Impact of muscle power and force on gait speed in disabled older men and women

Background. The purpose of this study was to explore the relationship between muscle power output at different external resistances and performance of functional tasks. The authors hypothesized that power at 40% skeletal muscle 1 repetition maximum (1RM), in which contraction velocity is high, would explain more of the variability in tasks such as level walking than would peak power or 1RM strength, in which contraction velocity is lower. 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% I RM, 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 (R-2 = .42 [regression coefficient = -.169 +/- .06] vs .43 [-.206 +/- .07]), CR (R-2 = .28 [-.154 +/- .057] vs .24 [-.152 + .070]) and HGV (R-2 = .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.