Objectives: To measure the activity profile, hydration status and core temperature of elite team sport athletes during matches in hot and cool conditions. Design: Thirty-five professional Australian footballers (age 25.9 ± 3.5 yrs; height 188.4 ± 7.8 cm; body mass 90.6 ± 8.8 kg), gave informed consent to participate in this study. Core temperature (Tc), hydration and running performance were compared in eight hot and eight cool matches classified via a rating of the risk of heat illness from the Wet Bulb Globe Temperature (WBGT). Methods: Core temperature was measured via an ingestible sensor before matches and after each quarter and player movement was recorded by 5 Hz GPS and expressed per period of the match (rotation), for distance; high-intensity running (HIR, 4.17-10.00 m s-1), sprinting (>4.17 m s-1) and maximal accelerations (2.78-10.00 m s-2). All data was compared for hot and cool matches and the magnitude of effects was analysed with the effect size (ES) statistic. Results: Core temperature was elevated from rest at all time-points during matches (37.3-39.4 °C), with small additional elevations after the first and third quarters in hot matches (ES: 0.39 ± 0.40 and 0.37 ± 0.42 respectively). In hot matches 12 players had Tc > 40 °C but only one in cool matches. Total distance was reduced in the latter parts of each half (-6.5%, -0.49 ± 0.58; and -6.7%, -0.57 ± 0.59), yet the high intensity tasks of sprinting and accelerating were preserved. Conclusions: Players tolerated core temperatures up to 40.5 °C during hot matches but reduced the volume of running undertaken, thus preserving the ability to undertake high intensity activities.
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