Explosive Force Production and Fractionated Reaction Time in Elderly Low and High Active Women

Abstract

A decrease in muscle mass and muscle power, as well as increased movement time with aging have been linked to increased fall risk and inability to perform activities of daily living. PURPOSE: The purpose of this study was to provide evidence that muscle power, the time course of muscle activation, and the neurological stimulation of muscle are directly related to physical activity patterns in older women. METHODS: 40 women between 65 and 84 years were assigned to High Active (HA, n=20) and Low Active (LA, n=20) groups based on a historical activity questionnaire and then performed a series of maximal, voluntary, isometric knee extensions. A reaction time task cued by a visual stimulus was used to measure total reaction time (RT), premotor time (PMT), motor time (MT), rate of torque development (RTD), peak torque (PT), and the rate (RER) and magnitude of initial EMG (oEMG). Torque output from the isokinetic dynamometer and surface EMG were sampled at 1000Hz using a data acquisition system and were analyzed offline using visual inspection of the data. RESULTS: Body mass normalized PT was 15% greater in the HA group (1.82 ± 0.48 Nm·kg-1)than in the LA group (1.54 ± 0.34 Nm·kg-1, p=0.042). Body mass normalized RTD was 26% greater in the HA group (9.4 ±3.1 Nm·s-1·kg-1) than in the LA group (7.0 ± 2.2 Nm·s-1·kg-1, p=0.008). There were no differences between the HA and LA groups for RT (.336 ± .04s, .361 ± .06s, p=. 12) or PMT (235 ± 40 ms, 244 ± 50ms, p=0.578). MT was 17% longer in the LA group (117 ± 20 ms) when compared to the HA group (100 ± 20 ms, p=0.006). The RER was 25% faster in the HA group (547 ± 172 %pEMG·s-1) than in the LA group (413 ± 50 %pEMG·s-1, p=0.039) and the normalized oEMG was 15% greater in the HA group (107 ± 32%) when compared to the LA group (91 ± 17%, p=0.048). CONCLUSION: These results indicate that older women with a history of vigorous activity can generate greater force, power, and motor output when rapid increases in muscle force are needed in comparison to their less active peers. This can be due to a number of factors including greater muscle mass, a better ability to recruit available motor units, increased motor unit firing rate, and lower fat mass relative to muscle mass in the active group. The lower body mass index of the HA group and 300% greater volume of activity are likely to account for the enhanced neuromuscular function seen in this group.