or lower extremities, and presence of cardiovascular symptoms. Body weight and height were measured. Limb dominance was determined by questionnaire, 12 and 17 of the 0 subjects were left-limb dominant for both the upper and lower extremities. The level of occupational and leisure activities was recorded according to the Saltin and Grimby scale. 13 Testing Protocol A Penny and Giles a hand-held myometer was used to measure the strength of 17 muscle groups in the upper and lower extremities and the neck. According to the manufacturer s specifications, the myometer used has a measurement range of 0 to 30kg with an overload rating to 40kg and an accuracy of 0.3kg, and is calibrated to within 0.1kg. The calibration was Arch Phys Med Rehabil Vol 81, May 00

2 654 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips checked at regular intervals using standard weights according to the method described by the manufacturer, and the accuracy and linearity of the force measurement was found to be within the manufacturer s specifications up to 35kg. The applicator of the myometer was padded to improve subject comfort during testing. All strength measurements were performed by one female examiner who was experienced with hand-held myometry. The muscle groups tested and the position of the limb segment and the applicator of the myometer are given in table 1 in the order in which the muscle groups were tested on each side of the body, and the side to be tested first was alternated between subjects. All tests were performed with the subject in supine and with the foot on the side not being tested resting lightly against a support. All muscle groups were tested with the limb segment in a gravity-neutral position except for the wrist extensors and the neck flexors, both of which were tested against gravity, because better stabilization of the subject could be provided in that position. Where appropriate, the examiner provided extra stabilization to the myometer by fixing the elbow of the arm holding the myometer against the region of the anterior iliac crest. The break method of testing was used for all tests. The applicator of the myometer was held against the limb segment, and the subject was asked to exert a maximum force against it. The examiner applied sufficient resistance to just overcome the force exerted by the subject, and the applicator was then immediately moved away from the limb segment and the measured force was recorded. Each trial lasted approximately 3 seconds, and 3 repetitions were performed, additional trials being performed if at least 2 of the repetitions were not within approximately 10% of each other. A 5-second rest was given between each repetition. An alteration to the protocol was made for testing the neck flexors, when only 1 trial was used because of the possible vulnerability of structures in the cervical spine to forward flexion performed against resistance. The maximum force recorded was used for analysis, and the value in kilograms recorded by the myometer was multiplied by 9.81 to convert it to Newtons. Reliability Study In 8 men and 12 women aged 23 to 39 years, the intrasession reliability was established using the 3 trials for each test (except for the neck flexors), and the testing protocol was repeated after an interval of 2 weeks to establish intersession reliability, using the maximum force of the 3 trials for each test. The same order of testing was followed except that the order in which the 2 sides were tested was reversed. ata Analysis For the reliability study, intraclass correlation coefficients (ICCs) ICC (1,3) and ICC (1,1) were calculated for the intrasession and intersession analyses, respectively, and 95% confidence intervals (CI) for the reliability coefficients were constructed. 14,15 Independent t tests and one-way analysis of variance (ANOVA) were used to examine differences in weight and height between genders, and a Wilcoxon rank sum test was used to examine differences in the Saltin and Grimby scales between genders. A regression analysis was performed on the data for each muscle group, using gender, age, weight, height, limb dominance, and side of testing as independent variables, performing a backward stepwise regression to determine which independent variables best explained differences in strength using an level of.05 and.15 to enter and remove variables, respectively. An ANOVA was performed to further analyze the effect of age on the strength of each muscle group, using Fisher s least-squares-difference (LS) for post-hoc testing. The mean and standard deviation (S) were calculated for each muscle group test, stratifying the data by gender, side of testing, and decade of age. To establish the lower limit of normal strength, the 5th percentile was calculated for each muscle group. The generally accepted method of using 2 Ss for a normal range was not applied because of the possibility of a positively skewed distribution in which a greater number of higher force values would increase the S. Because the purpose of obtaining normal data is to provide comparison with data from patients to determine if weakness is present, only the lower normal limit is of interest, and so, patients with force Table 1: Positions for Hand-Held Myometry Muscle Group Position of Limb Segment Myometer Position Stabilization Shoulder abductors Shoulder abducted, elbow flexed Immediately proximal to lateral epicondyle Nil 90 Shoulder external Arm at side, elbow flexed 90, Immediately proximal to ulnar styloid Nil rotators forearm mid-prone process Elbow extensors Shoulder abducted 10, elbow flexed Immediately proximal to ulnar styloid Nil 90, upper arm in contact with plinth during test, forearm supinated process Elbow flexors Shoulder abducted 10, elbow flexed Immediately proximal to proximal Nil 90, forearm supinated wrist crease Wrist extensors Shoulder abducted 10, elbow extended, forearm pronated and in Center of dorsum of hand Forearm stabilized on plinth by examiner contact with plinth, wrist extended, fingers flexed Hip flexors Hip and knee flexed 90, lower leg Immediately proximal to upper border Nil supported by examiner of patella Hip abductors Hip abducted, knee extended Immediately proximal to lateral femoral condyle Subject holding side of plinth, other leg stabilized by examiner Ankle dorsiflexors Hip abducted 10, knee extended, foot Center of dorsum of foot Nil plantigrade Neck flexors Head raised to 30 with chin tucked in Center of forehead Nil Arch Phys Med Rehabil Vol 81, May 00

3 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips 655 Gender ecade of Age (yrs) n Age (yrs) Table 2: Subject Characteristics Weight (kg) Height (cm) Physical Activity* Median (range) Occupational Activity* Median (range) Men (1-3) 2 (1-3) (1-3) 2 (1-3) (1-3) 2 (1-3) (1-3) 1 (1-3) (1-3) 1 (1-3) (1-3) 2 (1-3) Women (1-3) 2 (1-3) (1-3) 2 (1-3) (1-3) 1 (1-3) (1-3) 1 (1-3) (1-3) 1 (1-3) (1-3) 1 (1-3) * ata from Saltin and Grimby. 13 values less than the normal 5th percentile will be weaker than 95% of age- and gender-matched normal subjects. The SYS- TAT b statistical package was used for all statistical analyses. RESULTS The characteristics of the subjects, grouped by gender and decade of age, are given in table 2. Eight of the subjects experienced joint or soft tissue discomfort during testing, and the recorded force value for the associated muscle group test was not used. The men were significantly heavier ( p.001) and taller ( p.001) than the women. Men aged to 29 years weighed significantly less than the rest of the men (F 4.716, p.001). Women aged 30 to 39 years were significantly taller, and those aged 50 to 69 years were significantly shorter, than the rest of the women (F , p.001). There was no significant difference in the level of physical ( p.713) and occupational activity ( p.645) between the men and the women; the majority of subjects participated in some physical activity. Reliability The ICC values for intrasession and intersession reliability are given in tables 3 and 4, respectively. Intrasession reliability was excellent, ICC values being.95 for all muscle groups except for the ankle dorsiflexors. The intersession reliability was also good for the majority of the tests, the ICC values being.85 for all muscle groups except the nondominant hip abductors and the ankle dorsiflexors bilaterally; however, the reliability coefficients were lower than for the intrasession reliability. Regression Analysis Gender was a significant predictor for all muscle groups ( p.001), the maximum force in the women being, on average, 68% (range, 59% to 78%) of the maximum force in the men, and so the men and women were separated for the rest of the analyses. In the men, the side of testing was a significant predictor for the shoulder abductors, hip abductors, and ankle dorsiflexors. The dominant hip abductors and ankle dorsiflexors were, on average, 3.4% and 4.5% stronger, respectively, than the nondominant muscle groups, while the shoulder abductors on the nondominant side were, on average, 6.4% stronger than on the dominant side. For the remaining muscle groups, the differences between the 2 sides varied from 0.2% to 1.8%. In the women, the side of testing was a significant predictor of force in a greater number of muscle groups, the dominant side exerting greater force than the nondominant side (range, 4.7% to 8.0%) for the shoulder external rotators, elbow flexors, wrist extensors, hip flexors, and ankle dorsiflexors. Similar to the men, the nondominant shoulder abductors exerted 3.2% greater Table 3: Intrasession Reliability of Force (N) Measured With Hand-Held Myometry Muscle Group Side n ICC (1,3) ICC 95% CI Trial 1 Trial 2 Trial 3 Shoulder abductors , N , Shoulder external rotators , N , Elbow extensors , N , Elbow flexors , N , Wrist extensors , N , Hip flexors , N , Hip abductors , N , Ankle dorsiflexors , N , Abbreviations:, dominant; N, nondominant. Arch Phys Med Rehabil Vol 81, May 00

4 656 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips Table 4: Intersession Reliability of Force (N) Measured With Hand-Held Myometry Muscle Group Side n ICC (1,1) ICC 95% CI Test 1 Test 2 Shoulder abduc , tors N , Shoulder external , rotators N , Elbow extensors , N , Elbow flexors , N , Wrist extensors , N , Hip flexors , N , Hip abductors , N , Ankle dorsi , flexors N , Neck flexors , Abbreviations:, dominant; N, nondominant. force than the dominant muscles. There was minimal difference between the dominant and nondominant sides for the elbow extensors (1.9%) and the hip abductors (0.4%). Although the plots of the residuals for the regression models in either gender demonstrated a random pattern for all muscle groups, indicating that there was no violation of statistical assumptions, it was decided to separate the dominant and nondominant sides for each gender, particularly because weakness is often asymmetric in patients with neuromuscular disease. A further regression analysis was performed using age, weight, height, and limb dominance as independent variables, and the models of best fit for each muscle group are given in table 5. In the men, age was a significant predictor for all muscle groups except the nondominant shoulder abductors, as was weight except for the dominant ankle dorsiflexors. Height was a significant predictor for the elbow extensors bilaterally and the dominant shoulder external rotators. Limb dominance was a significant predictor for the shoulder abductors, elbow extensors, and ankle dorsiflexors bilaterally, the dominant elbow flexors, and the nondominant wrist extensors and hip abductors, the left-limb-dominant subjects exerting, on average, greater forces than the right-limb-dominant subjects. In the women, age was a significant predictor for all muscle groups except the shoulder external rotators bilaterally, the dominant elbow flexors, and the nondominant ankle dorsiflexors. Weight was also a significant predictor for most muscle groups except the dominant hip flexors, and the nondominant hip abductors and ankle dorsiflexors. Height was a significant predictor for the shoulder external rotators, elbow extensors, and hip abductors bilaterally and the dominant elbow flexors. Limb dominance was a significant predictor for fewer muscle groups than in the male subjects, those being the nondominant shoulder abductors, the dominant hip flexors, and the ankle dorsiflexors bilaterally. To determine whether using the 5th percentile value as the normal lower limit was appropriate, the percentage difference between the normal limit given by the 2 Ss below the mean and that given by the 5th percentile was calculated for each muscle group. In the men, the average difference was 0.6%. There was a greater average difference of 6.1% for the women, the distribution of the data being positively skewed for 73.3% of the tests. The use of the 5th percentile value as a normal lower limit was therefore considered more appropriate for the women, and thus, the same method was used for the men. The mean, S, and 5th percentile values are given for each muscle group for the dominant and nondominant sides separately for each decade of age in the men and women in tables 6 and 7, respectively. Age was a significant predictor of force for all muscle groups except for the nondominant shoulder abductors in the men, and the shoulder external rotators bilaterally, the dominant elbow flexors, and the nondominant ankle dorsiflexors in the women. The decade of age at which there was a significant decrease in strength using an ANOVA is shown in tables 6 and 7, the p values for Fisher s LS post-hoc testing ranging from.033 to.001 for the significant results. For the majority of muscle groups in the men, there was a significant decrease in strength in the 60-to-69-year decade relative to the -to-29-year decade, although for the hip abductors bilaterally and the neck flexors, strength significantly decreased in the 50-to-59-year decade. Strength decreased significantly at an earlier age for the Muscle Group Side Table 5: Regression Equations for Hand-Held Myometry Men Women Regression Equation R 2 Regression Equation R 2 Shoulder abductors A 1.84W 15.34L A 0.69W.171 N W 12.L A 0.95W 8.54L.183 Shoulder external rotators A 1.44W 1.36H W 0.54H.063 N A 1.51W W 0.44H.069 Elbow extensors A 1.39W 0.96H 8.11L A 0.86W 0.68H.160 N A 1.25W 0.80H 8.90L A 0.97W 0.99H.195 Elbow flexors A 0.76W 7.84L W 0.79H.219 N A 1.28W A 0.91W.162 Wrist extensors A 0.37W A 0.54W.077 N A 0.81W 13.25L A 0.79W.154 Hip flexors A 0.28W A 8.92L.233 N A 0.36W A 0.36W.164 Hip abductors A 0.73W A 0.52W 1.09H.170 N A 0.60W 1.32L A 1.61H.230 Ankle dorsiflexors A 8.39L A 0.54W 12.36L.297 N A 0.38W 11.94L H 8.76L.117 Neck flexors A 1.08W A 0.34W.212 Abbreviations:, dominant; N, nondominant; A, age; W, weight; H, height; L, limb dominance. Arch Phys Med Rehabil Vol 81, May 00

5 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips 657 Table 6: Force (N) Using Hand-Held Myometry in Men Muscle Group ecade n Side 5th Percentile Shoulder abductors N N N N * 188 N Shoulder external rotators Elbow extensors -29 Elbow flexors -29 Wrist extensors -29 Hip flexors N N N N N N N N N N N N N N * 231 N * N N N N * 214 N * N N * 2 N N N * 178 Table 6: Force (N) Using Hand-Held Myometry in Men (Cont d) Muscle Group ecade n Side 5th Percentile Hip abductors -29 N N N * 5 N * N Ankle dorsiflexors -29 N N * N N N Neck flexors * Abbreviations:, dominant; N, nondominant. * Age of significant decrease in muscle strength compared with subjects aged to 29 years. dominant hip flexors (40 to 49 years) and the nondominant ankle dorsiflexors (30 to 39 years). The average decrease in mean strength for the upper and lower extremity muscle groups was similar, being 8.7% and 9.3%, respectively, compared with the mean strength of the - to 29-year-old subjects. As in the men, muscle strength in the women significantly decreased at the slightly younger age of 50 to 59 years for the hip abductors bilaterally and the neck flexors, and also for the hip flexors bilaterally. The nondominant shoulder abductors and the wrist extensors bilaterally significantly decreased in strength in the 60-to-69-year decade, and the ankle dorsiflexors bilaterally from the 40-to-49-year decade. The decrease in mean strength when compared with the -to-29-year decade was generally greater than for the men, there being an average decrease of 13% and 10% for the upper and lower extremity muscle groups, respectively. ISCUSSION The intrasession and intersession reliability for the break tests using hand-held myometry performed in this study were high and compare favorably with other reports of the reliability of hand-held myometry in normal subjects, although previous studies have used make tests when examining reliability. 9,16 For the intrasession testing in the present study, there was a trend of decreasing force measured across the 3 trials that was not statistically significant. It is probable that the short rest interval between trials contributed to this trend, and it is recommended that a longer intertrial rest interval be used in future studies, particularly if the average rather than the peak force is used for analysis. Although the reliability testing in the present study was performed at the beginning of the study with younger subjects for convenience, previous studies have reported excellent intrarater reliability in older subjects, 8,17 sug- Arch Phys Med Rehabil Vol 81, May 00

6 658 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips Table 7: Force (N) Using Hand-Held Myometry in Women Muscle Group ecade n Side 5th Percentile Shoulder abductors -29 N N N N N * 117 Shoulder external rotators Elbow extensors -29 Elbow flexors -29 Wrist extensors -29 Hip flexors N N N N N N N N N N N N N N N N N N N * 121 N * N N N * 133 N 160 * N Table 7: Force (N) Using Hand-Held Myometry in Women (Cont d) Muscle Group ecade n Side 5th Percentile Hip abductors -29 N N N * 161 N * N Ankle dorsiflexors -29 N N * 192 N 4 31* N N Neck flexors * Abbreviations:, dominant; N, nondominant. * Age of significant decrease in muscle strength compared with subjects aged to 29 years. gesting that age does not influence reliability of hand-held myometry. As concluded in other studies, 16,18- the short lever arm used when testing the ankle dorsiflexors may have contributed to the lower intrasession and intersession ICC values reported for this muscle group in the present study, particularly given that the ankle dorsiflexors are very strong muscles, thus making testing more difficult for the examiner. It was also observed during the testing that because of the high force exerted, it was difficult to perform a break test without the foot moving into a small degree of inversion as well as the intended plantar flexion, and this may have affected the perpendicular alignment of the transducer of the myometer to the line of force exerted by the subject. When comparing the force values recorded using break testing in the present study with the make test results reported by Bohannon 9 for those muscle groups tested in the same positions, the make forces for the ankle dorsiflexors in Bohannon s study were 1.1 to 1.5 times the break forces measured in the present study, which is the reverse of the trend for other muscle groups, in which the force measured using break tests was 1.04 to 2 times greater than that measured with make tests, which is in agreement with results of previous studies comparing make and break testing. 7,21 It is considered likely that the possible nonperpendicular alignment of the myometer transducer to the line of force exerted by the subject contributed to an inaccurate recording of the maximum force when testing the ankle dorsiflexors, and thus, the results have therefore been interpreted with caution. This muscle group will not be included in the following discussion about the factors that may influence maximum strength of the remaining muscle groups tested. Limited comparisons only of the results in the present study can be made with those of previous studies reporting normal values for hand-held myometry using break testing. In a Arch Phys Med Rehabil Vol 81, May 00

7 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips 659 study of 100 normal subjects, 10 only the elbow flexors and extensors, hip flexors, and ankle dorsiflexors were tested in the same positions as in the present study. The 5th and 50th percentiles were reported separately for men and women; however, the results for the right and left sides were averaged, and the percentiles were calculated for the total age range of to 60 years. Although these factors limit any comparison, in general, for subjects aged to 60 years, the 5th percentile values in the present study for the elbow flexors and extensors and the hip flexors were greater than those reported by van der Ploeg and coworkers. 10 Bäckman and colleagues 11 used a Penny & Giles myometer to perform break tests in 128 normal subjects for 6 of the 9 muscle groups tested in the present study, testing 3 of the muscle groups against gravity; however, the mean and S values for only the nondominant side for each decade of age for men and women separately were reported. In a smaller study of 30 men and 30 women aged 16 to 78 years, break tests using the Penny & Giles myometer were performed in 6 muscle groups, testing 4 of the muscle groups in the same positions as in the present study 22 ; however, as the mean and S values were presented for the total age group for each gender, the different age range of the subjects limits the comparison of these results with the results of the present study. As reported in other studies, 8,9 the results of the initial regression analyses for the myometry tests demonstrated that gender was a significant predictor of strength for all muscle groups, holding the effect of weight and height constant. The difference in strength between men and women is not surprising, given the previously reported difference in muscle crosssectional area between men and women, and the results of the present study are similar to previous reports that the mean force values for women are approximately two thirds of the force values for men. 10,11,27 Men and women are considered separately for the remainder of the discussion. etailed analysis of the difference in muscle strength between dominant and nondominant muscle groups has not been performed in previous studies using hand-held myometry, although the dominant side has been reported to be stronger than the nondominant side in a number of muscle groups. 11,27 It is unclear why there was a significant effect for the side of testing for more muscle groups in the women than in the men in the present study, given that the levels of occupational and physical activity were similar for both groups. Similarly, it is unclear why the shoulder abductors were considerably stronger on the nondominant than the dominant side in both the men and women, given that, in the other muscle groups for which side of testing was a significant predictor, there was greater muscle strength on the dominant side. Although all of the differences between sides of testing were small and 8%, the mean force values for each muscle group for the dominant and nondominant sides have been reported separately. In 2 of the previous studies reporting myometry values for normal subjects, limb dominance was recorded; however, no data were provided as to the number of left-limb-dominant subjects who were included in the total sample. 8,9 In a study using a cable tensiometer to test the isometric strength of the elbow flexors, knee extensors and flexors, and a hand-grip dynamometer to measure grip strength in 32 right- and 32 left-limb-dominant men, the left-limb-dominant subjects exerted a slightly greater force than the right-limb-dominant subjects for the knee extensors and flexors and grip strength on the dominant side. 28 In the present study, given that the rightand left-limb-dominant men were similar in characteristics such as age, weight, and activity levels, the reason for the greater strength of the left-limb-dominant subjects is not known, although the small sample size (n 10) may mean that the sample is in some way biased. Because the interaction terms of limb dominance with each of the other significant independent variables were not significant for any of the regression models, it was decided not to exclude the left-limb-dominant subjects. Body weight was found to have a low correlation with muscle strength in the majority of muscle groups tested in 2 previous studies using myometry in smaller groups of normal subjects 10,11 ; however, in contrast, a strong relationship was found between weight and strength in 2 larger studies, in which weight was a significant predictor of force in the regression model for each muscle group tested, and normal values for absolute force and force normalized for weight were reported. 8,9 Values for force normalized for weight were not calculated in the present study, because the main purpose for obtaining normal values was for comparison with force values in patients with inflammatory myopathy. The majority of these patients are not their average weight because of the influence either of muscle atrophy due to the disease process or of weight gain due to the side effects of long-term corticosteroid use; therefore, using weight-adjusted force values would give misleading results. None of the aforementioned studies have reported height as a significant predictor for muscle strength except for 2 muscle groups in the study by Andrews and coworkers. 8 In comparison, in a study measuring strength of the hip and trunk flexors and extensors using a cable tensiometer, a high correlation between strength and height was found. 29 In the present study, given the significant correlations between weight and height for most decades of age in both the men and women, it was not unexpected that height was a significant predictor of force for a number of muscle groups. Age was a significant predictor of force in the majority of muscle groups tested in both the men and women. Given that both the men and women were similar with respect to activity levels, it is not known why the women demonstrated a greater decline in strength compared with the men, although it has been suggested that hormonal changes in postmenopausal women may influence strength. 30 In both the men and women, the decrease in strength for the lower-extremity muscle groups occurred at the younger decade of 50 to 59 years compared with the upper-extremity muscle groups, which did not decline in strength until the 60-to-69-year decade. The earlier decline in strength in the lower-extremity muscle groups found in the present study is in agreement with the results of previous studies using different instruments to measure isometric strength In one of the studies reporting myometry values for normal subjects aged to 60 years, there was no significant effect for age in most muscle groups, 10 while when older subjects were included in a different study, a decline in strength occurred in the 60-to-69-year decade for the majority of muscle groups in both men and women. 11 In the 2 previous studies reporting myometry values for larger groups of normal subjects, 8,9 age was a significant predictor of force for all muscle groups tested; however, there was no analysis performed to determine at what age strength significantly declined. In a secondary report using the data for the women previously reported, Bohannon 34 performed a detailed analysis of the effect of age on the strength of 3 muscle groups in the upper extremity and 3 in the lower extremity. It was found that strength declined in all of the muscle groups, but that the age of onset and degree of decline varied, and by examining the coefficients in the regression equations, it was determined that the rate of decline across decades of age was significantly different for the dominant and nondominant sides. The results of the present Arch Phys Med Rehabil Vol 81, May 00

8 660 NORMAL HAN-HEL MYOMETRY FORCE VALUES, Phillips study support the conclusion that the decline in muscle strength with age varies for different muscle groups, given that there were 5 muscle groups in the men and 7 muscle groups in the women that did not show a significant change in muscle strength with age, and that in those muscle groups that did decline in strength with age, the change occurred at varying decades of age. There have been a number of reports describing the changes that occur in muscle physiology with aging, and most studies have reported that changes occur after the approximate age of 60 years. There is a reduced muscle cross-sectional area, with a decrease in the total number of muscle fibers and preferential atrophy of the type II fast-twitch muscle fibers, and there is some fiber type grouping, implying that there is a degree of denervation and reinnervation occurring, and a decline in the number of functional motor units is also observed It has been reported that these changes may vary for different muscle groups, but that men and women appear to demonstrate similar trends. 38 There are certain limitations to the present study that must be considered. The normal values reported are specific to using a Penny & Giles hand-held myometer and as such may not necessarily be interchangeable with results using different hand-held myometers, although a previous study has reported that the results using two different hand-held myometers could be used interchangeably. 39 The reliability results reported in the present study were established using only one examiner; however, there have been several previous studies that have established acceptable interexaminer reliability for hand-held myometry. 17,40,41 Although the experienced female examiner in the present study was able to adequately perform break tests for all muscle groups tested in both men and women by using a combination of strength, body weight, and leverage in a controlled manner, it is possible that other examiners may have difficulty adequately performing break tests, 1,42,43 and, in these circumstances, make tests should be performed. In a previous study, 44 we demonstrated that the use of hand-held myometry (both make and break testing) underestimated maximum force of the knee extensors but not the knee flexors when compared with the results using a computerized dynamometer, and so, larger muscle groups such as the knee extensors were not tested in the present study. In the clinical setting in which quantitative measurement of the strength of lowerextremity muscle groups in addition to the hip muscles may be required, the use of other methods such as computerized dynamometry is recommended. CONCLUSION Reliable lower limits of force, using the 5th percentile, measured using break testing with a hand-held myometer have been established for 5 muscle groups in the upper extremity, 2 in the lower extremity, and the neck flexors for male and female normal subjects aged to 69 years. 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