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Heat Balance and Cumulative Heat Storage during Intermittent Bouts of Exercise

KENNY, GLEN P.1; DORMAN, LUCY E.1; WEBB, PAUL2; DUCHARME, MICHEL B.1,3; GAGNON, DANIEL1; REARDON, FRANCIS D.1; HARDCASTLE, STEPHEN G.4; JAY, OLLIE1

Medicine & Science in Sports & Exercise: March 2009 - Volume 41 - Issue 3 - pp 588-596
doi: 10.1249/MSS.0b013e31818c97a9
Basic Sciences

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.

1Laboratory of Human Bioenergetics and Environmental Physiology, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, CANADA; 2Yellow Springs, OH; 3Defence R&D Canada, Quebec City, Quebec, CANADA; and 4CANMET-MMSL Natural Resouces Canada, Sudbury, Ontario, CANADA

Address for correspondence: Glen P. Kenny, Ph.D., School of Human Kinetics, University of Ottawa, 125 University, Montpetit Hall, Ottawa, Ontario, Canada K1N 6N5; E-mail: gkenny@uottawa.ca.

Submitted for publication May 2008.

Accepted for publication August 2008.

©2009The American College of Sports Medicine