Physiological Demands and Contributors to Variance of Core Temperature in 162 Recreational Runners during Distance Running in a Warm-Humid Environment

PURPOSE: To determine the physiological demands and their associations with body core temperature (Tc) of tropical recreational runners during mass participation distance running in a warm-humid environment (Dry bulb Temperature: 27.2±0.4°C, relative humidity: 87±2%). METHODS: 162 individuals participated in a 21km (n=84) or 10km (n=78) race. Participants demographics were recorded in a pre-race questionnaire, and in-race measurements of environmental conditions, heart rate (HR, n=115), Tc (n=102), four-site skin temperature (Tsk, n=34) and fluid balance (n=36) were assessed. Real-time monitoring of HR, Tc and geolocation was conducted via a multi-user dashboard. Race split and finishing times were extracted from official results. Correlation and multiple linear regression analyses were performed between various parameters and peak Tc. RESULTS: Participants achieved peak HR (21km: 183±9(154-209)bpm, 10km: 180±10(156-204)bpm), Tc (21km: 39.4±0.6(38.3-40.8)°C, 10km: 39.3±0.6(38.2-41.1)°C) and Tsk (21km: 34.0±0.6(32.8-35.4) °C, 10km: 33.7±0.8(31.9-35.3) °C). Mean Tc was higher (p<0.05) in 21km (38.7±0.5(37.9-39.8)°C) than in 10km (38.5±0.4(37.2-39.9)°C). Tsk exhibited an 'inverted-U' profile in the 21km but plateaued in the 10km race. Body mass loss was -2.5±1.1(-5.5 to -0.7)% and -1.3±0.7(-2.4 to +0.5)% for the 21km and 10km participants, respectively. Starting Tc (18%), mean HR (13%), Body Surface Area (11%), and average speed (9%), but not age, estimated maximal aerobic capacity nor finishing time, to peak Tc. CONCLUSIONS: Recreational runners experienced high cardiovascular and thermal demands. We observed an 'inverted-U' Tsk profile in the 21km race in contrast to a plateau commonly described in laboratory-based findings. Starting Tc, mean HR, Body Surface Area, and average speed were independently associated with inter-individual differences in peak Tc. Real-time monitoring and contributors of peak Tc may inform future development of targeted strategies to optimise safety of recreational populations competing in the heat.