Purpose: The aim of this study was to investigate heat balance during thermal transients caused by successive exercise bouts. Whole-body heat loss (H˙L) and changes in body heat content (ΔHb) were measured using simultaneous direct whole-body and indirect calorimetry.
Methods: Ten participants performed three successive bouts of 30-min cycling (Ex1, Ex2, and Ex3) at a constant rate of heat production of ∼500 W, each separated by 15-min rest (R1, R2, and R3) at 30°C.
Results: Despite identical rates of heat production during exercise, the time constant (τ) of the exponential increase in H˙L was greater in Ex1 (τ = 12.3 ± 2.3 min) relative to both Ex2 (τ = 7.2 ± 1.6 min) and Ex3 (τ = 7.1 ± 1.6 min) (P < 0.05). ΔHb 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 τ of the exponential decrease in H˙L was the same during R1 (τ = 6.5 ± 1.1 min), R2 (τ = 5.9 ± 1.3 min), and R3 (τ = 6.0 ± 1.2 min). ΔHb during R1 (−82 ± 48 kJ), R2 (−91 ± 48 kJ), and R3 (−88 ± 54 kJ) were the same. The cumulative ΔHb was consequently greater at the end of Ex2 and Ex3 relative to the end of Ex1 (P < 0.05). Likewise, cumulative ΔHb was greater at the end of R2 and R3 relative to R1 (P < 0.05).
Conclusion: 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.