To investigate the influence of explosive type strength training on isometric force- and relaxation-time and on electromyographic and muscle fibre characteristics of human skeletal muscle, 10 male subjects went through progressive training which included primarily jumping exercises without extra load and with light extra weights three times a week for 24 weeks. Specific training-induced changes in force-time curve were observed and demonstrated by great (P less than 0.05-0.01) improvements in in parameters of fast force production and by a minor (P less than 0.05) increase in maximal force. The continuous increases in fast force production during the entire training were accompanied by and correlated with the increases (P less than 0.05) in average IEMG-time curve and with the increase (P less than 0.05) in the FT:ST muscle fibre area ratio. The percentage of FT fibres of the muscle correlated (P less than 0.05) with the improvement of average force-time curve during the training. The increase in maximal force was accompanied by significant (P less than 0.05) increases in maximum IEMGs of the trained muscles. However, the hypertrophic changes, as judged from the anthropometric and muscle fibre area data, were only slight during the training. It can be concluded that in training for fast force production considerable neural and selective muscular adaptations may occur to explain the improvement in performance, but that genetic factors may determine the ultimate potential of the trainability of this aspect of the neuromuscular performance.