Changes in neuromuscular performance in voluntary and reflex contraction during strength training in man

Abstract

Eleven male subjects went through heavy resistance strength training 3 times a week for 16 weeks. The training program consisted mainly of dynamic exercise for the knee extensor muscles with loads of 80%-120% of one maximum repetition in the squat lift. The investigation was undertaken to examine effects of strength training on neuromuscular performance both in voluntary and reflex contractions with special interest in their possible relationships. In addition to a large (P less than 0.001) increase in maximal isometric force and a more (P less than 0.05) economical activation of the knee extensor muscles, significant (P less than 0.05) improvements were noted in isometric force-time parameters, which were related (P less than 0.05) to the increase in the fast-twitch/slow-twitch (FT/ST) muscle fiber area ratio. No changes were observed in reflex time components, but the relative change in reflex electromechanical delay (EMD) was related (P less than 0.05) to the relative change in the FT/ST area ratio. A significant (P less than 0.05) decrease in the peak-to-peak amplitude of the reflex electromyogram (EMG), was noted during the training, and a decrease (P less than 0.05) in reflex EMG/force ratio was related (P less than 0.02) to the change in maximal integrated electromyogram (iEMG)/force ratio of the voluntary contraction. The decrease of reflex EMG may indicate a change in sensitivity of the muscle spindle. The interrelationship between the changes in EMG/force ratios of the reflex and voluntary contractions suggests that the mechanical response of individual muscle fibers of the respective motor units has improved.