title: Heat balance and cumulative heat storage during intermittent bouts of exercise.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Laboratory of Human Bioenergetics and Environmental Physiology, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada. gkenny@uottawa.ca
authors.forename: Glen P
authors.initials: GP
authors.lastname: Kenny
authors.forename: Lucy E
authors.initials: LE
authors.lastname: Dorman
authors.forename: Paul
authors.initials: P
authors.lastname: Webb
authors.forename: Michel B
authors.initials: MB
authors.lastname: Ducharme
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.forename: Francis D
authors.initials: FD
authors.lastname: Reardon
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Hardcastle
authors.forename: Ollie
authors.initials: O
authors.lastname: Jay
mesh_terms: D000328:Adult; D001831:Body Temperature / Q000502:physiology*; D002151:Calorimetry / Q000379:methods; D005080:Exercise Test / Q000379:methods*; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology; D012881:Skin Temperature / Q000502:physiology; D022722:Thermogenesis / Q000502:physiology*