LAUSR

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-weeks of aerobic training (i.e. partial HA) followed by 6-days of HA in 38°C/65%RH (i.e. complete HA). On separate days, esophageal temperature (Tes), arm (LSRarm) and back (LSRback) sweat rate, and whole-body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30%RH) and 60-min uncompensable (37°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°C, P=0.05) and POST-HA (36.60±0.27°C, P=0.001) compared to PRE-TRN (36.99±0.19°C), howeve rΔTes was similar in all trials (PRE-TRN:0.40±0.23°C; POST-TRN:0.42±0.20°C; POST-HA:0.43±0.12°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/cm2/min, P<0.001) and POST-HA (0.75±0.16 mg/cm2/min, P<0.001) compared to PRE-TRN (0.61±0.15 mg/cm2/min). Despite matched evaporative heat balance requirements, steady-state WBSR (31st-45thmin) 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 to PRE-TRN (11.7±0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77±0.22°C, P=0.05) and POST-HA (36.62±0.15°C, P=0.03) compared to PRE-TRN (36.86±0.24°C). But, ΔTes was smaller POST-TRN (0.77±0.19°C, P=0.05) and POST-HA (0.75±0.15°C, P=0.04) compared to PRE-TRN (1.10±0.32°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 to 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.