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1 Human Physiology Laboratory, Department of Health Sciences, Sargent College of Health and Rehabilitation Science, Boston University, Massachusetts.
2 Department of Physical Medicine and Rehabilitation, Spaulding Rehabilitation Hospital, Harvard Medical School, Boston, Massachusetts.
Address correspondence to Roger A. Fielding, Human Physiology Laboratory, Department of Health Sciences, Sargent College of Health and Rehabilitation Science, Boston University, 635 Commonwealth Ave., Boston, MA 02215. E-mail: fielding{at}bu.edu
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Abstract |
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Methods. Participants were men and women (n = 48; ages 65–91 years) with physical disability as evidenced by 2 or more deficits on the Medical Outcomes Study Short Form physical function subscale or a score of 9 or less on the Established Populations for the Epidemiologic Studies of the Elderly short physical performance battery. Muscle strength (1RM) was measured using a bilateral leg press exercise, and power output was determined by selecting the highest power output from 6 different contraction velocities: 40%, 50%, 60%, 70%, 80%, and 90% 1RM. Functional performance tasks consisted of habitual gait velocity (HGV) and stair climb (SC) and chair rise (CR) performance. Separate linear regression models were fit for each of the 3 dependent variables (SC, CR, HGV) using 1RM strength, power at 70% 1RM, and power at 40% 1RM as independent variables. All models were adjusted for age, body mass, and sex.
Results. Lower extremity power at 70% and 40% 1RM demonstrated greater associations with SC and HGV than did 1RM strength, whereas power at 40% 1RM demonstrated similar or stronger associations with all functional tasks compared with 1RM strength. Power at 40% 1RM explained the same or more of the variability in SC (R2 =.42 [regression coefficient = –.169 ±.06] vs.43 [–.206 ±.071]), CR (R2 =.28 [–.154 ±.057] vs.24 [–.152 +.070]) and HGV (R2 =.59 [.214 +.37] vs.51 [.223 ±.049]) compared with power at 70% 1RM. Power at 40% 1RM explained more of the variability in the lower intensity (HGV) compared with the higher intensity (SC or CR) functions.
Conclusions. Power output at 40% of 1RM explained more of the variability in HGV than did power at 70% 1RM, suggesting that measures such as HGV that require a lower percentage of maximal strength to perform might be more sensitive to differences in contraction velocity. Because HGV is highly predictive of subsequent disability, future studies should evaluate the determinants of muscle power output at low external resistances.
During dynamic concentric contractions, power output can vary based on the magnitude of force generated and the velocity of muscle shortening. We hypothesized that functions such as level walking that require a lower percentage of maximum force to perform might be more sensitive to differences in power output at low external resistances than differences in peak power or strength in older persons because of differences in the velocity of shortening during these tests.
The purpose of the current study was to explore the relationship between muscle power and functional performance by evaluating power output at low and high external resistances during bilateral leg press exercise in community-dwelling older persons. Because, during the performance of some functional tasks, such as level walking, the necessary force generated is low and the velocity of shortening is higher, we hypothesized that power at 40% 1RM (when shortening velocity is higher) would explain more of the variability in functional performance (chair rise, stair climb, habitual walking) than would power at 70% 1RM (when shortening velocity is lower). In addition, to confirm our previous observations, we evaluated the relationship between lower extremity functional performance and 1RM strength.
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METHODS |
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Study Population
Participants were recruited from the Boston area through advertisements in local senior citizens publications, visits to senior residences, and through the Harvard Research Cooperative on Aging volunteer database. Eligible persons had to be older than 65 years, living in the community, able to walk with or without an assistive device, and report 2 or more deficits on the physical function subscale of the Medical Outcomes Study Short Form (MOS SF-36) (12) or score 9 or fewer points on the Established Populations for the Epidemiologic Studies of the Elderly short physical performance battery (13). Eligible participants completed a medical history questionnaire and underwent a physical examination and medical screening by the study physician. Participants were excluded from participation if they had acute or terminal illness, myocardial infarction in the past 6 months, unstable cardiovascular disease or other medical conditions as assessed during the screening history and physical examination, upper or lower extremity fracture in the past 6 months, upper or lower extremity amputation, cognitive impairment according to the Folstein Mini-Mental State Examination (score 
23) (14), or current participation in regular, strenuous exercise sessions (more often than once a week). All participants gave written informed consent. The Boston University Institutional Review Board approved the study.
Participant Characteristics
Body mass was recorded on a standard platform scale to the nearest 0.1 kg with the participant fully clothed. Height was measured to the nearest 0.5 cm with a scale stadiometer. Psychosocial variables, including measures of cognition, depressive symptoms, and physical activity, were also assessed. Global cognitive function was assessed using the Folstein Mini-Mental State Examination, an instrument with scores ranging from 0 (severe dementia) to 30 (normal). The Geriatric Depression Scale is a questionnaire containing 30 yes/no questions administered to assess depression during the previous week (15). A score greater than 9 indicates the increasing severity of clinical depression in community-dwelling older persons. Habitual occupational, household, and leisure physical activity was estimated using the Physical Activity Scale for the Elderly (16,17). This scale is a valid and reliable instrument for assessing physical activity in elderly persons, with higher scores indicating higher levels of participation in physical activities (11). The numbers of medical diagnoses and daily medications were assessed via medical history questionnaires and the physical examination.
Outcome Measures
Muscle strength.--
Muscle strength of the lower extremities was quantitatively assessed by the 1RM measure of bilateral leg press using Keiser pneumatic resistance training equipment (Keiser Sports Health Equipment, Fresno, CA). The 1RM is defined as the maximum load that can be moved 1 time only throughout the full range of motion while maintaining proper form (18). The recumbent leg press apparatus was adjusted to provide support to the spinal column during 1RM assessments. In the starting position, the seat was positioned to place the knee joint between 90° and 100° of flexion. In this position, the hips were in a similar degree of flexion. Participants were instructed to extend both legs fully but without locking the knee joint. They performed the concentric phase, maintained full extension, and performed the eccentric phase of each repetition over 2, 1, and 2 seconds, respectively. The examiner progressively increased the resistance for each repetition until the participant could no longer move the lever arm 1 time through the full range of motion. The reliability of this measure was excellent as calculated using the intraclass correlation coefficient (ICC) (0.98).
Muscle power.—After the 1RM was measured, bilateral leg press peak muscle power was measured using the same pneumatic resistance machines used for 1RM testing. Measurement of muscle power tests using the pneumatic resistance machines has previously been described and validated by our laboratory and has excellent reliability in this population (19,20). Power was assessed at several percentages (40%, 50%, 60%, 70%, 80%, and 90%) of the 1RM. Beginning at 40%, participants performed the lift at each established percentage of 1RM as fast as possible through the full range of motion. Only 1 maximal attempt was made at each resistance level, with 30 to 45 seconds of rest given between each repetition. The software engineered for the testing equipment calculated work and power during the concentric phase of each repetition by sampling the system pressure (equivalent to force [piston area is constant]) and position 400 times per second. To eliminate noisy data generated at the start and end of the motion, data obtained between 5% and 95% of the concentric phase were used to calculate velocity and power. After each repetition, work, velocity, and power data were stored and then displayed on the output screen. The highest power achieved was recorded as the peak power. Because of changes in our technical capabilities, we were able to simultaneously measure both power output and velocity of movement from the computerized output in a subset of participants (n = 17) assessed in the latter part of the study, and those data are also reported. We selected power at 40% of 1RM and 70% of 1RM as outcome variables because they represented the external resistance at which the velocity was highest and the external resistance at which power output tended to peak, respectively. The measured power outputs at 40% and 70% of the 1RM were both excellent (ICCs = 0.85 and 0.88, respectively).
Functional Performance Measures
Stair climb.--
A standard 8-stair flight (stair height = 19 cm), with handrails on both sides, was used for this test. The participant was instructed to ascend the stairs as quickly and safely as possible. If necessary, the handrails could be used on either (preferred) side. The stopwatch was started when the participant moved his or her feet to begin climbing the stairs and was stopped when both feet landed on the top (eighth) step. Time was recorded to the nearest 0.01 second, and left/right handrail use was also noted. Two trials were attempted on the same day and the average was recorded. The reliability for this test is excellent (ICC = 0.97).
Chair rise time.-- A chair with arms and a seat height of 0.43 meters from the floor was placed against a wall for support and safety purposes. Each participant was instructed to sit in the chair with his or her back against the chair back, provided that both feet remained flat on the floor. Each was asked to place his or her arms across the chest. One completed chair rise was defined as moving from a starting seated position to standing fully upright and returning to the seated position. The participant was then instructed to perform 10 repetitions. The elapsed time to the nearest 0.01 second was recorded, as was whether the arms were used at any time during completion. As a result of participant fatigue, a single test was performed.
Habitual gait velocity.-- Habitual gait velocity was measured using an Ultra timer (DCPB Electronics, Glasgow, Scotland). Participants were fitted with an emitting device, with the examiner standing immediately behind them. They were instructed to walk away from the examiner at their normal walking velocity, "as if you were walking down the street to go to the store." Velocity was measured with a sonar transducer after the participant had walked 2 meters to avoid bias from acceleration and deceleration. These tests were each performed twice, and the average of two trials was used for analysis. The reliability for this test is excellent (ICC = 0.96).
Statistical Analyses
Descriptive statistics were calculated for all participants. Differences in power and velocity across varying external resistances were assessed using a one-way analysis of variance for repeated measures. When significance was found, post hoc comparisons were made with power and velocity at 70% 1RM. Significance was set at p <.05, with a Bonferroni adjustment for 5 multiple comparisons to p <.01. In the first part of the regression analyses, we fit 3 unadjusted linear regression models for each of the 3 dependent variables (stair climb time, chair rise time, and habitual gait velocity): 1 using 1RM as the independent variable, 1 using power at 70% 1RM as the independent variable, and 1 using power at 40% 1RM as the independent variable to confirm the relationships between impairment and functional performance. Next, we fit 2 unadjusted regression models for each of the 3 dependent variables: 1 using power at 70% 1RM as the independent variable and 1 using power at 40% 1RM as the independent variable. Each model was fit with a logarithmic curve to describe the nature of the relationship between impairment and function. Finally, we fit 2 final multivariate linear models for each of the 3 dependent variables, adjusting for the variables of age, body mass, and sex: 1 using power at 70% 1RM as the independent variable and 1 using power at 40% 1RM as the independent variable. Age, body mass, and sex were chosen as adjustment variables from a pool of variables including the Folstein Mini-Mental State Examination, Geriatric Depression Scale, the Physical Activity Scale for the Elderly, and medications per day due to their association (p <.15) with each of the functional performance measures. Because the impairment and function data were positively skewed, log transformations were performed on each dependent variable to obtain a normal distribution about the regression line, and then the independent variable was transformed to achieve linearity. Statistical significance for all regression models was accepted at p <.05. All data were analyzed using SPSS statistical software (Chicago, IL).
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RESULTS |
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DISCUSSION |
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We hypothesized that both skeletal muscle power at 70% 1RM and power at 40% 1RM would explain more of the variability in functional performance than would 1RM strength. In all unadjusted bivariate linear models run in the current study, leg power (both 70% and 40% 1RM) demonstrated a stronger relationship with stair climb time, chair rise time, and habitual gait velocity than did muscular strength. In the final adjusted model (age, body mass, and sex), leg power had a stronger relationship with stair climb time and gait velocity than did 1RM strength. Power output at the highest velocity of shortening that we measured (40% 1RM) showed similar or stronger associations with all functional tasks compared with 1RM strength. Although skeletal muscle strength has been shown to improve functional performance (5,24,25), the findings in the current study and others from our laboratory underscore the potentially greater influence of skeletal muscle power on functional performance than strength (22,23).
In addition, the relationship between skeletal muscle power and functional performance is best described as curvilinear. Both quadratic and logarithmic models explained a greater proportion of the variance in the 3 dependent variables (chair rise, stair climb, and gait velocity) than did the linear model. This finding is consistent with those of previous studies that have reported a curvilinear association of impairments such as strength (1,21,26) and peak power (22) with functional performance. The significance of the curvilinear relationship between muscle power and gait speed may be related to the concept of functional threshold (21). Functional threshold suggests that there is some level of function above which differences in a particular impairment (e.g., lower extremity power) have little effect on functional performance. In our study, power had a functional threshold in relation to gait speed. Beyond this threshold, persons with higher levels of power apparently do not walk incrementally faster.
Our second hypothesis was that power at 40% 1RM would explain more of the variability in tasks such as walking than would power at 70% 1RM. Unadjusted quadratic and logarithmic models and adjusted multivariate linear models showed that lower extremity power at 40% 1RM was a better predictor of functional performance than power at 70% 1RM, especially for the habitual gait velocity measure. This finding is interesting because of the highly predictive value of gait velocity on future disability. Guralnik and colleagues (27) found that gait speed alone, based on the Established Populations for the Epidemiologic Studies of the Elderly short physical performance battery, was a significant predictor of mobility-related disability and activities of daily living disability in community-dwelling older adults at both 1 and 4 years of follow-up. Strength and functional performance have been shown to be significantly related to some functional tasks, such as stair climb and chair rise performance (2–4), but not all, such as walking tasks (3,7,8). Our findings suggest that if skeletal muscle power at 40% 1RM is the best predictor of walking performance, then the velocity component (highest at 40% 1RM compared with power at 70% 1RM and 1RM) may be a more critical determinant than other impairment measures that are traditionally more related to strength. Thus, activities such as level walking that require a much lower percentage of maximal strength to perform compared with stair climb or chair rise might be more sensitive to reductions in velocity or power output at low external resistances with advancing age.
Potential limitations in the current study also must be considered. First, measures of 1RM and power were obtained using leg press, but associations between these measures and gait speed might have been greater had measures of ankle strength and power been included in the analysis. Second, the sample size was relatively small. A larger and more functionally diverse sample would have allowed further evaluation of the impairment–mobility relationships across varying magnitudes of the different measures of strength and power. Future studies could include measures of leg and ankle power as well as function in a larger cohort to examine these relationships.
Conclusion
Lower extremity skeletal muscle power obtained at high velocities (40% 1RM) explained as much, or more, of the variability in functional performance than did skeletal muscle power at 70% 1RM. Furthermore, lower extremity power at 40% 1RM explained a greater proportion of the variance in habitual gait velocity compared with either stair climb time or chair rise time. Future studies should address the effect of training at higher velocities on impairment measures such as strength and peak power and how these relate to various functional performance measures.
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Acknowledgments |
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Received January 23, 2003
Accepted June 6, 2003
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