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1 Department of Physical Medicine and Rehabilitation, Harvard Medical School, Spaulding Rehabilitation Hospital, Boston, Massachusetts.
2 Hebrew Rehabilitation Center for Aged, Boston, Massachusetts.
3 Laboratory of Clinical Epidemiology, INRCA Geriatric Department, Florence, Italy.
4 Epidemiology, Demography and Biometry Program
5 Longitudinal Studies Section, Clinical Research Branch, Gerontology Research Center, National Institute on Aging, Bethesda, Maryland.
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Abstract |
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Methods. An analysis was conducted using data from the InCHIANTI study, a population-based cohort of 1032 adults living in Italy. To assess the relationships between impairments in power and strength and mobility performance, we created separate multivariate linear and log-transformed regression models as well as separate logistic regression models.
Results. Subjects were age 
65 years, predominately female (54%), with a mean age of 74.2 years, and most had mild–moderate mobility limitations (Short Physical Performance Battery score 10.5 ± 2.1). Though leg extension power and isometric hip extension strength were closely associated, in several separate multivariate linear regression models, leg power consistently explained more of the variance than strength did in several measures of physical performance. Differences were even larger when observed in curvilinear models of power and strength. Using separate multivariate logistic regression models to examine the odds ratios for mobility limitations in persons with low muscle power versus high muscle power, and in persons with low muscle strength versus high muscle strength, we found that both factors influenced risk for mobility limitations, but low power was associated with a 2–3-fold greater likelihood than low strength.
Conclusion. These findings identified muscle power to be a more influential proximal determinant of physical performance than impairments in strength and emphasized muscle power as an important determinant of mobility skills in older adults.
Muscle power reflects the ability to generate muscular work per unit of time, and is more simply understood as the product of force and velocity (power = force x velocity). Though related to muscle strength, muscle power is a separate attribute declining more precipitously after age 50 (9). Theoretically, muscle power may be related to mobility in many ways such as rapidly generating force to maintain balance following a perturbation or while performing a time-dependent task such as crossing a street before the light changes. Prior investigations have shown that impairments in muscle power are important factors limiting mobility in nursing home residents (10,11) and in community-dwelling elders (12–14). These more recent studies evaluating both muscle power and strength impairments and their relation to important mobility tasks have suggested that muscle power may be a more critical attribute than strength (12–14). In one of these investigations, evidence suggested that, like strength, the relationship between muscle power and mobility performance is best characterized as curvilinear (14). A limitation of these investigations was that they were conducted in relatively small groups of subjects limiting the scope of the analyses and the generalizability of their results.
The InCHIANTI study is a large population-based study of elders living in the Tuscany (Chianti) region of Italy. The study's purpose is to identify factors that underlie limitations in mobility among older adults living in the community (15). This longitudinal study has over 1400 subjects (n = 1453) with an excellent response rate of >90%. We evaluated baseline data from the InCHIANTI study in order to investigate the respective influences of muscle power and muscle strength on mobility performance among community-dwelling older adults. We hypothesized that (a) impairments in muscle power would describe more of the variance in mobility performance than would impairments in strength; (b) that these relationships would be curvilinear; and (c) that the likelihood of having moderate-to-severe mobility limitations would be greater for persons with low muscle power than those with low muscle strength.
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Methods |
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65 (n = 1032), and excluded any subjects who did not complete hip strength, knee strength, and leg power testing, leaving 839 subjects for the purposes of our analyses. A full description of the InCHIANTI methods are presented elsewhere (15); however, a brief description of the measures we utilized to represent muscle strength and power, mobility, and adjustment variables are described below. Leg power, hip strength, and knee strength served as our measures of impairment. Leg power was obtained via a leg power rig as described by Bassey (17). Subjects sat in a chair and unilaterally depressed a foot lever, in the horizontal plane, attached to a flywheel. Power output was derived from the acceleration of the flywheel and was recorded in Watts. Isometric hip extensor strength and knee extensor strength were measured via a handheld isometric dynamometer. This method of strength measurement has been demonstrated to be both reliable and valid for the purposes of research (16). For all strength and power measures, the highest of 2 repetitions was utilized. Though each measurement was obtained bilaterally, the correlation coefficients between sides were considered excellent (r =.89–.93), so only data recorded from the right leg was used for the analyses.
Stair climb time, total Short Physical Performance Battery (SPPB) score and each of the components of the SPPB (habitual gait speed, balance, and chair rise time) served as functional measures of mobility performance. Stair climb time was measured as the fastest time of 2 trials to climb a single flight of stairs. The SPPB is a composite score of performance [0–12] based on 3 functional tasks (18). On each task, subjects can score between 0–4, with the higher values representing the best performance. To measure walking speed, 2 photocells connected with a recording chronometer were placed at the beginning and the end of 4-meter course established at the study clinic. Participants were instructed to stand with both feet touching the starting line and to begin walking at their usual pace after a verbal command. The time between the activation of the first and the second photocell was recorded. The average of 2 walks was used to compute a measure of walking speed. Use of aids (canes or walkers) was allowed for this test. Chair rise represented the time to complete 5 chair rises as quickly as possible. As a balance measure, the SPPB balance component score was used with responses ranging between 0–4. Based on their subsequent risk for disability, mobility limitations have been characterized as being mild (score 10), moderate (score 7–9), and severe (score 4–6) (18).
Age, weight, height, and gender are common covariates with the potential to influence the relationships between our independent and dependent variables, and served as our adjustment variables.
A 4-stage statistical analysis was performed. First, mean values and the distribution for all variables were determined. Second, to evaluate the relationship between our impairment (hip strength, leg strength, and leg power) and mobility measures (SPPB, stair climb time, gait speed, balance, and chair rise), 15 separate multivariate linear regression analyses were conducted utilizing each impairment measure as the independent variable and each mobility measure as the dependent variable. Third, to evaluate whether the relationship between impairment and mobility measures could be characterized as curvilinear, we created both quadratic and log-transformed multivariate regression models (15 separate models for each type of analysis). We compared the separate quadratic and log transformed models evaluating adjusted R2 and AIC (Akaike's information criterion) values as well as visually inspecting residuals of the squared and log terms. We determined that the log-transformed models fit the data best and utilized those for our analysis. Fourth, to contrast the magnitude to which worsening impairment levels were associated with poor mobility, we performed separate logistic regression models. Using SPPB score 
9 as the dependent variable and dividing each impairment variable into quartiles (with the highest quartile as the reference value), odds ratios (OR) for predicting the dependent variable were calculated using 95% confidence intervals (CI).
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10 (n = 662) as the dependent variable in the first model and SPPB score 9 (n = 177) as the dependent variable in the second model. For all 3 impairment measures, there was an approximate twofold difference in slope of the relationship between muscle power or strength and mobility (as measured by the SPPB) among subjects with poorer mobility, compared with the subgroup with good mobility (hip strength, ß =.048 vs.029; knee strength, ß =.045 vs.025; leg power, ß =.008 vs.004).
Logistic regression models were employed to evaluate poor physical performance (SPPB 9) according to quartiles of leg power, hip strength, and knee strength. As seen in Figure 1, within all 3 categories of impairment, approximately one half of all subjects with poor performance were in the lowest category (leg power 54.6%, hip strength 50.0%, and 51.7% knee strength) and approximately one quarter were in the second lowest category (leg power 27.8%, hip strength 26.7%, and knee strength 21.6%). Adjusted ORs for the separate models are presented in Figure 2. The ORs for poor performance in the lowest quartile of leg power (OR 8.9, 95% CI 4.0, 20.1) are two- to threefold higher when compared with the lowest quartile of hip strength (OR 2.9, 95% CI 1.5, 5.6) and knee strength (OR 4.5, 95% CI 2.2, 9.2). Similar data were found with regard to the second lowest quartile.
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Discussion |
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The differences between leg power and leg strength within separate linear regression models are consistent with our prior investigations in mobility-limited elders utilizing different methods of obtaining leg power and leg strength (pneumatic exercise equipment) (14). Consistent with prior investigations evaluating leg strength and performance of mobility tasks (6–8), the relationship between the impairment and functional measures were best characterized as curvilinear. In all cases, curvilinear models accentuated the differences between leg power and leg strength, with the exception of chair rise where the variance explained by leg power and hip strength were equivalent in both linear and curvilinear models.
There are a variety of factors that may explain the lack of difference between strength and power in their association with chair rise. These findings suggest that the performance of this task is more related to force production, the common component of power and strength, than to the rate at which that force is produced (velocity), which is represented as a component of power. Biomechanical analyses of a single chair rise may help to clarify the importance of leg strength as measured through lower extremity joint torques (19). The performance of 5 chair rises may reflect more of a strength endurance test negating the influences of shorter-term differences in rate of force production between subjects. This is supported in that similar blunting of differences between strength and power at the knee and hip has been seen with longer duration tasks such as the 6-minute walk test (20). Additionally, these findings may be related in part to our measure of leg power. In our previous report, utilizing similar statistical methodology, leg power described more of the variance in chair rise time than did strength (leg power R2 =.24 vs leg strength R2 =.20) (14), yet this study used measures of peak leg power measured at 
70% of the 1-repetition maximum. By design, Bassey's leg rig does not provide the ability to know at which percentage of the 1-repetition maximum an individual's power is being measured. Though never formally compared with other methods of obtaining lower-extremity power in frail elders, anecdotal experience suggests that the leg rig may represent power generation at relatively lower forces, where velocity is higher, whereas chair rise may require muscle power production at higher forces with lower velocity, weakening their association. Further exploration of the relationships between different mobility tasks and muscle power generation at low versus high forces would help clarify such speculation.
In contrast to chair rise, the highest associations between functional performance and muscle power and the greatest differences between muscle power and strength were seen with habitual gait speed and stair climb time. Therefore, it is possible that, from a clinical perspective, gait speed and stair climb performance may be a surrogate means of identifying muscle power capacity, perhaps useful in an ambulatory care or home setting. Such hypotheses would be worth confirming through future clinical investigations.
There are a number of physiologic mechanisms by which declines in muscle power may occur. Age-related changes in skeletal muscle including drop-out of type 2 muscle fibers (21) and qualitative changes in skeletal muscle fiber force production properties (22,23) may contribute. Potential neurologic factors include the age-related decline in motor unit firing rate (24) and alterations in agonist/antagonist coactivation during multijoint actions (25). Though yet to be appropriately elucidated, further inquiry into the subsystems underlying declines in muscle power with aging is certainly warranted.
Potential limitations of our study exist. We conducted a cross-sectional analysis of cohort data. In the future, the respective roles of muscle power and strength impairments in predicting incident mobility limitations and disability will be more clearly defined through analyses of longitudinal data, which would more strongly support causal relationships. It should also be pointed out that, based on their SPPB score and habitual gait speed, the InCHIANTI participants are a relatively high-functioning cohort manifesting mild mobility limitations. This may reflect differences in baseline physical activity and availability of health services between community-dwelling older adults in Italy and the United States as well as the chosen methods for obtaining gait speed.
Differences between strength and power as discriminating factors for mobility performance are likely greatest at the lower level of functional performance. The fact that the cohort was at such a high functional level may have reduced our ability to discriminate larger differences between leg power and leg strength within the linear and log-transformed regression models. Given the age of the InCHIANTI participants, longitudinal data are likely to reveal greater heterogeneity due to functional decline with aging. In order to address more-specific questions regarding strength and power among those individuals with poorer mobility performance, future investigations using follow-up data or within more functionally diverse populations would be helpful. Lastly, as opposed to leg press, it has been suggested that more distal power generation at the ankle and knee may be more closely related to mobility performance as it pertains to balance and gait (20,26,27). Future investigations comparing power and strength at these sites may demonstrate that our findings are in fact conservative estimates of the differences between muscle power and strength.
To our knowledge, this is the first investigation to quantify the respective influence of impairments in leg power and leg strength in their association with poor mobility performance. The SPPB is predictive of disability, institutionalization, and mortality (4,18,28). Scores of 9 or lower have been characterized as moderate-to-severe mobility limitations (18), imparting a high risk for subsequent disability. Low strength is recognized as an important impairment contributing to mobility limitations (as defined by the SPPB) and leading to subsequent disability (29,30). The recognition that low power exerts a two- to threefold greater risk for mobility limitations emphasizes muscle power impairments as important proximal factors associated with and possibly leading to disability. It underscores the importance in developing interventions for mobility-limited elders, which accentuate muscle power such as resistance training, which is conducted with a "high-velocity" component to the repetitions, therefore accentuating both force production and velocity of movement. Additionally, this study serves as an important rationale for investigations of the physiologic mechanisms that underlie changes in muscle power generation with aging.
Conclusion
Through linear, curvilinear, and categorical statistical models, impairments in muscle power were identified as more-influential proximal determinants of mobility performance than were impairments in strength. These findings have important implications for future physiological and disablement research addressing the needs of mobility-limited elders.
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Acknowledgments |
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Address correspondence to Jonathan F. Bean, MD, MS, Geriatric PM&R, Hebrew Rehabilitation Center for Aged, 1200 Centre Street, Boston, MA 02131. E-mail: bean{at}mail.hrcs.harvard.edu
Received January 28, 2003
Accepted March 10, 2003
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