Steadiness During Plantar Flexion Is Associated With Postural Sway In Young But Not Elderly Adults

Abstract

Fluctuations in motor output during steady isometric contractions with a muscle or muscle group are often greater in elderly than young adults, but the functional significance of force fluctuations during isolated muscle contractions is not determined. PURPOSE: The present study examined the association between force fluctuations during isolated contractions with plantar flexor muscles and postural sway during quiet standing in young and elderly adults. METHODS: Twenty young (28.1 ± 4.0 yrs) and 20 elderly (69.7 ± 2.8 yrs) subjects performed a force-matching task with unilateral isometric plantar flexion (PF) at 2.5%, 5%, 10%, 15%, and 20% of subjects‘ maximal voluntary contraction (MVC) for 30 s. The knee joint was fully extended and the ankle joint was in the neutral position during PF. They also maintained quiet standing (QS) on a force platform for 30 s as a separate task. To quantify the variability of motor output, the coefficient of variation (CV) of force during PF and the CV of center of pressure displacement during QS were calculated. Surface electromyogram (EMG) was recorded from the soleus, medial gastrocnemius and lateral gastrocnemius muscles during PF and QS. Power spectrum density of the rectified EMG was calculated to characterize the muscle activation strategies. RESULTS: In young subjects, a significant (P < 0.05) positive correlation was found in the CV between QS and PF at lower contraction intensities (2.5% and 5% of MVC). There was no correlation between QS and PF at any contraction intensity in elderly adults. During PF, the EMG power around 10 Hz was significantly (P < 0.05) greater in the medial gastrocnemius in elderly than young adults at low contraction intensities (2.5% and 5% of MVC). There was no difference in the EMG power distribution in other muscles between young and elderly adults during PF. During QS, the EMG power around 10 Hz was significantly (P < 0.05) greater in all the muscles in elderly than young adults. CONCLUSION: The variability in postural sway during quiet standing was correlated to the force fluctuations during isolated plantar flexion at low contraction intensities in young but not elderly adults. The presence or absence of association between postural sway and force fluctuations may be influenced by the muscle activation strategy around 10 Hz. Supported by NIH Grant NS052480