Mechanism of fatigue during maximal cycling exercise

Numerous research models have been designed to investigate potential mechanisms leading to fatigue during short-term maximal exercise. However, the neuromuscular mechanisms responsible for fatigue are still a matter of debate and interest, particularly in a human exercising model. Two likely mechanisms include excitation/relaxation and force-velocity kinetics. In this study short cycle cranks (120 mm) were used to accentuate fatigue associated with muscle excitation/relaxation and long cranks (220 mm) were used to emphasize contractile (force-velocity) origins of fatigue. Fatigue index (peak power - min power / peak power) was used to quantify fatigue. Ten competitive cyclists (7 male, 3 female) cycled maximally for 30 seconds on crank lengths of 120 mm at 136 rpm and 220 mm at 110 rpm using an isokinetic cycling protocol. Power data (averaged over a complete revolution of the cranks) were recorded at 10 Hz with a power meter. Peak power did not differ between cranks (901 ± 309 W for 120 mm, and 898 ± 3 1 1 W for 220 mm,/? = 0.873). Fatigue index differed significantly between cranks (57.5 ± 8.4% for 120 and 51.1 ± 11.3% for 220, p < 0.01). Work performed also differed significantly between cranks (18.0 ± 5.5 kJ for 120 and, 19.2 ± 5.9 kJ for 220 p < 0.01). These results suggest that fatigue during a maximal short-term exercise stems mainly from kinetics of excitation and relaxation of the sarcomere. Improving fatigue resistance during maximal exercise may require improvements in the processes of excitation and relaxation.