Purpose: Prolonged exercise reduces the capacity of the neuromuscular system to produce force, which is known as fatigue. The purpose of this study was to examine the time course of neural and contractile processes during a 20-km running bout.
Methods: Eight experienced runners (mean ± SD: age = 31 ± 6 yr, V˙O2max = 60.1 ± 2.2 mL·kg−1·min−1) completed an all-out self-paced 20-km treadmill run. Isometric knee extensor torque and EMG responses of the vastus lateralis (VL) in response to percutaneous electrical stimulation and voluntary contraction were measured before and after 5, 10, 15, and 20 km of exercise.
Results: Participant's RPE, measured using the Borg 6-20 scale, increased steadily throughout the run to a value of 18 ± 1 at exercise termination. Maximal voluntary contraction (MVC) of the knee extensors only decreased during the final 5 km of running, with a 15% ± 12% (P = 0.02) decrease at 20 km. Vastus lateralis EMG during an MVC was reduced after 15 km (−18% ± 21%, P = <0.01) and 20 km (−20% ± 22%, P = 0.03). A significant correlation (r = 0.71, P = 0.048) was observed between the final reduction in MVC and the maximal EMG. Voluntary activation, estimated by the twitch interpolation technique, decreased by 13% ± 6% at 20 km (P = < 0.01), and this was significantly correlated (r = 0.70, P = 0.049) with MVC loss. There were no significant changes in the amplitude of the electrically evoked muscle action potential (M-wave) or potentiated twitch during or after the 20-km run.
Conclusions: A reduction in knee extensor MVC only occurs during the final 5 km of a 20-km self-paced run. Impaired voluntary activation and neural drive but not contractile processes are responsible for this decreased strength.