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- MATERIALS AND METHODS
OBJECTIVES: To test the hypothesis that peak power of the ankle flexors is related to physical functioning in older women with functional limitations.
DESIGN: A cross-sectional study.
SETTING: University-based human physiology laboratory.
PARTICIPANTS: Thirty-four older women (75.4 ± 5.1 years, 67.8 ± 11.3 kg, body mass index 27.4 ± 4.5) with self-reported functional limitations.
MEASUREMENTS: Plantarflexion (PF) and dorsiflexion (DF) peak power and isometric strength with physical performance (stair climb time, repeated chair rise time, maximal and habitual gait velocity) were determined. An isokinetic dynamometer was used to measure isometric strength, isokinetic peak torque and power of PF and DF at five angular velocities (30°, 60°, 90°, 120°, and 180°·sec−1), and isometric strength.
RESULTS: Peak torque for both PF and DF declined with increasing velocity of movement (PF: P < .0001; DF: P < .0001), whereas peak power increased with increasing velocity up to 120°·sec−1. The strongest univariate associations were found between chair rise time and DF peak power (r = 0.50; P < .002), stair climb time and DF peak power (r = 0.49; P < .003), habitual gait velocity and PF isometric strength (r = 0.53; P < .001), and maximal gait and PF isometric strength (r = 0.47; P < .005). Multivariate regression analysis revealed that DF and PF peak power along with the physical functioning and general health scores from the Medical Outcomes Study Short Form were independent predictors of chair and stair climb performance.
CONCLUSION: These data suggest that ankle muscle power together with self-reported measures of health and physical functioning are essential components of functional mobility in older women with functional limitations.
Advancing age is associated with a progressive decline in skeletal muscle mass.1 Concomitant with this decline in skeletal muscle mass have been observed changes in exercise capacity, including declines in cardiorespiratory fitness,2 muscle strength, and peak power.3
The projected increase in the older population, especially women, together with the observation that women are more likely to become disabled as they age, have made understanding age-associated functional limitations in women an important public health issue.4,5 Several studies have established the role of lower extremity muscle strength (maximum force generating capacity) as a predictor of functional limitations in disabled and nondisabled older people.6,7 It has been proposed that peak muscle power or the maximum capacity to perform muscular work per unit time is a more critical variable than strength in helping to understand the relationship between muscle impairments, functional limitations, and subsequent disability. Peak muscle power has been shown to decline earlier and more precipitously with advancing age.3 Peak lower extremity power has also been associated with functional limitations and falling risk in institutionalized older people.8,9 Recently, peak power of the leg extensor muscles was identified as an independent predictor of self-reported disability using the Functional Status Survey in community-dwelling frail older women.10
The plantarflexor (PF) and dorsiflexor (DF) muscles of the ankle play an important role in functional activities, including generation of torque during gait and chair rising.7,11,12 In addition, low peak torque of the PF muscle has been linked to falling risk in institutionalized older individuals.8 However, no studies have examined the relationships between PF and DF strength and peak power and performance-based functional tasks in older individuals.
Therefore, the primary objective of the present study was to establish the predictors of performance-based functional tasks among independent, community-dwelling older women with self-reported functional limitations. We specifically tested the hypotheses that strength and power of the ankle PF and DF muscle groups would be independent predictors of performance-based functional tasks—stair climb time, chair rise time, and maximal gait velocity.
- Top of page
- MATERIALS AND METHODS
The main findings of the present study were that PF power was an independent predictor of chair rise performance, and DF power was an independent predictor of stair climb performance. Interestingly, neither PF nor DF strength was an independent predictor of chair rise or stair climb performance, but PF strength was an independent predictor of both measures of gait quality (habitual and maximal gait velocity). In addition, both the physical functioning and general health subscales of the MOS SF36 were equally strong independent predictors of function.
DF peak power had the highest degree of univariate association of any of the factors with the functional performance measures. Stepwise regression models revealed that PF peak power was a significant independent predictor of chair rise time and DF peak power was a significant independent predictor of stair climb time. Both of these observed relationships are consistent with the biomechanical involvement of the PF in chair rising functions7 and the DF in stair climbing.19 Reduced PF and DF power has been linked to an increased risk of falling in institutionalized older individuals.8 Interestingly, when entered into the same regression model, neither PF strength nor DF strength was identified as an independent predictor of stair climb and chair rising functions. This suggests that both the force and velocity-generating components of ankle movement are important factors related to stair climbing and chair rising performance. Peak power of the leg extensor muscles has previously been shown to correlate with chair rising, stair climbing, and gait functions in frail institutionalized men and women.9 Furthermore, we have recently noted that peak power of the leg extensors is an independent predictor of functional status in community-dwelling frail older women.10 Peak muscle power may be a more critical variable than muscle strength in explaining decrements in physical function in older people because of its well-described rapid decline with advancing age.3,20 The present study extends this work by demonstrating that peak power of the ankle flexors is correlated to function and is an important independent predictor of performance of specific functional tasks (e.g., stair climbing and chair rising).
PF isometric strength and balance were independent predictors of habitual gait velocity, whereas PF strength was the sole independent predictor of maximal gait velocity. These results are consistent with reports on the relationship between knee extensor strength, balance, and gait speed in older women with severe walking disability.6 Gait speed alone has also recently been shown to be a good predictor of subsequent disability in two large cohorts of older individuals.21 The present data suggest that peak power of the ankle flexors is not predictive of gait velocity and this may be explained by the relatively slow velocity of movement achieved even during maximal gait in our study population.
Self-reported measures of functional limitations were also related to our measures of functional performance. Specifically, the physical functioning and general health subscales were independent predictors of physical performance in our study population. These data are consistent with previous reports on the relationships between impairments, function, and disability in older people.22,23 Other psychosocial variables may in fact have been related to function in older women with self-reported disability. However, our analysis of these relationships is limited by our relatively small sample size.
Linear associations were observed between measures of ankle power and strength and several of the tests of functional performance we employed. Although previous studies have observed a curvilinear relationship between impairment and function,22,24 this apparent inconsistency may be a result of the relatively small sample size employed in the present study. In addition, the specific measures of impairment and function utilized, and the relatively narrow range of age and level of functional limitations among participants in the present study, may have also accounted for these differences.
The absolute peak torques and the declines in peak torque observed with increasing velocity were similar to previous studies on older individuals.25,26 In addition to observations regarding the relationships between ankle power and function, the reliability of isokinetic and isometric measures of ankle torque and power was consistent with previous reports in young and older individuals and ranged from good to excellent.27–29 Furthermore, the reliability of our physical performance measures was also good to excellent, confirming a previous report on the suitability of performance-based measures in older individuals.30
In summary, the results of this study support the concept that muscle power generated by the ankle DFs and PFs are important predictors of stair climb and chair rise performance. These data suggest that ankle muscle power is an essential component of functional mobility in older women with functional limitations. In addition, for specific functional tasks such as gait, isometric strength of the ankle PFs is a strong independent predictor. Furthermore, self-reported measures of general health status and physical functioning were also predictors of functional performance. In addition to strategies aimed at improving general health, function, and strength, interventions targeted at improving function in older people should include exercises designed to increase peak power of the ankle PFs and DFs.