Fixed-intensity Vs Self-paced Exercise In The Heat

Abstract

PURPOSE: To compare the physiological and behavioral outcomes of fixed-intensity and self-paced exercise in uncompensable heat stress. METHODS: Eight well-trained male cyclists completed (work-matched) fixed-intensity (FI) and self-paced (SP) cycling exercise bouts in a hot (40.6 ± 0.2°C) and dry (relative humidity: 23 ± 3%) environment estimated to elicit 70% of VO2max. RESULTS: Exercise intensity (i.e. power output) decreased (P<0.05) over time in SP, which resulted in longer (P<0.05) exercise duration (FI: 20.3 ± 3.4 min, SP: 23.2 ± 4.1 min). According to the heat strain index, the modification of exercise intensity in SP improved (P<0.05) the compensability of the thermal environment which, relative to FI, was likely a result of the reductions (P<0.05) in metabolic heat production. Consequently, the rate of rise in core body temperature was higher (P<0.01) in FI (0.108 ± 0.020°C/min) than in SP (0.082 ± 0.016°C/min). Interestingly, the increase (P<0.05) in heart rate (mean starting heart rate: 158 ± 11 bpm, mean finishing heart rate: 182 ± 10 bpm) and decreases (P<0.05) in cardiac output (mean starting cardiac output: 33.1 ± 3.9 L/min, mean finishing cardiac output: 30.4 ± 4.8 L/min) and stroke volume (mean starting stroke volume: 211 ± 24 ml; mean finishing stroke volume: 168 ± 27 ml) over time were independent of exercise modality (P>0.05). However, core body temperature (FI: 39.4 ± 0.3°C, SP: 39.1 ± 0.4°C), blood lactate (FI: 2.9 ± 0.8 mmol/L, SP: 2.3 ± 0.7 mmol/L), and perceived exertion (FI: 18 ± 2, SP: 16 ± 2) were all higher (P<0.05) in FI compared to SP at exhaustion / completion. CONCLUSION: These findings indicate that when exercise is self-paced, behavioral modification of metabolic heat production improves the compensability of the thermal environment and reduces thermoregulatory strain. Therefore, under uncompensable heat stress body temperature regulation is dependent upon whether exercise intensity is fixed or self-selected. Acknowledgement: The presentation of this abstract was funded by the Gatorade Sports Science Institute.