Fourteen male subjects (20-30 yr) accustomed to weight training went through progressive strength training of combined concentric and eccentric contractions three times per week for 16 wk. The training was followed by the 8-wk detraining period. The training program consisted mainly of dynamic exercises for leg extensors with the loads of 80-120% of one maximum concentric repetition. Significant improvements in muscle function were observed in early conditioning; however, the increase in maximal force during the very late training period was greatly limited. Marked improvements (P less than 0.001) in muscle strength were accompanied by significant (P less than 0.01) increases in the neural activation (IEMG) of the leg extensor muscles. The relationship between IEMG and high absolute forces changed (P less than 0.01) during the training period. The occurrence of these changes varied during the course of training. It was concluded that the early change in strength may be accounted for largely by neural factors with a gradually increasing contribution of hypertrophic factors as the training proceeds. It was suggested that the magnitudes and occurrence of these changes may vary due to the differences in conditioning periods, in individual muscles of muscle groups, in subject material, and in conditioning methods. During detraining, the decrease in muscle force seemed to be explainable also by the neural and muscular adaptations caused by the inactivity.