The aim of the present study was to simultaneously examine skeletal muscle heat production and ATP turnover in humans during dynamic exercise with marked differences in aerobic metabolism. This was done to test the hypothesis that efficiency is higher in anaerobic than aerobic ATP resynthesis. Six healthy male subjects performed 90 s of low intensity knee-extensor exercise with (OCC) and without thigh occlusion (CON-LI) as well as 90 s of high intensity exercise (CON-HI) that continued from the CON-LI bout. Muscle heat production was determined by continuous measurements of muscle heat accumulation and heat release to the blood. Muscle ATP production was quantified by repeated measurements of thigh oxygen uptake as well as blood and muscle metabolite changes. All temperatures of the thigh were equalized to approximately 37 degrees C prior to exercise by a water-perfused heating cuff. Oxygen uptake accounted for 80 +/- 2 and 59 +/- 4 %, respectively, of the total ATP resynthesis in CON-LI and CON-HI, whereas it was negligible in OCC. The rise in muscle temperature was lower (P < 0.05) in OCC than CON-LI (0.32 +/- 0.04 vs. 0.37 +/- 0.03 degrees C). The mean rate of heat production was also lower (P < 0.05) in OCC than CON-LI (36 +/- 4 vs. 57 +/- 4 J s-1). Mechanical efficiency was 52 +/- 4 % after 15 s of OCC and remained constant, whereas it decreased (P < 0.05) from 56 +/- 5 to 32 +/- 3 % during CON-LI. During CON-HI, mechanical efficiency transiently increased (P < 0.05) to 47 +/- 4 %, after which it decreased (P < 0.05) to 36 +/- 3 % at the end of CON-HI. Assuming a fully coupled mitochondrial respiration, the ATP turnover per unit of work was calculated to be unaltered during OCC (approximately 20 mmol ATP kJ-1), whereas it increased (P < 0.05) from 21 +/- 4 to 29 +/- 3 mmol ATP kJ-1 during CON-LI and further (P < 0.05) to 37 +/- 3 mmol ATP kJ-1 during CON-HI. The present data confirm the hypothesis that heat loss is lower in anaerobic ATP resynthesis than in oxidative phosphorylation and can in part explain the finding that efficiency declines markedly during dynamic exercise. In addition, the rate of ATP turnover apparently increases during constant load low intensity exercise. Alternatively, mitochondrial efficiency is lowered as exercise progresses, since ATP turnover was unaltered during the ischaemic exercise bout.