Early Phase Adaptations in Strength, Muscle Size, and Co-activation Following Resistance Training in Untrained Women

The present study examined the effects of four days of training on peak torque (PT), muscle thickness (MT), and electromyographic co-activation (Co- Act). Ten untrained women (age=21.6±1.0 years) completed four days of maximal, reciprocal, concentric, isokinetic forearm extension (FE) and flexion (FF) training with six sets of 10 repetitions at 180°·s^-1. The subjects completed pre-training and post-training visits, which consisted of collecting FE and FF PT at 180°·s^-1, triceps brachii (TB) and biceps brachii (BB) MT, and FE and FF EMG Co-Act (ratio of antagonist to agonist). Separate two-way ANOVAs were used to analyze PT (Movement×Time), MT (Muscle×Time), and EMG Co-Act (Movement×Time). Individual changes from pre-training to post-training were compared to minimal difference (MD) values. For PT, a significant (p=0.040) main effect for Time indicated an increase from pre-training (19.53±5.45 N·m) to post-training (22.47±5.93 N·m). Ten and 20% of the subjects exhibited changes greater than the MD for FE and FF PT, respectively. There was no interaction (p=0.743) or main effect of Time (p=0.106) for MT. There was a decrease (p=0.011) in FF EMG Co-Act (-7.78%), whereas FE EMG Co-Act (+2.32%) did not change (p=0.598). Zero subjects exhibited changes greater than the MD for FF EMG Co-Act. The mean increases in PT were likely due to neural adaptations since there were no changes in MT. Although a mean decrease in FF EMG Co-Act suggested it may have contributed to mean increases in FF PT, none of the subjects expressed "real" changes (change values greater than the MD) in FF EMG Co-Act.