Central and peripheral fatigue kinetics during exhaustive constant-load cycling

The kinetics of central and peripheral fatigue development during an intensive constant-load cycling exercise was evaluated to better understand the mechanisms of task failure. Thirteen males cycled to exhaustion at 80% of maximal power output in intermittent bouts of 6 min of exercise with 4-min break between bouts to assess quadriceps fatigue with maximal voluntary contractions and single (1 Hz), paired (10 and 100 Hz) potentiated and interpolated magnetic stimulations of the femoral nerve (TwQ). Surface electromyographic signals (EMG) of the quadriceps muscles were recorded during stimulations and cycling. Total cycling duration (TCD) was 27 min 38 s+/-7 min 48 s. The mechanical response evoked by magnetic stimulation decreased mostly during the first half of TCD (TwQ1 Hz reduction: -34.4+/-12.2% at 40% TCD and -44.8+/-9.2% at exhaustion; P<0.001), while a reduction in maximum voluntary activation was present toward the end of exercise only (-5.4+/-4.8% and -6.4+/-5.6% at 80% TCD and exhaustion, respectively; P<0.01). The increase in quadriceps EMG during cycling was significantly correlated to the TwQ reduction for the rectus femoris (r2=0.20 at 1 Hz, r2=0.47 at 100 Hz, all P=0.001). We conclude that peripheral fatigue develops early during constant-load intense cycling and is compensated by additional motor drive, while central fatigue appears to be associated with task failure.