METABOLIC HEAT-PRODUCTION DURING FATIGUE FROM VOLUNTARY REPETITIVE ISOMETRIC CONTRACTIONS IN HUMANS

The effect of repetitive isometric knee extensions on the energy cost of contraction was examined. The rate of temperature rise (dT/dt) was determined in test contractions at 30 and 50% of maximal voluntary con traction (MVC) force before and during 30% MVC repetitive isometric ex ercise (RIE) to exhaustion and regularly in a 30-min postexercise reco very period (n = 9). Pulmonary O-2 uptake and muscle temperature (T-mu s) were determined at regular intervals. During the 30% MVC test contr actions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing l inearly by 68% during exercise. In the 50% MVC test contractions, dT/d t rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test cont ractions at both force levels did not decrease significantly throughou t the 30-min postexercise recovery period. The rise in dT/dt was paral leled by a 76% increase in O-2 uptake. In contrast, T-mus rose initial ly and then leveled off. The present data indicate that RIE induced a gradual rise in the rate of energy turnover associated with isometric force production. Neither increased T-mus nor recruitment of less econ omic type II fibers can fully explain the increased energy cost. We su ggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occu rring during fatigue from submaximal voluntary RIE.