Journal of Gerontology: MEDICAL SCIENCES 2001, Vol. 56A, No. 12, M767–M770
Copyright 2001 by The Gerontological Society of America
A New Approach of Measuring Muscle Impairment During a Functional Task: Quadriceps Muscle Activity Recorded During Chair Stand Jennifer S. Brach,1,2 Andrea M. Kriska,2 Anne B. Newman,2,3 and Jessie M. VanSwearingen 1 Departments of 1Physical Therapy, 2Epidemiology, and 3Medicine—Section of Geriatric Medicine, University of Pittsburgh, Pennsylvania.
Background. Biologic changes are expected to occur prior to disability. Compared with physical disability measures, measures of muscle impairment may be an earlier indicator of functional decline. The purpose of this study was to describe a new approach of measuring muscle impairment during a functional task. Methods. Right quadriceps muscle activity was recorded using surface electromyography (sEMG) from 160 older women (age 73.9 3.9 years, mean SD). Specific patterns of muscle activity during the chair stand task were determined using an exploratory principal components factor analysis (PCFA). Muscle activity parameters were validated by comparison to the Physical Performance Test, gait speed, and the Functional Status Questionnaire. Results. The PCFA indicated two factors (magnitude and timing) that represented important components of quadriceps muscle activity during chair stand, explaining 68.6% of the variance in performance. The slope of the rise of muscle activity represents a combination of the magnitude and timing components of muscle activity. Compared with women with a slope 1, women with a slope 1 walked faster (1.17 m/s vs 1.09 m/s; p .02) and reported less difficulty with activities of daily living (ADL) (98.6 vs 95.8; p .003) and instrumental ADL (97.3 vs 92.2; p .001). Conclusions. Quadriceps muscle activity recorded during chair stand is a valid and reliable measure of muscle performance during a functional task. As a biologic measure of muscle activation, sEMG may identify muscle impairment, which could indicate functional decline earlier than measures of functional status.
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preclinical state of disability is characterized by the development of impairment or early functional limitations that are not yet clinically apparent (1,2). Individuals with preclinical disability are believed to be at high risk for future disability. A lower extremity battery has been used successfully in predicting future disability in individuals who were nondisabled at baseline (3). However, some individuals identified as being “at risk for future disability” were likely already disabled at baseline given their walking speed (0.77 m/s), which was lower than the average gait speed of older adults (1.2–1.3 m/s) (4,5). Ideally, we would like to identify individuals at risk for future decline before they have difficulty with a functional task. In a healthier population that is walking at or near the desired gait speed of 1.2 to 1.3 m/s, the lower extremity battery may demonstrate a ceiling effect. An impairment-based measure of muscle could be used to identify individuals with preclinical disability among nondisabled older adults. We describe assessing lower extremity muscle performance during chair stand using surface electromyography (sEMG). The sEMG can be recorded during the chair stand task, thus providing information about muscle recruitment and pattern activation as the individual uses their muscle during a meaningful task (6). The purpose of this study is to describe a new approach of measuring muscle impairment during a functional task in
community-dwelling older women. We describe patterns of quadriceps muscle activity as measured by sEMG during a chair stand task and determine the validity and the test-retest reliability of the measure. METHODS Subjects Participants were 171 older women volunteering in a follow-up study to a randomized controlled exercise trial (7– 9). To be included in the present study, the women had to be able to rise from a sitting position without the use of their arms. Measures Muscle Activation During Chair Stand Task.—Muscle activity was recorded from the right quadriceps muscle during the task of standing from a chair (Myosystem 1200; Noraxon, Scottsdale, AZ). The raw EMG signal was fullwave rectified and integrated with a 15-millisecond “sliding window” of integration. The electrodes were placed 3 inches proximal to the superior border of the patella. Participants were asked to stand from the chair with their arms crossed over their chest and then to return to the seated position. Peak muscle activity, total muscle activity (area), and time to peak muscle activity for both phases of the chair M767
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Figure 1. Surface electromyography parameters recorded from the quadriceps muscle during a chair stand task.
stand task (standing and sitting) were recorded for the right quadriceps muscle (Figure 1). To determine the test-retest reliability, a subsample of the participants (n 15) underwent two tests, at least 30 minutes apart, on the same day. Functional Status.—Physical Performance Test (PPT) scores, self-selected gait speed, and the Functional Status Questionnaire (FSQ) were recorded for each of the participants. The PPT is a comprehensive, performance-based measure of physical performance of daily activities (10). Gait speed was measured while the participants walked at a self-selected pace on an instrumented walkway (GaitMat II; EQ, Chalfont, PA) (11). The FSQ is a self-report comprehensive measure of functional status in ambulatory care patients with basic activities of daily living (ADL) and instrumental activities of daily living (IADL) subscales (12). Data Analysis.—The sEMG variables (peak, area, and time to peak) recorded during the standing and sitting component of the chair stand were subjected to an exploratory principal-components factor analysis (PCFA) to determine specific patterns of quadriceps muscle activity during the chair stand task. The exploratory PCFA was calculated with an eigenvalue of one and a varimax rotation of the component loadings. The validity of the patterns of sEMG muscle activity was examined by comparison to the seven-item PPT, gait speed, and the ADL and IADL subscales of the FSQ. Construct validity of the sEMG measure was examined by describing the sEMG parameters in relation to the individual’s perceived difficulty of standing from a chair taken from the response
to one question from the FSQ. To determine the test-retest reliability of the sEMG parameters, Pearson Product Moment correlations were calculated. RESULTS sEMG data were available on 160 of the 171 women who participated. Eleven women were excluded from the study because they were unable to stand from the chair without using their arms (n 3) or because the sEMG equipment was unavailable at the time of testing (n 8). The mean (SD) age, height, and weight of the 160 women were 73.9 3.9 years, 158.7 7.3 cm, and 68.1 13.4 kg, respectively. All subjects had a higher peak muscle activity and a greater area under the curve with the standing component compared with the sitting component (Table 1). The time to peak muscle activity was longer for the sitting component of the task. The sEMG parameters were quite variable Table 1. Surface Electromyography Parameters Recorded During a Chair Stand Task Parameter Stand Peak, V Time, ms Area under the curve Sit Peak, V Time, ms Area under the curve Note: SD standard deviation.
Mean
SD
Range
228.5 369.5 96.8
112.7 364.1 68.9
46–888 28–3036.3 7.8–551.0
143.9 438.1 37.2
88.4 221.1 24.1
31–681 27.5–1092.3 3.3–151.9
SURFACE ELECTROMYOGRAPHY AND CHAIR STAND
Table 2. Principal-Components Factor Analysis for Surface Electromyography Variables: Rotated Factor Loadings Variable
Magnitude
Peak—stand Peak—sit Area under the curve—stand Area under the curve—sit Time—stand Time—sit
0.885* 0.834* 0.713* 0.842* 0.065 0.101
*Rotated factor loadings from principal-component factor analysis, representing 1 magnitude and factor 2 timing.
across subjects, as demonstrated by the relatively large range in the values and the large standard deviations. The exploratory PCFA identified two factors, which represent the magnitude and timing of the muscle activity (Table 2). The two factors explain 68.6% (magnitude 48.0%, timing 20.6%) of the variance in quadriceps muscle activity recorded during the chair stand task. Taken together, the magnitude and timing components of the quadriceps muscle activity can be summarized as one measure, the slope of the rise of muscle activity. To validate the sEMG as a measure of muscle performance, the women were divided into two groups (slope 1 or slope 1) on the basis of the slope of the rise of muscle activity during the standing component of the chair stand task (Table 3). Compared with women who had a slope 1, women with a slope 1 walked faster (1.17 m/s vs 1.09 m/s) and reported less difficulty with ADL (98.6 vs 95.8) and IADL (97.3 vs 92.2). Women who reported no difficulty with the chair stand task on the FSQ had a higher peak muscle activity with standing, a steeper slope with standing, and greater peak muscle activity with sitting compared with the women who reported some difficulty with the task (Table 4). The testretest reliability determined for the slope of the rise of muscle activity during standing was moderate (Pearson’s r .55, p .03). DISCUSSION We describe a new approach to measuring muscle impairment using sEMG during a functional task. On the basis of the factor analysis, a combination of the sEMG parameters representing the magnitude and timing of muscle activity (i.e., slope of the rise of the muscle activity) best ex-
Table 3. Physical Performance Measures by Slope of the Rise of Muscle Activity Parameter Gait speed, m/s Seven-item PPT score FSQ score ADL IADL
Table 4. Surface Electromyography Parameters by Perceived Difficulty of Chair Rise
Timing 0.0084 0.144 0.528 0.284 0.866* 0.527*
Slope 1 (n 65)
Slope 1 (n 95)
p Value
1.17 (0.17) 24.8 (2.2)
1.09 (0.22) 24.5 (2.2)
.02 .56
98.6 (3.7) 97.3 (6.4)
95.8 (8.1) 92.2 (11.7)
.003 .001
Notes: Values are means (standard deviations). PPT Physical Performance Test; FSQ Functional Status Questionnaire; ADL activities of daily living; IADL instrumental activities of daily living.
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Parameter Stand Peak, V Time to peak, ms Area under the curve Slope of rise Sit Peak, V Time to peak, ms Area under the curve Slope of rise
No Difficulty (n 129)
Some Difficulty (n 31)
p Value
237.0 (115.7) 331.6 (283.8) 95.1 (56.3) 1.3 (1.3)
193.0 (93.0) 527.2 (571.9) 103.8 (107.4) 0.6 (0.5)
.05 .07 .67 .001
151.1 (92.3) 438.8 (222.5) 38.4 (22.3) 0.5 (0.7)
114.2 (62.5) 435.2 (219.0) 32.0 (30.5) 0.4 (0.6)
.04 .94 .18 .42
Note: Values are means (standard deviations).
plained muscle performance during a functional task, which suggests that the absolute value of muscle activity (i.e., magnitude) is not the most representative characteristic of muscle’s role in the functional task but that a pattern of muscle activity may be more representative of muscle performance during chair stand. On the basis of the disablement model of the World Health Organization (13), we would expect biologic changes (i.e., changes in muscle recruitment) to occur prior to a disability (i.e., difficulty standing from a chair). To begin to understand if the sEMG measure during chair stand represents physical performance differently from the self-report, we explored the range and variance of the sEMG slope among persons with no difficulty in rising from a chair. In our sample of women performing at a high functional level, the women who reported having no difficulty with the chair stand task varied greatly on the slope of the sEMG activity with standing (mean 1.30, SD 1.32; range 0.12–7.51). Of the women who reported no difficulty standing from a chair, 45% had a slope less than the mean value of the slope of muscle activity for the women who reported difficulty standing from a chair (0.64). Gait speed, a commonly used measure to predict disability, did not vary as greatly as the slope of muscle activity with standing in women who did not report difficulty standing from a chair (mean 1.14, SD 0.19; range 0.48–1.63). Of the women who reported no difficulty with standing from a chair, 30% had a gait speed less than the mean gait speed for the women who reported difficulty standing from a chair (1.06 m/s). The women who reported no difficulty in chair stand but were identified by a low sEMG slope would not have been recognized as at risk for future disability on the basis of mean gait speed. Using the National Institute on Aging Short Physical Performance Battery (3,14), 97.7% of the women who reported no difficulty with standing from a chair in this study would have scored a 4 (highest quartile of scoring) on the gait component of the battery. Future studies are warranted to determine if individuals with a low slope of the rise of muscle activity with standing are at increased risk for future functional decline. Muscle recruitment, an impairment measure, may provide an earlier indicator of risk for functional decline than other physical performance measures.
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Acknowledgments This study was funded by a grant from the National Institute on Aging (Grant AG14753). At the time this work was completed Jennifer S. Brach was funded in part by the National Institutes of Health (Public Health Service Grant TG32AG00181), the Foundation for Physical Therapy, and the Geriatric Section of the American Physical Therapy Association. Data were presented at the American Physical Therapy Association, Combined Sections Meeting, New Orleans, LA, February 2000. Address correspondence to Jennifer S. Brach, PhD, PT, GCS, Department of Physical Therapy, University of Pittsburgh, 6035 Forbes Tower, Pittsburgh, PA 15260. E-mail:
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Received November 16, 2000 Accepted December 14, 2000 Decision Editor: John E. Morley, MB, BCh
