Heat Balance and Cumulative Heat Storage during Intermittent Bouts of Exercise

The aim of this study was to investigate heat balance during thermal transients caused by successive exercise bouts. Whole-body heat loss (H x L) and changes in body heat content (Delta Hb) were measured using simultaneous direct whole-body and indirect calorimetry. Ten participants performed three successive bouts of 30-min cycling (Ex1, Ex2, and Ex3) at a constant rate of heat production of approximately 500 W, each separated by 15-min rest (R1, R2, and R3) at 30 degrees C. Despite identical rates of heat production during exercise, the time constant (tau) of the exponential increase in H x L was greater in Ex1 (tau = 12.3 +/- 2.3 min) relative to both Ex2 (tau = 7.2 +/- 1.6 min) and Ex3 (tau = 7.1 +/- 1.6 min) (P < 0.05). Delta Hb during Ex1 (256 +/- 76 kJ) was greater than during Ex2 (135 +/- 60 kJ) and Ex3 (124 +/- 78 kJ) (P < 0.05). During recovery bouts, heat production was the same, and the tau of the exponential decrease in H L was the same during R1 (tau = 6.5 +/- 1.1 min), R2 (tau = 5.9 +/- 1.3 min), and R3 (tau = 6.0 +/- 1.2 min). Delta Hb during R1 (-82 +/- 48 kJ), R2 (-91 +/- 48 kJ), and R3 (-88 +/- 54 kJ) were the same. The cumulative Delta Hb was consequently greater at the end of Ex2 and Ex3 relative to the end of Ex1 (P < 0.05). Likewise, cumulative Delta Hb was greater at the end of R2 and R3 relative to R1 (P < 0.05). The proportional decrease in the amount of heat stored in the successive exercise bouts is the result of an enhanced rate of heat dissipation during exercise and not due to a higher rate of heat loss in the recovery period. Despite a greater thermal drive with repeated exercise, the decline in the rate of total heat loss during successive recovery bouts was the same.