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Neural adaptations to fatigue: implications for muscle strength and training
D A Gabriel1, J R Basford, K N An
1Biomechanics Laboratory, Brock University, St. Catharines, Ontario, Canada L2S 3A1. dgabriel@arnie.pec.brocku.ca
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.
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