Effects of running exercise with increasing loads on tibialis anterior muscle fibres in mice.

Cross-sectional areas and succinate dehydrogenase (SDH) activities of type identified fibres in the deep, middle and superficial regions of the tibialis anterior muscle in mice were examined after 4 weeks of voluntary running exercise with increasing loads. Nineteen-week-old male mice were assigned randomly to either a control or exercise group. The mean cross-sectional areas of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. The mean SDH activities of type IIx and type IIb fibres in the middle region and of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. These results suggest that voluntary running exercise with increasing loads causes hypertrophy and/or an increase in the SDH activity of fibres in the specific muscle region where fibres with a high threshold and a low-oxidative enzyme activity are distributed, and these fibres are recruited to adapt to changes in exercise conditions.