Abstract

In an attempt to evaluate central and peripheral contributions to muscle fatigue, changes in force development and electromyographic (EMG) signal after sustained isometric plantar flexion (IPF) were examined. Sustained IPF of maximal effort was continued until the contraction force was reduced to values of 50% of maximum voluntary contraction (MVC). Maximal IPFs of 3 s durations were done before (pre- IPF) and after (post-IPF) the sustained IPF. Post IPF was done by immediately finished sustained IPF. The contraction force and the EMG signals of the gastrocnemius in pre- and post-IPFs were measured, and the rate of force development (RFD) and the rate of EMG signal development (RED) were calculated. The evoked torque was electrically stimulated by posterior tibial nerve. The peak force, average force, RFDs in post-IPF were significantly smaller than those in pre-IPF. The root mean square and median power frequency (MdPF) of the EMG were not changed in post-IPF. The electrically evoked torque was increased after sustained IPF. These results suggested that the early phase of post-IPF fatigue was the almost purely muscular origin, while the late phase of IPF fatigue contained the influence of central nervous system, possibly due to the difficulty of large motor neurons recruitment after the sustained IPF.