Heat production in human skeletal muscle at the onset of intense dynamic exercise

1. We hypothesised that heat production of human skeletal muscle at a given high power output would gradually increase as heat liberation per mole of ATP produced rises when energy is derived from oxidation compared to phosph ocreatine (PCr) breakdown and glycogenolysis. 2. Five young volunteers performed 180 s of intense dynamic knee-extensor e xercise (similar to 80 W) while estimates of muscle heat production, power output, oxygen uptake, lactate release, lactate accumulation and ATP and PC r hydrolysis were made, Heat production was determined continuously by (i) measuring heat storage in the contracting muscles, (ii) measuring heat remo val to the body core by the circulation, and (iii) estimating heat transfer to the skin by convection and conductance as well as to the body core by l ymph drainage. 3. The rate of heat storage in knee-extensor muscles was highest during the first 45 s of exercise (70-80 J s(-1)) and declined gradually to 14 +/- 10 J s(-1) at 180 s. The rate of heat removal by blood was negligible during the first 10 s of exercise, rising gradually to 112 +/- 14 J s(-1) at 180 s . The estimated rate of heat release to skin and heat removal via lymph flo w was < 2 J s(-1) during the first 5 s and increased progressively to 24 +/ - 1 J s(-1) at 180 s. 4. The rate of heat production increased significantly throughout exercise, being 107% higher at 180 s compared to the initial 5 s, with half of the i ncrease occurring during the first 38 s, while power output remained essent ially constant. 5. The contribution of muscle oxygen uptake and net lactate release to tota l energy turnover increased curvilinearly from 32% and 2%, respectively, du ring the first 30 s to 86% and 8%, respectively, during the last 30 s of ex ercise. The combined energy contribution from net ATP hydrolysis, net PCr h ydrolysis and muscle lactate accumulation is estimated to decline from 37% to 3% comparing the same time intervals. 6. The magnitude and rate of elevation in heat production by human skeletal muscle during exercise in vivo could be the result of the enhanced heat li beration during ATP production when aerobic metabolism gradually becomes do minant after PCr and glycogenolysis have initially provided most of the ene rgy.