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Body core temperature (T c ) monitoring is crucial for minimizing heat injury risk. However, validated strategies are invasive and expensive. Although promising, aural canal temperature (T ac ) is susceptible to environmental influences. This study investigated whether incorporation of external auricle temperature (T ea ) into an ear-based T c algorithm enhances its accuracy during multiple heat stress conditions. Twenty males (mean ± SD; age = 25 ± 3 years, BMI = 21.7 ± 1.8, body fat = 12 ± 3%, maximal aerobic capacity (VO 2max ) = 64 ± 7 ml/kg/min) donned an ear-based wearable and performed a passive heating (PAH), running (RUN) and brisk walking trial (WALK). PAH comprised of immersion in hot water (42.0 ± 0.3 °C). RUN (70 ± 3%VO 2max ) and WALK (50 ± 10%VO 2max ) were conducted in an environmental chamber (T db  = 30.0 ± 0.2 °C, RH = 71 ± 2%). Several T c models, developed using T ac , T ea and heart rate, were validated against gastrointestinal temperature. Inclusion of T ea as a model input improved the accuracy of the ear-based T c algorithm. Our best performing model (T rf3 ) displayed good group prediction errors (mean bias error = - 0.02 ± 0.26 °C) but exhibited individual prediction errors (percentage target attainment ± 0.40 °C = 88%) that marginally exceeded our validity criterion. Therefore, T rf3 demonstrates potential utility for group-based T c monitoring, with additional refinement needed to extend its applicability to personalized heat strain monitoring., (© 2024. The Author(s).)

Background: Excessive heat exposure can lead to hyperthermia in humans, which impairs physical performance and disrupts cognitive function. While heat is a known physiological stressor, it is unclear how severe heat stress affects brain physiology and function., Methods: Eleven healthy participants were subjected to heat stress from prolonged exercise or warm water immersion until their rectal temperatures (T re ) attained 39.5°C, inducing exertional or passive hyperthermia, respectively. In a separate trial, blended ice was ingested before and during exercise as a cooling strategy. Data were compared to a control condition with seated rest (normothermic). Brain temperature (T br ), cerebral perfusion, and task-based brain activity were assessed using magnetic resonance imaging techniques., Results: T br in motor cortex was found to be tightly regulated at rest (37.3°C ± 0.4°C (mean ± SD)) despite fluctuations in T re . With the development of hyperthermia, T br increases and dovetails with the rising T re . Bilateral motor cortical activity was suppressed during high-intensity plantarflexion tasks, implying a reduced central motor drive in hyperthermic participants (T re  = 38.5°C ± 0.1°C). Global gray matter perfusion and regional perfusion in sensorimotor cortex were reduced with passive hyperthermia. Executive function was poorer under a passive hyperthermic state, and this could relate to compromised visual processing as indicated by the reduced activation of left lateral-occipital cortex. Conversely, ingestion of blended ice before and during exercise alleviated the rise in both T re and T br and mitigated heat-related neural perturbations., Conclusion: Severe heat exposure elevates T br , disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance., (Copyright © 2023. Production and hosting by Elsevier B.V.)

Objectives: Compare the effects of ice slurry ingestion at low and normal doses on endurance capacity and exertional heat stress-induced gastrointestinal perturbations., Design: Randomised, cross-over design., Methods: Twelve physically active males completed four treadmill running trials, ingesting ice slurry (ICE) or ambient drink (AMB) at 2 g·kg -1 (Normal; N) or 1 g·kg -1 (Low; L) doses every 15-min during exercise and 8 g·kg -1 (N) or 4 g·kg -1 (L) pre- and post-exercise. Pre-, during and post-exercise serum intestinal fatty-acid binding protein ([I-FABP]) and lipopolysaccharide ([LPS]) concentrations were determined., Results: Pre-exercise gastrointestinal temperature (T gi ) was lower in L + ICE than L + AMB (p < 0.05), N + ICE than N + AMB (p < 0.001) and N + ICE than L + ICE (p < 0.001). Higher rate of T gi rise (p < 0.05) and lower estimated sweat rate (p < 0.001) were observed in N + ICE than N + AMB. Rate of T gi rise was similar at low dose (p = 0.113) despite a lower estimated sweat rate in L + ICE than L+AMB (p < 0.01). Time-to-exhaustion was longer in L + ICE than L + AMB (p < 0.05), but similar between N + ICE and N + AMB (p = 0.142) and L + ICE and N + ICE (p = 0.766). [I-FABP] and [LPS] were similar (p > 0.05)., Conclusions: L + ICE elicited a lower heat dissipation compensatory effect with similar endurance capacity as N + ICE. Ice slurry conferred no protection against exertional heat stress-induced gastrointestinal perturbations., Competing Interests: Declaration of interest statement None., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)