Purpose This study aimed to characterize the thermal and cardiovascular strain of professional cyclists during the 2019 Tour Down Under and determine the associations between thermal indices and power output, and physiological strain. Methods Gastrointestinal temperature (Tgi), heart rate (HR), and power output were recorded during the six stages (129-151.5 km) of the Tour Down Under in =22 male participants. Thermal indices included dry-bulb, black-globe, wet-bulb, and wet-bulb-globe (WBGT) temperature; relative humidity (RH), Heat Index; Humidex; and universal thermal climate index. The heat stress index (HSI), which reflects human heat strain, was also calculated. Results Dry-bulb temperature was 23°C-37°C, and RH was 18%-72% (WBGT: 21°C-29°C). Mean Tgi was 38.2°C-38.5°C, and mean peak Tgi was 38.9°C-39.4°C, both highest values recorded during stage 3 (WBGT: 27°C). Peak individual Tgi was =40.0°C in three stages and =39.5°C in 14%-33% of cyclists in five stages. Mean HR was 131-147 bpm (68%-77% of peak), with the highest mean recorded in stage 3 (P = 0.005). Mean power output was 180-249 W, with the highest mean recorded during stage 4 (P < 0.001; 21°C WBGT). The thermal indices most strongly correlated with power output were black-globe temperature (r = -0.778), RH (r = 0.768), universal thermal climate index (r = -0.762), and WBGT (r = -0.745; all P < 0.001). Mean Tgi was correlated with wet-bulb temperature (r = 0.495), HSI (r = 0.464), and Humidex (r = 0.314; all P < 0.05), whereas mean HR was most strongly correlated with HSI (r = 0.720), along with Tgi (r = 0.599) and power output (r = 0.539; all P < 0.05). Conclusions Peak Tgi reached 40.0°C in some cyclists, although most remained <39.5°C with an HR of ~73% of peak. Power output was correlated with several thermal indices, primarily influenced by temperature, whereas Tgi and HR were associated with the HSI, which has potential for sport-specific heat policy development.
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