Training and detraining adaptations in electromyographic, muscle fibre and force production characteristics of human leg extensor muscles, with special reference to prolonged heavy resistance and explosive type strength training

Abstract

To investigate prolonged training-induced changes in electromyographic, muscle fibre and force production characteristics of leg extensor muscles and in serum hormones, eleven trained male subjects went through heavy resistance (loads between 70 and 120 %) and ten comparable male subjects through explosive type (jumping exercises with low loads) strength training for 24 weeks. Great increase in maximal strength (26.8-30.2 %, p<.001) during heavy resistance strength training was accompanied by significant (p<.05-.01) increases in the neural activation (IEMG) of the trained muscles during the periods with higher training loads. Explosive force development was slight during the entire training. Large muscular hypertrophy of especially FT fibre type (p<.001) occurred primarily during the first half of the training. Individual alterations in serum testosterone/cortisol ratio correlated (p<.01) with changes in muscular strength during the last training month. Great (24.1-32.5 %, p<.001) improvement in rapid force production during explosive type strength training correlated (p<.05-.001) with significant (p<.05-.01) increases noted in the neural activation of the trained muscles observable both during rapid isometric and concentric contractions and during high velocity stretch-shortening cycle exercises. Maximal strength development and hypertrophic changes were relatively slight. Selected decreases (p<.05-.001) occurred in IEMG, muscle fibre areas and in muscular strength during the following detraining periods for 12 weeks. The present findings indicate that training-induced enhancements in various aspects of force production are explainable by specific adaptations in the neuromuscular system and in endogenous hormone balance taking place in part with regard to type, loading and duration of strength training and perhaps to subject material.