The “disablement pathway,” as originally described by Nagi and adapted by Jette, progresses from impairments to functional limitations to disability.1,2 Functional limitations, as manifested by measures of lower extremity physical performance, have been shown to be predictive of disability, the most distal disablement outcome.3,4 As a result, many investigators have theorized that impairments in strength serve as a more proximal determinant of physical performance.5–7 It has been demonstrated that strength impairments are related to function as measured by a variety of physical performance measures, in a curvilinear pattern.6,8,9
More recently, it has been theorized that impairments in muscle power are more influential than strength on function.10 Muscle power is a related but different attribute than strength and is defined as the ability to perform muscular work per unit of time (power = work/time). In simpler terms, if strength is defined as the ability to exert force, power is defined as the ability to exert force quickly (power = force × velocity). Muscle power has been reported to be associated with nursing home resident's physical performance. Bassey et al. found that leg power, measured by using the Nottingham Leg Rig, was correlated with gait speed, chair-rise time, and stair-climb time.11 More recently, in community-dwelling older women, leg power was shown to be predictive of self-reported disability.12 The influence of muscle power on the physical performance of community-dwelling older people has yet to be demonstrated. Furthermore, although power has been theorized as potentially more influential on function than strength, the relative contribution of these attributes has yet to be elucidated.
Thus, the purpose of this study is to understand the influence of impairments in muscle power on the function (physical performance) of mobility-limited, community-dwelling older people. We hypothesize that muscle power is a predictor of function, as defined by physical performance, and that its influence on function is greater than that of muscle strength.
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The major finding of this study is that, in older people with mild to moderate mobility limitations, leg power is a significant predictor of physical performance. This is the first study of community-dwelling mobility-limited people to demonstrate that leg power impairments serve as a major determinant of function. The nature of this relationship appears to be curvilinear. Furthermore, though leg power and leg strength are related attributes being strongly correlated (r = .89), our results underscore differences as they relate to physical performance. Leg power, in comparison to strength, appears to exert a greater influence on function.
As previously stated, in mobility-limited older people, a relationship between function, as measured by physical performance, and disability has been demonstrated. Guralnik et al. have demonstrated that, in mobility-limited older people, the risk of disability, institutionalization, and mortality can be stratified based on SPPB score or habitual gait speed.4,16 Strength has been hypothesized as a primary physiological mediator on the disablement pathway.8 In contrast, our findings demonstrate that, within a cohort of older people with mild to moderate mobility limitations, leg power is different and exerts more influence than strength. In fact, using a curvilinear model, leg power generally predicted 15% to 50% of the variance in physical performance. Specifically in the case of habitual gait speed, in the unadjusted linear model, over one-quarter of the variance was predicted by leg power. This is a very significant finding, given that habitual gait speed is strongly predictive of disability, highlighting the importance of leg power as a proximal outcome on the disablement pathway. In reviewing all of the respective performance measures, it appears that, in comparison to strength, power may exert the greatest influence on stair-climb time, chair-stand time, tandem gait, and habitual gait. In contrast, the influence of power and strength on maximal gait appears to be more consistent.
The relationship between leg power and physical performance may be best described as curvilinear. The quadratic models explained more of the variance for all six dependent variables, although it only reached significance in three (stair-climb time, maximal gait, and SPPB). This finding is consistent with previous studies evaluating the relationship between motor impairments in strength and physical performance.6,8,9 The fact that the leg power quadratic models did not reach statistical significance for all of the functional outcomes can possibly be explained by an insufficient sample size. Other studies demonstrating a significant curvilinear relationship between strength and physical performance had sample sizes that were 4.5 to 20 times larger.6,8,9 In contrast, we did not find strong evidence for a curvilinear relationship between leg strength and the functional measures. It is possible that the point of change in slope for the regressions occurs at a different point for leg strength than it does for leg power. Along with larger sample sizes, the previous studies demonstrating curvilinearity included subjects who had little to no demonstrable functional limitations. By design, such subjects were excluded from our study. Taken together, these facts suggest that the changes in slope of the relationship between leg strength and physical performance occur at a higher functional level than for leg power. These results and hypotheses will need further confirmation through future research. Regardless, our findings underscore differences between leg power and leg strength as they relate to function.
Age, BMI, and number of chronic conditions (a measure of health status) were found to be significant adjustment variables for some of the physical performance measures. This is consistent with previous reports in similar populations.21 Additionally, in contrast to previous reports evaluating strength, adjusting for weight and gender did not influence the results.6,9,22 This finding, within a relatively small sample size, may reflect further differences between leg power and leg strength.
Despite the importance of its results, our study has limitations worthy of discussion. The relative homogeneity of the population places limits on the generalizability of the results. The majority of participants were college-educated Caucasians and represented a subset of mobility-limited older people. As stated previously, the failure to detect significant associations were likely due to our small sample size. Our methods should be applied to larger, more demographically and functionally diverse populations. As opposed to many previous studies that evaluated motor impairments, we did not examine specific measures of strength and power such as double leg press or right/left KE in our analysis.6,7 Instead we created composite scores of leg strength and power by summing all three measures. This represents a strength rather than a limitation of our methodology. Biomechanical analyses of gait and chair rise demonstrate that the performance of such tasks involve an interplay between muscle actions at both the hips and knees.23,24 Thus, our composite score better represents the diverse muscle actions composing our performance measures. Furthermore, this approach has been used by other investigators and is recognized as an enhanced means of statistically representing the relationship between impairment and function.9,22 We focused solely on lower extremity power and strength. The inclusion of upper extremity measures should be included in future investigations. Lastly, peak muscle power is not a measure easily obtainable on standard exercise equipment, thus limiting the translation of our findings to common clinical settings. However, using the specialized equipment available in this study, power measurements were no more challenging or time consuming than strength measurements. Recognizing the potential importance of measuring muscle power, in the future, greater availability of appropriate exercise equipment will be needed.
In conclusion, these findings from a cohort of community-dwelling mobility-limited older people identify impairments in leg power as important determinants of physical performance measures predictive of disability. Although related to leg strength, leg power is recognized as a separate attribute that exerts a greater influence on physical performance. These findings identify leg power as an important proximal determinant on the disablement pathway. They raise important questions for future geriatric and rehabilitative research.