The Effects of a Resistance Training Program on Average Motor Unit Firing Rates

The purpose of this study was to examine the effects of an 8-week resistance training program on the relationship between average motor unit firing rates and recruitment threshold in the vastus lateralis. Eleven untrained men (mean ± SD age = 22.5 ± 4.2 years) volunteered to perform resistance training three times per week for 8 weeks. At the end of each week during the training, the subjects were tested for unilateral isometric strength of the dominant leg extensors. The subjects also performed a 6-second isometric muscle action at 80% of the maximum voluntary contraction while surface electromyographic (EMG) signals were detected from the vastus lateralis. The surface EMG signals were then decomposed into individual motor unit action potential trains, and the recruitment threshold and average firing rate for each motor unit were calculated. The results showed that despite increases in isometric strength, the resistance training program had no effect on the linear slope coefficient for the average motor unit firing rate versus recruitment threshold relationship. Thus, the resistance training did not bring the average motor unit firing rates of the high-threshold motor units up to a level comparable to those of the low-threshold motor units.