Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions

摘要

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38 degrees C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (T-es), arm (LSRarm), and back (LSRback) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37 degrees C/30% RH) and 60-min uncompensable (37 degrees C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post-heat acclimation (POST-HA). For compensable heat stress trials, resting Tes was lower POST-TRN (36.74 +/- 0.27 degrees C, P < 0.05) and POST-HA (36.60 +/- 0.27 degrees C, P < 0.001) compared with PRE-TRN (36.99 +/- 0.19 degrees C); however, Delta T-es was similar in all trials (PRE-TRN:0.40 +/- 0.23 degrees C; POST-TRN: 0.42 +/- 0.20 degrees C; POST-HA:0.43 +/- 0.12 degrees C, P < 0.97). While LSRback was unaltered by HA (P < 0.94), end-exercise LSRarm was higher POST-TRN (0.70 +/- 0.14 mg.cm(-2).min(-1), P < 0.001) and POST-HA (0.75 +/- 0.16 mg.cm(-2).min(-1), P < 0.001) compared with PRE-TRN (0.61 +/- 0.15 mg.cm(-2).min(-1)). Despite matched evaporative heat balance requirements, steady-state WBSR (31st-45th min) was greater POST-TRN (12.7 +/- 1.0 g/min, P < 0.02) and POST-HA (12.9 +/- 0.8 g/min, P < 0.004), compared with PRE-TRN (11.7 +/- 0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77 +/- 0.22 degrees C, P < 0.05) and POST-HA (36.62 +/- 0.15 degrees C, P < 0.03) compared with PRE-TRN (36.86 +/- 0.24 degrees C). But Delta T-es was smaller POST-TRN (0.77 +/- 0.19 degrees C, P < 0.05) and POST-HA (0.75 +/- 0.15 degrees C, P < 0.04) compared with PRE-TRN (1.10 +/- 0.32 degrees C). LSRback and LSRarm increased with HA (P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 +/- 2.4 g/min, P < 0.001; POST-HA: 16.8 +/- 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 +/- 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat.