Relationship between muscle QO2 and fatigue during repeated isokinetic contractions

The percent slow-twitch fibers (%ST) of the vastus lateralis muscle and selected indicators of maximum aerobic power (QO2, VO2 max, and leg extensor VO2 max) were compared with the rate of fatigue development during 45 s of repeated maximum isokinetic leg extension-flexion contractions (180 degrees/s) on 13 males. Subjects with muscle respiratory capacities (QO2) above the mean (2,472 microliter O2 X h-1 X g-1) displayed a significantly slower rate of fatigue development than those subjects with QO2 values below the mean. In addition, indicators of maximum aerobic power were correlated with the percent of maximum power and power/ml fat-free thigh volume at 5, 15, 30, and 45 s during the fatigue test. The correlations increased progressively during the fatigue test and became significant at 45 s. This trend was not observed when the %ST fibers were compared with the percent of maximum power and power/ml fat-free thigh volume at these time intervals. These data suggest that 1) the muscle tissue's capacity for aerobic metabolism influences the rate of fatigue development even during a strenuous short-term isokinetic exercise task, and 2) factors other than fiber type may interact to confound the prediction of fiber type by noninvasive means.