Changes in twitch contractile characteristics of plantarflexor muscles during repeated fatiguing submaximal static contractions

Changes in twitch contractile properties of human plantarflexor (PF) muscles during repeated fatiguing submaximal static contractions were studied in 22 young men. To determine the contractile properties of PF muscles the posterior tibial nerve in popliteal fossa was stimulated by supramaximal square wave pulses of 1 ms duration after maximal voluntary contraction (MVC) of 5 s duration (in post-activation potentiation condition). The fatiguing exercise consisted of three trials of submaximal static contractions (60% MVC force), which were performed with inter-trial rest intervals of 2 min. MVC force and twitch characteristics were measured in pre-exercise condition, after each trial and after 5 min and 10 min recovery period. MVC force decreased significantly after the 2(nd) and 3(rd) trial and then recovered to the initial level after 5 min of fatiguing exercise. It has been suggested that muscle fatigue in repeated submaximal contractions resulted both in a significant decline in potentiated twitch maximal force and a prolongation of twitch contraction and half-relaxation time. During the 10 min post-fatigue recovery period the twitch maximal force was significantly declined (post-contractile depression) and time course of twitch was shortened.