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The histological changes associated with exercise-induced muscular hypertrophy have variously been interpreted as due to fibre-splitting or satellite-cell activity. If due to fibre-splitting, than a marked fall in the number of nuclei per unit volume of muscle must occur. To see if this is the case, we have examined post-mortem specimens of extensor digitorum longus muscles in rats exercised by swimming for up to 30 min twice daily for 35 days. The mean cross-sectional area of muscle fibres was unaltered by exercise but mean length of capillary per unit volume of muscle was increased by 55% and number of nuclei per unit volume of muscle by 30%, both changes being significant. Mean nuclear volume increased by approximately 75%. Thus the muscles did adapt to exercise and new myonuclei were formed, contrary to expectations if fibre-splitting were the essential change. In this study the fibres seemed not to have increased in size and further studies, perhaps using autoradiographic analysis of tritiated thymidine-labelled satellite-cell nuclei, are needed to clarify the precise mechanisms involved.

It is known that some anabolic steroids are capable of increasing the sizes of skeletal muscles or increasing their mechanical performance. The present study was undertaken to establish quantitatively the alterations which occur in skeletal muscle fibres as a result of the injection of 19-nortestosterone (nandrolone phenyl proprionate). Specimens of diaphragm were obtained from the experimental and control animals and prepared for electron microscopy and stereological analyses. Following low doses of the steroid stereological analyses have shown that in Type I and in intermediate fibres their mitochondrial volume proportions increase though their myofilament contents decrease. With higher doses, stereological analyses have shown that the mitochondrial volume proportions were increased particularly in intermediate fibres whilst the myofilament content of Type I and II fibres was significantly reduced.