Central & peripheral fatigue in male cyclists after 4, 20 & 40 km time-trials

Abstract

Purpose: Few studies have assessed neuromuscular fatigue after self-paced locomotor exercise; moreover, none have assessed the degree of supraspinal fatigue. This study assessed central and peripheral fatigue after self-paced exercise of different durations. Methods: Thirteen well-trained male cyclists completed 4 km, 20 km and 40 km simulated time-trials (TTs). Pre- and immediately post-TT (< 2.5 min), twitch responses from the knee-extensors to electrical stimulation of the femoral nerve and transcranial magnetic stimulation of the motor cortex were recorded to assess neuromuscular and corticospinal function. Results: Time to complete 4 km, 20 km and 40 km was 6.0±0.2 min, 31.8±1.0 min and 65.8±2.2 min, at average exercise intensities of 96%, 92% and 87% of V̇O2max, respectively. Exercise resulted in significant reductions in maximum voluntary contraction, with no difference between TTs (–18%, –15% and –16% for 4, 20 and 40 km respectively). Greater peripheral fatigue was evident after the 4 km (40% reduction in potentiated twitch) compared to the 20 km (31%) and 40 km TTs (29%). In contrast, longer TTs were characterized by more central fatigue, with greater reductions in voluntary activation measured by motor nerve (–11% and –10% for 20 km and 40 km vs. –7% for 4 km) and cortical (–12% and –10% for 20 km and 40 km vs. –6% for 4 km) stimulation. Conclusions: These data demonstrate fatigue after self-paced exercise is task-dependent, with a greater degree of peripheral fatigue after shorter, higher intensity (~6 min) TTs and more central fatigue after longer, lower intensity TTs (>30 min).