Relationship between neuromuscular fatigue, muscle activation and the work done above the critical power during severe-intensity exercise

Abstract

New findings: What is the central question of this study? Does the work done above critical power (W') or muscle activation determine the degree of peripheral fatigue induced by cycling time trials performed in the severe-intensity domain? What is the main finding and its importance? Peripheral fatigue increased when power output and muscle activation increased, whereas W' did not change between the time trials. Therefore, no relationship was found between W' and exercise-induced peripheral fatigue such as previously postulated in the literature. In contrast, we found a significant association between EMG amplitude during exercise and exercise-induced reduction in the potentiated quadriceps twitch, suggesting that muscle activation plays a key role in determining peripheral fatigue during severe-intensity exercise.

Abstract: In order to determine the relationship between peripheral fatigue, muscle activation and the total work done above critical power (W'), 10 men and four women performed, on separated days, self-paced cycling time trials of 3, 6, 10 and 15 min. Exercise-induced quadriceps fatigue was quantified using pre- to postexercise (15 s to 15 min recovery) changes in maximal voluntary contraction (MVC) peak force, voluntary activation and potentiated twitch force (QT). Voluntary activation was measured using the interpolated twitch technique, and QT was evoked by electrical stimulations of the femoral nerve. Quadriceps muscle activation was determined using the root mean square of surface EMG of vastus lateralis (VL_RMS ), vastus medialis (VM_RMS ) and rectus femoris (RF_RMS ). Critical power and W' were calculated from the power-duration relationship from the four time trials. Mean power output and mean VL_RMS , VM_RMS and RF_RMS were greater during shorter compared with longer exercise periods (P < 0.05), whereas no significant between-trial change in W' was found. The magnitude of exercise-induced reductions in QT increased with the increase in power output (P < 0.001) and was associated with mean VL_RMS, VM_RMS and RF_RMS (P < 0.001, r² > 0.369) but not W' (P > 0.150, r² < 0.044). Reduction in voluntary activation tended (P = 0.067) to be more pronounced with the lengthening in time trial duration, whereas no significant between-trial changes in MVC peak force were found. Our data suggest that peripheral fatigue is not related to the amount of work done above the critical power but rather to the level of muscle activation during exercise in the severe-intensity domain.

Keywords: critical power; exercise performance; muscle activation; neuromuscular fatigue; recovery from fatigue.