HÄKKINEN, K., ALÉN, M. & KOMI, P.V. 1985. Changes in isometric force‐ and relaxation‐time, electromyographic and muscle fibre characteristics of human skeletal muscle during strength training and detraining. Acta Physiol Scand 125, 573–585. Received 26 January 1985, accepted 9 May 1985. ISSN 0001–6772. Department of Biology of Physical Activity and Department of Health Sciences, University of Jyväskylä, Jyväskylä, Finland.

Eleven male subjects (20–32 years) accustomed to strength training went through progressive, high‐load strength training for 24 weeks with intensities ranging variably between 70 and 120% during each month. This training was also followed by a 12‐week detraining period. An increase of 26.8% (P < 0.001) in maximal isometric strength took place during the training. The increase in strength correlated (P < 0.05) with significant (P < 0.05–0.01) increases in the neural activation (IEMG) of the leg extensor muscles during the most intensive training months. During the lower‐intensity training, maximum IEMG decreased (P < 0.05). Enlargements of muscle‐fibre areas, especially of fast‐twitch type (P < 0.001), took place during the first 12 weeks of training. No hypertrophic changes were noted during the latter half of training. After initial improvements (P < 0.05) no changes or even slight worsening were noted in selected force‐time parameters during later strength training. During detraining a great (P < 0.01) decrease in maximal strength was correlated (P < 0.05) with the decrease (P < 0.05) in the maximum IEMGs of the leg extensors. This period resulted also in decreases (P < 0.05) of the mean muscle‐fibre areas of both fibre types. It was concluded that improvement in strength may be accounted for by neural factors during the course of very intensive strength training. Selective training‐induced hypertrophy also contributed to strength development but muscle hypertrophy may have some limitations during long‐lasting strength training, especially in highly trained subjects.