Improved tolerance of peripheral fatigue by the central nervous system after endurance training

The purposes of this study were to evaluate the effect of endurance training on central fatigue development and recovery. A control group was compared to a training group, which followed an 8-week endurance-training program, consisting in low-force concentric and isometric contractions. Before (PRE) and after (POST) the training period, neuromuscular function of the knee extensor (KE) muscles was evaluated before, immediately after and during 33 min after an exhausting submaximal isometric task at 15 % of the maximal voluntary contraction (MVC) force. After training, the trained group performed another test at iso-time, i.e., with the task maintained until the duration completed before training was matched (POST2). The evaluation of neuromuscular function consisted in the determination of the voluntary activation level during MVCs, from peripheral nerve electrical (VA(PNS)) and transcranial magnetic stimulations (VA(TMS)). The amplitude of the potentiated twitch (Pt), the evoked [motor evoked potentials, cortical silent period (CSP)] and voluntary EMG activities were also recorded on the KE muscles. Before training, the isometric task induced significant reductions of VA(PNS), VA(TMS) and Pt, and an increased CSP. The training period induced a threefold increase of exercise duration, delayed central fatigue appearance, as illustrated by the absence of modification of VA(PNS), VA(TMS) and CSP after POST2. At POST, central fatigue magnitude and recovery were not modified but Pt reduction was greater. These results suggest that central fatigue partially adapts to endurance training. This adaptation principally translates into improved tolerance of peripheral fatigue by the central nervous system.