Exercise capacity is influenced by thermal balance at onset of active/inactive phases of circadian cycle

Abstract

The aim of this study was to verify if circadian phase at which exercise is performed influences performance and thermoregulatory adjustments.Wistar rats (n=8), in a L/D cycle of 14/10h with free access to water and food, were used. To establish active and inactive phase of each animal, spontaneous locomotor activity and core temperature (Tc) were continuously recorded during 48h with an implanted telemetric sensor. After acclimatized to running animals were submitted to incremental exercise until fatigue during both the onset of active and inactive phases of circadian Tc rhythm. Time to fatigue (TF), Tc and skin tail temperature (Ttail) were monitored during exercise. Workload (W) and TF were used as performance index.Results confirmed that onset of active phase was followed by peak Tc (p<0.05) while trough was observed just after inactive phase onset (p<0.05). This was accompanied by superior TF and W during inactive compared to active phase (p<0.05). Exercise capacity differences were followed by higher Tc and Ttail before exercise at active phase (p<0.05). After fatigue Ttail remained higher at active phase (p<0.05).We have demonstrated that exercise capacity was indirectly influenced by time of the day through a circadian‐related modulation of heat production and dissipation before and during exercising until fatigue.Financial support: CAPES, CNPq and FAPEMIG.