Influence of Subtypes of Fast-Twitch Fibers on Isokinetic Strength in Untrained Men

The purpose of the present study was to investigate the influence of the subtypes of fast-twitch (FT) fibers on the peak torque per unit cross-sectional area in working muscle. Muscle biopsies were taken from the right vastus lateralis of nine healthy untrained men. Using staining for myofibrillar ATPase after preincubation at pH 4.3 and 4.6, muscle fibers were classified as slow-twitch (ST), FTa, and FTb. Cross-sectional area of the leg extensor muscles was measured by ultrasonic method. Maximum voluntary strength for the leg extensor muscles was determined as peak torques with constant angular velocity using an isokinetic dynamometer. The peak torques at various angular velocities (30, 60, 120, and 180 degrees/s) indicated statistically significant positive correlations to the percent number and area of FTa fibers in m. vastus lateralis. Other fiber types, such as ST or FTb, however, were not correlated significantly to the peak torques at all angular velocities. A significant positive correlation of 0.766 (180 deg/s) was observed between peak torque per unit cross-sectional area of the leg extensor muscle and the percent number of FTa fibers. These results suggest that in untrained men, the percent number of FTa fibers plays a more important role in producing high peak torques than the percent number of FTb and ST and that even if muscle cross-sectional area is equal, a muscle with a high percent number of FTa fibers can exert higher isokinetic strength.