Do greater rates of body heat storage precede the accelerated reduction of self-paced exercise intensity in the heat?
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To reevaluate the previous hypothesis that greater reductions in self-paced exercise intensity in the heat are mediated by early differences in the rate of body heat storage (S).
Eight trained volunteers cycled in 19 °C/1.8 kPa (COOL), 25 °C/1.2 kPa (NORM), and 34 °C/1.6 kPa (HOT), while maintaining an RPE of 16. Potential differences in S following the onset of exercise were assessed by comparing rates of esophageal temperature change (ΔT es/Δt); and estimated S values using a traditional two-compartment thermometric model (S therm) of changes in rectal (T re) and skin (T sk) temperature, and partitional calorimetry (S cal).
After 15 min of exercise, workload decreased more in HOT vs. COOL (P = 0.03), resulting in a shorter time (HOT: 40.7 ± 14.9 min; COOL: 53.5 ± 18.7 min; P = 0.04) to 70 % of initial workload. However, there were no preceding differences in ΔT es/Δt between conditions (P = 0.18). S therm values were different between HOT and COOL during the first 5 min of exercise (P < 0.05), primarily due to negative S therm values (−32 ± 15 kJ min−1) in COOL, which according to partitional calorimetric measurements, required improbably high (~56 kJ min−1) rates of evaporation when no sweating on the back and thigh was observed until after 7.6 ± 1.5 min and 4.8 ± 1.7 min of exercise, respectively. S cal values in the first 5 min of exercise confirmed S was actually positive in COOL (+21 ± 8 kJ min−1) and not negative. Different S therm values following the onset of exercise at different environmental temperatures are simply due to transient differences in the rate of change in T sk.
Reductions in self-paced exercise intensity in the heat are not mediated by early differences in S following the onset of exercise.
KeywordsCore temperature Exercise performance Partitional calorimetry Rating of perceived exertion Thermoregulation
Effective radiant surface area (ND)
Body surface area (m2)
Convective heat exchange (W m−2)
Convective heat exchange via respiration (W m−2)
Evaporative heat loss from the skin (W m−2)
Evaporative heat loss via respiration (W m−2)
Caloric equivalent of carbohydrates (kJ LO−2)
Caloric equivalent of lipids (kJ LO−2)
Electromyography (% of initial EMG)
Area-weighted emissivity of the skin (ND)
Area-weighted clothing factor (ND)
Combined heat transfer coefficient (W m−2 °C−1)
Convective heat transfer coefficient (W m−2 °C−1)
Radiative heat transfer coefficient (W m−2 °C−1)
Rate of metabolic heat production (W m−2)
Integrated electromyography (% of initial iEMG)
Local sweat rate on the lower back (mg cm−2 min−1)
Local sweat rate on the thigh (mg cm−2 min−1)
Metabolic energy expenditure (W m−2)
Physical Activity Readiness Questionnaire
Ambient water vapor pressure (kPa)
Radiant Heat exchange (W m−2)
Thermal resistance of clothing (m2 °C W−1)
Respiratory exchange ratio (ND)
Rating of perceived exertion
Rate of body heat storage (kJ min−1)
Rate of body heat storage measured with partitional calorimetry (kJ min−1)
Rate of body heat storage estimated with a traditional 2-compartment thermometry model (kJ min−1)
Stefan-Boltzmann constant (5.67 × 10−8W m−2 °C−4)
Ambient air temperature (°C)
Change in mean body temperature (°C)
Temperature of the clothing (°C)
Esophageal temperature (°C)
Rate of oesophageal temperature change (°C min−1)
Operative temperature (°C)
Mean radiant temperature (°C)
Rectal temperature (°C)
Mean skin temperature (°C)
Peak rate of oxygen consumption (L min−1)
Rate of oxygen consumption (L min−1)
Air velocity (m s−1)
Rate of external work (W m−2)
Peak rate of external work (W)
The authors thank the volunteers for their participation and effort. This research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada (#386143-2010, held by O. Jay). Funding for the metabolic cart and dew point sensor were provided by a Canadian Foundation for Innovation (CFI) Infrastructure grant (co-applicant: O. Jay). N.M. Ravanelli was supported by a University of Ottawa Faculty of Health Sciences Research Bursary. M.N. Cramer is supported by a Postgraduate Scholarship (doctoral) from NSERC and an Excellence Scholarship from the University of Ottawa. Y. Molgat-Seon was supported by a University of Ottawa Master’s Scholarship. A.N. Carlsen is supported by NSERC and the University of Ottawa, Faculty of Health Sciences.
Conflict of interest
The authors report no competing interests, financial or otherwise.
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