Influence of the Thermal Environment on Work Rate and Physiological Strain during a UCI World Tour Multistage Cycling Race

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 (T-gi), 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 degrees C-37 degrees C, and RH was 18%-72% (WBGT: 21 degrees C-29 degrees C). Mean T-gi was 38.2 degrees C-38.5 degrees C, and mean peak T-gi was 38.9 degrees C-39.4 degrees C, both highest values recorded during stage 3 (WBGT: 27 degrees C). Peak individual T-gi was >= 40.0 degrees C in three stages and >= 39.5 degrees 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 degrees 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 T-gi 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 T-gi (r = 0.599) and power output (r = 0.539; all P < 0.05). Conclusions: Peak T-gi reached 40.0 degrees C in some cyclists, although most remained <39.5 degrees C with an HR of similar to 73% of peak. Power output was correlated with several thermal indices, primarily influenced by temperature, whereas T-gi and HR were associated with the HSI, which has potential for sport-specific heat policy development.

Automated summary

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. The results showed that the gastrointestinal temperature, heart rate, and power output of cyclists were associated with various thermal indices.