Contractile impairment after quadriceps strength training via electrical stimulation.

The purpose of this study was to investigate the neural and muscular changes associated with electrical stimulation (ES) training and subsequent detraining. Twenty healthy active men were randomized to receive (intervention group) or not (control group) 4 weeks of ES strength training followed by 4 weeks of detraining. Quadriceps ES training sessions (20 minutes per session, 4 sessions per week) were completed under isometric loading conditions. Quadriceps maximal voluntary contraction (MVC) strength, activation level, maximal electromyographic (EMG) activity, and excitation-contraction coupling properties were assessed before training, after training, and after detraining. Maximal voluntary contraction strength did not change after training but significantly increased after detraining (+21.5%; p < 0.05). Activation level (+7.3%) and maximal EMG activity (+27.9%) increased significantly after training and remained elevated after detraining (p < 0.05). Vastus lateralis M-wave amplitude did not change during the study period, whereas quadriceps contractile properties were significantly impaired after training but then recovered to pre-training values after detraining. We conclude that the maximal force-generating capacity of the quadriceps was unchanged after 4 weeks of ES strength training because of the interplay between neural (increased activation) and muscular (contractile impairment) changes. On the other hand, recovered contractile function and preserved activation after 4 weeks of detraining resulted in significant MVC strength increases. Quadriceps strength training via ES may induce overreaching and delayed adaptations and therefore should be used with caution. These findings may help in conceiving effective ES strength training programs for physically active subjects.