No evidence of higher peak heat tolerance in the afternoon or with opportunity for thermal behaviour during low-intensity work in uncompensable heat.

Abstract

Heat tolerance is known to be limited by high brain temperature, but most findings pertain to strenuous exercise following artificially altered baseline temperature or with limited opportunity for behaviour. Therefore, we investigated the hypotheses that thermal perceptions, hyperventilation and cerebrovascular responses would be exacerbated in: (1) the afternoon relative to morning; and (2) the absence of opportunity for behavioural thermoregulation. Thirteen habitually active participants (seven females) cycled at 25% peak power wearing a water-perfused suit (∼48°C) until volitional tolerance on three occasions, once in the morning (AM) and twice in the afternoon (PM), with or without access to face fanning with mist (∼1.7 m/s). Heating took ∼2 h. Core temperature (T_c) was 0.35°C lower in AM at baseline (95% confidence interval: -0.55°C, -0.16°C; P < 0.001, Hedges' g = 1.673) but not at tolerance (39.10°C ± 0.71°C vs. 39.11°C ± 0.50°C; P = 0.947, g = 0.024), hence the rise was greater (95% confidence interval: 0.00°C, 0.80°C; P = 0.047, g = 0.651). The fan did not confer higher T_c tolerance (95% confidence interval: -0.44°C, 0.24°C; P = 0.764, g = 0.050). The modelled T_c thresholds for heat discomfort and hyperventilation were ∼0.23°C lower in AM, but sensitivities did not differ reliably or with fan availability (P ≥ 0.098). Internal carotid artery flow was similar across time of day (P = 0.904) and showed little effect of heating. Changes in estimated intracranial pressure did not differ reliably between conditions. In conclusion, thermal reserve was smaller in PM by virtue of higher baseline, and psychophysical, ventilatory and cerebrovascular responses were not reliably different. For most individuals, an opportunity for behaviour did not improve discomfort, hyperventilation or tolerance.