Postexercise thermal and cardiovascular responses following fast and slow heating exercise protocols

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Exercise-induced hyperthermia can result in significant cardiovascular and thermoregulatory disturbances which can persist for a prolonged period. However, it is unclear if this response differs as a function of the time to reach hyperthermia. Therefore, we compared cardiovascular and thermoregulatory responses of subjects rendered hyperthermic by either a slow heating (SH) or fast heating (FH) exercise protocol performed in the heat (40°C). On separate days, 7 subjects either walked (4 mph, 2% grade, SH) or ran (7.5 mph, 2% grade, FH) on a treadmill until rectal temperature (Tre) increased to 39.5°C. Subjects then rested in the upright seated position in thermoneutral conditions (30°C) until Tre returned to 38.0°C. Heart rate (HR) and mean arterial pressure (MAP) were assessed continuously. Change in plasma volume (ΔPV) from baseline rest was measured at end of recovery. The time to reach Tre of 39.5°C was significantly longer during SH (76.5±6.5 min) compared to FH (35.5±3.3 min, p<0.05). However, the time to reach the Tre recovery of 38.0°C was significantly greater during FH compared to SH (82.5±9.2 vs. 56.2±6.5 min, p<0.05). Despite a longer recovery during FH, a similar reduction in MAP (FH:−9.6±1.7 vs. SH: −10.1±3.4 mmHg) was observed at end-recovery which was paralleled by similar elevations in HR (FH:83±2 vs. SH:87±5 bpm). Further, ΔPV was also similar between conditions (FH: −5.1±1.9 vs. SH: −5.8±2.8%) albeit reduced from end-exercise values (FH:9.0±1.1% vs. SH:11.8±1.5%,p<0.05). Different exercise-intensity ramping protocols performed to a similar level of thermal strain influence core temperature recovery time but not the level of cardiovascular strain. Support: Natural Sciences and Research Council of Canada.