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Work Rate during Self-paced Exercise is not Mediated by the Rate of Heat Storage

Friesen Brian J.; Périard, Julien D.; Poirier, Martin P.; Lauzon, Martin; Blondin, Denis P.; Haman, Francois; Kenny, Glen P.
Medicine & Science in Sports & Exercise: Post Acceptance: September 08, 2017
doi: 10.1249/MSS.0000000000001421
Original Investigation: PDF Only

ABSTRACTPURPOSETo date, there have been mixed findings on whether greater anticipatory reductions in self-paced exercise intensity in the heat are mediated by early differences in rate of body heat storage. The disparity may be due to an inability to accurately measure minute-to-minute changes in whole-body heat loss. Thus, we evaluated whether early differences in rate of heat storage can mediate exercise intensity during self-paced cycling at a fixed rate of perceived exertion (RPE of 16; hard-to-very-hard work effort) in COOL (15°C), NORMAL (25°C) and HOT (35°C) ambient conditions.METHODSOn separate days, nine endurance-trained cyclists exercised in COOL, NORMAL and HOT conditions at a fixed RPE until work rate (measured after first 5-min of exercise) decreased to 70% of starting values. Whole-body heat loss and metabolic heat production were measured by direct and indirect calorimetry respectively.RESULTSTotal exercise time was shorter in HOT (57±20 min) relative to both NORMAL (72±23 min, P=0.004) and COOL (70±26 min, P=0.045). Starting work rate was lower in HOT (153±31 W) compared to NORMAL (166±27 W, P=0.024) and COOL (170±33 W, P=0.037). Rate of heat storage was similar between conditions during the first 4 min of exercise (all P>0.05). Thereafter, rate of heat storage was lower in HOT relative to NORMAL and COOL until 30-min of exercise (last common time-point between conditions; all P<0.05). Further, rate of heat storage was significantly higher in COOL compared to NORMAL at 15- (P=0.026) and 20-min only (P=0.020). No differences were measured at end-exercise.CONCLUSIONSWe show that rate of heat storage does not mediate exercise intensity during self-paced exercise at a fixed RPE in cool to hot ambient conditions.

PURPOSE

To date, there have been mixed findings on whether greater anticipatory reductions in self-paced exercise intensity in the heat are mediated by early differences in rate of body heat storage. The disparity may be due to an inability to accurately measure minute-to-minute changes in whole-body heat loss. Thus, we evaluated whether early differences in rate of heat storage can mediate exercise intensity during self-paced cycling at a fixed rate of perceived exertion (RPE of 16; hard-to-very-hard work effort) in COOL (15°C), NORMAL (25°C) and HOT (35°C) ambient conditions.

METHODS

On separate days, nine endurance-trained cyclists exercised in COOL, NORMAL and HOT conditions at a fixed RPE until work rate (measured after first 5-min of exercise) decreased to 70% of starting values. Whole-body heat loss and metabolic heat production were measured by direct and indirect calorimetry respectively.

RESULTS

Total exercise time was shorter in HOT (57±20 min) relative to both NORMAL (72±23 min, P=0.004) and COOL (70±26 min, P=0.045). Starting work rate was lower in HOT (153±31 W) compared to NORMAL (166±27 W, P=0.024) and COOL (170±33 W, P=0.037). Rate of heat storage was similar between conditions during the first 4 min of exercise (all P>0.05). Thereafter, rate of heat storage was lower in HOT relative to NORMAL and COOL until 30-min of exercise (last common time-point between conditions; all P<0.05). Further, rate of heat storage was significantly higher in COOL compared to NORMAL at 15- (P=0.026) and 20-min only (P=0.020). No differences were measured at end-exercise.

CONCLUSIONS

We show that rate of heat storage does not mediate exercise intensity during self-paced exercise at a fixed RPE in cool to hot ambient conditions.

Address for correspondence: Glen P. Kenny, Ph.D., University of Ottawa, School of Human Kinetics, 125 University Private, Montpetit Hall, Room 367, Ottawa, Ontario, Canada, K1N 6N5, (613) 562-5800 ext. 4282, (613) 562-5497 (fax), E-mail: gkenny@uottawa.ca

This research was supported by the Natural Sciences and Engineering Research Council of Canada (Discovery grant, RGPIN-06313-2014 and Discovery Grants Program - Accelerator Supplement, RGPAS-462252-2014) (all funds held by Dr. Glen P. Kenny). G.P. Kenny is supported by a University of Ottawa Research Chair. M.P. Poirier is supported NSERC Canada Alexander Graham Bell Graduate Scholarship (CGS-D). B.F. is supported by an Ontario Graduate Scholarship. No conflicts of interest, financial or otherwise, are declared by the author(s). The results of the present study do no constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

Accepted for Publication: 1 September 2017

© 2017 American College of Sports Medicine