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 contraction (MVC) force before and during 30% MVC repetitive isometric exercise (RIE) to exhaustion and regularly in a 30-min postexercise recovery period (n = 9). Pulmonary O2 uptake and muscle temperature (Tmus) were determined at regular intervals. During the 30% MVC test contractions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing linearly by 68% during exercise. In the 50% MVC test contractions, dT/dt rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test contractions at both force levels did not decrease significantly throughout the 30-min postexercise recovery period. The rise in dT/dt was paralleled by 76% increased in O2 uptake. In contrast, Tmus rose initially 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 Tmus nor recruitment of less economic type II fibers can fully explain the increased energy cost. We suggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occurring during fatigue from submaximal voluntary RIE.