The purpose of this study was to investigate the influence of sprint running addition in endurance training on skeletal muscle. Eighteen male Fischer rats were classified into control (C), endurance trained (E) and endurance and sprint trained (ES) group at 4 weeks of age. Training groups ran on treadmill 6 days / week for 9 weeks. E group animals were run continuously for 120 min at 35 m / min. ES group animals were run for 90 min at 35 m / min and 10 intervals at 80 m /min lasting 30 sec sprints interposed with 2.5 min rest periods. Our results are as follows : 1) There were increase in muscle weight / body weight in both trained groups. 2) In soleus muscle (SOL) from both trained groups, glycolytic enzyme activities were significantly increased. As compared with C group oxidative enzyme activities increased by about twofold in SOL, plantaris (PLA), extensor digitorum longus (EDL) and Gastrocnemius (GAS) muscles from both trained groups. 3) A significant increase in the mitochondrial volumes were observed in ES trained SOL muscle and in both trained PLA muscles. 4) A decrease in SOL and PLA twitch contraction times were observed from trained groups. Despite of these changes, fatigability was not altered. 5) After 9 weeks training, the number of fibre type compositions in the muscles studied did not observed the transformation between slow and fast fibre types. 6) Three types of fast-twitch fibres were classified based on differences in ATPase activity and, in addition, on metabolic characteristics as judged from the histochemically assessed SDH activity : type II a, type II b-FOG, and type II b. The relative number of type II a in PLA increased and in GAS deep and superficial regions increased with decrease of II b-FOG from E group. A significant increase in the II b-FOG with decrease of type II b of PLA and GAS superficial region.
　These data indicate that the type II b-FOG may represent a trasitional state, and an increase composition of II b-FOG suggest that the adaptation in muscle following endurace training.

DESCENTE SPORTS SCIENCE Vol.13/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE