The effects of temperature, concentration of ATP and pH in solution on shortening velocity of skinned muscle fibers were examined. The skinned fibers were prepared from tibialis anterior muscle of adult male rats. The fibers were classified into myosin subgroups by its reaction for myofibrilla ATPase and for enzymes of energy metabolism. The both histochemical fiber classifications were in good agreement. The shortening velocities for SO fibers were quite below those obtained from fast-twitch fibers, and the velocities for FG fibers were about 1.2-1.5 times greater than that of FOG fibers. There were no characteristic differences between FG and FOG fibers with respect to the shortening velocity changes depending on the temperature, concentration of ATP and pH.
　The results indicate that there are no differences of ATPase properties between the subgroups of fast-twitch fibers. However, FG fibers had ATPase activity that were consistently higher than that of FOG fibers.

DESCENTE SPORTS SCIENCE Vol.8/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE