Effects of fatigue induced by neuromuscular electrical stimulation on postural control

The aim was to investigate the effects of fatigue induced by electrical stimulation superimposed onto voluntary muscular contractions on postural abilities and strategy (n=8). Two groups of subjects performed a fatiguing of the triceps surae, the first by voluntary muscular contractions (group VOL) and the second (n=8) by electrical stimulation superimposed onto voluntary muscle contractions (group VOL+ES). Balance was measured before and after the completion of both fatiguing tasks, by using a force platform which recorded centre of foot pressure (COP). Spatio-temporal COP parameters were used to evaluate the postural abilities, and a frequency analysis of the COP excursions (Fast Fourier Transform) was performed to estimate the postural strategy (low frequencies mostly account for visuo-vestibular regulation and high frequencies for proprioceptive regulation). In the post-fatigue condition, spatio-temporal COP parameters were not modified in either group. However, the frequency analysis showed that the high frequency band increased more in group VOL than in group VOL+ES. Calf-muscles fatigue did not impair postural control but it generated a change of the contribution of the proprioceptive information (myotatic loops) greater in group VOL than in group VOL+ES. Calf-muscles fatigue induced a modification of the postural strategy that may be exerted more strongly after voluntary muscular contractions than after electrical stimulation superimposed onto voluntary muscular contractions.