Neural adaptations to fatigue: implications for muscle strength and training

PURPOSE: This paper investigates the neural mechanisms responsible for the increase in strength that occurs during serial isometric contractions. METHODS: A three-session design was used. Thirteen subjects (N = 13) were asked to perform five maximal isometric elbow extension strength trials to serve as baseline. After a 5-min rest, the subjects were administered a 30-trial fatigue protocol. This process was repeated two more times at 2-wk intervals. Elbow extension torque and surface electromyography (EMG) of the triceps and biceps brachii were monitored concurrently. The criterion measures were elbow extension torque, root-mean-square EMG amplitude, and mean power frequency (MPF). RESULTS: Intraclass reliability ranged from good to excellent. Within each experimental session, the fatigue protocol resulted in a decrease in maximal isometric elbow extension torque as well as biceps and triceps EMG amplitude and MPF (P < 0.05). However, the mean of the 30 trials and the magnitude of the linear decrease in elbow extension torque increased across the three sessions (P < 0.05). Biceps and triceps EMG amplitude increased and MPF decreased as the number of sessions increased (P < 0.05). CONCLUSIONS: These findings suggest that the fatigue protocol served as a training stimulus to down regulate motor-unit firing frequency.