Submaximal motor unit firing rates after 8 wk of isometric resistance training

Purpose: The purpose of this study was to test the hypothesis that average motor unit firing rates change in parallel with the contractile properties of vastus lateralis following 8 wk of isometric resistance training. Method s: The firing rates from more than 400 motor units of vastus lateralis were obtained during voluntary isometric contractions of 50% MVC, before and ag ain after training in male subjects (N = 10) and their untrained controls ( N = 10). Single motor unit spike trains were recorded with tungsten microel ectrodes. Results: Training resulted in a 36% (P < 0.05) increase in MVC. W e also found significant increases (P < 0.05) in maximal twitch amplitude ( +17%), time to peak tension (+9%) and the maximal instantaneous rate of con traction (+20%) in the trained leg of the experimental group. Neither the m aximal integrated EMG nor the rate of increase of integrated EMG was differ ent after training. There were no significant changes in any of these measu res from the untrained leg or the control group. Average firing rates were not different after training despite the increase in twitch contractile spe ed. Conclusion: These findings suggest that the control properties of the n ervous system are not altered despite sizable changes in the contractile pr operties of muscle following 8 wk of resistance training.