Are All Heat Loads Created Equal?

PURPOSE: We evaluated physiological responses during exercise at a fixed evaporative requirement for heat balance (Ereq) but varying combinations of metabolic and environmental heat load.

METHODS: Nine healthy, physically active males (age: 46 ± 8 yr) performed four experimental sessions consisting of 75 min of semirecumbent cycling at various ambient temperatures. Whole-body dry heat loss (direct calorimetry) was monitored continuously as was heat production (indirect calorimetry), which was adjusted to achieve an Ereq of 400 W. The resultant metabolic heat productions and ambient temperatures for the sessions were as follows: (i) 440 W and 30°C (440 [30]), (ii) 388 W and 35°C (388 [35]), (iii) 317 W and 40°C (317 [40]), and (iv) 258 W and 45°C (258 [45]). Whole-body evaporative heat loss was determined via direct calorimetry. Esophageal (Tes) and mean skin (Tsk) temperatures as well as HR were monitored continuously. Mean body temperature (Tb) was calculated from Tes and Tsk. Physiological strain index (PSI) was determined from Tes and HR.

RESULTS: End-exercise evaporative heat loss and Tb were similar between conditions (both P ≥ 0.48). Tes was greater in 440 [30] (37.67°C ± 0.04°C) and 388 [35] (37.58°C ± 0.07°C) relative to both 317 [40] (37.35°C ± 0.06°C) and 258 [45] (37.20°C ± 0.07°C; all P ≤ 0.05). Further, Tsk was different between each condition (440 [30], 33.85°C ± 0.16°C; 388 [35], 34.53°C ± 0.08°C; 317 [40], 35.67°C ± 0.07°C; and 258 [45], 36.54°C ± 0.08°C; all P < 0.01). In 440 [30], HR was elevated by about 13 and 18 bpm relative to 317 [40] and 258 [45], respectively (both P < 0.01). Finally, PSI was greater in both 440 [30] and 388 [35] compared with 317 [40] and 258 [45] (all P ≤ 0.04).

CONCLUSIONS: Exercise at a fixed Ereq resulted in similar evaporative heat loss and Tb. However, the Tes, Tsk, HR, and PSI responses varied depending on the relative contribution of metabolic and environmental heat load.