Hypobaric-hypoxic exposure and histochemical responses of soleus muscle fibers in the rat.

Hypoxia-acclimatized rats had a higher percentage of fast-twitch oxidative glycolytic fibers and a lower percentage of slow-twitch oxidative fibers in the soleus muscle than age-matched normoxic-rats. However, there was no difference in the fiber type composition of the soleus muscle between hypoxic- and normoxic-male rats after hypoxic-rats were bred at the sea level atmospheric pressure for 5 weeks. Therefore, it is suggested that a higher percentage of fast-twitch oxidative glycolytic fibers in the muscle is found only under hypoxic conditions and is due to an acute response to hypoxia.