Twenty four male Sprague-Dawley rats, 35 days old, were randomly assigned to one of four groups: 2 resting control groups and 2 swimming groups.
　The hypoxia control and hypoxia swimming groups were housed for one week at 550 torr, followed by 4 weeks at 460 ton.
　The sea-level control and the sea level swimming groups were housed for 5 weeks at 760 ton.
　The swimming program consisted of endurance swimming at a 30 minutes a day, 6 days a week, for 5 weeks.
　At ten weeks of age, fiber composition of their hind-limb muscles were histochemically studied. Samples of the soleus, EDL, plantaris and tibialis ant. muscles were sectioned (10μm thick) by cryostat at -20°C.
　The sections were stained with succinate dehydrogenase (SDH) and myosin adenosine triphosphatase (ATPase), and the muscle fiber were classified in to the three types: FG, FOG, SO.
　In soleus muscle, hypoxia and swimming induced significant increase in FOG fibers, and decrease in SO fibers.
　Sea-level control group had the lowest percentage in FOG fibers compared to the other groups.
　EDL and plantaris muscles had a significantly higher percentage of FOG fibers in the hypoxia and swimming groups than in the sea-level control group.
　These results obtained in the present experiments indicated that hypoxia is a triggerring factor for the conversions of muscle fiber types and the increase in oxidative capacity.

DESCENTE SPORTS SCIENCE Vol.7/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE