Central Regulation and Neuromuscular Fatigue during Exercise of Different Durations

The aim of this study was to determine if exercise time trials (TT) of different durations would cause different levels of peripheral and central fatigue during exercise.

Twelve trained subjects (11 men, one woman) performed TT lasting 3, 10, and 40 min with repetitive self-paced concentric right knee extension at 60°·s on an isokinetic dynamometer. Neuromuscular function was assessed before, during, and immediately after the TT using voluntary and electrically evoked forces.

Maximal voluntary contraction force, evoked peak force for single stimulus, and rating of perceived exertion reached similar levels at termination of all TT. Evoked peak force for paired stimuli of 100 Hz decreased more for the 40-min TT compared with the 3-min TT (-42% ± 15% vs -37% ± 13%, P < 0.05), and central fatigue was significant for the 40-min TT and 10-min TT but not for the 3-min TT. Single stimulus and paired stimuli of 100 Hz decreased, whereas voluntary electromyography normalized to M-wave for self-paced contractions increased during the end-spurt in all TT.

These data demonstrate that the extent of peripheral and central fatigue that contribute to reductions in force of single-limb dynamic contractions depend on the duration and intensity of self-paced exercise. There was no evidence for a critical threshold in peripheral fatigue that was common to all TT.