Solving Heat Stress in Sport: Intermittent Cold Air Exposure Mitigates Deterioration of the Autonomic Nervous System in American Football Athletes.

Objectives: Cooling interventions mitigating heat-related declines in autonomic nervous system (ANS) function are understudied in American football.

Methods: This study exposed fifty, collegiate male athletes during their 2023 season to two bouts of cold air via air-conditioning prior to (60 mins) and during (5 min) training sessions in a hot and humid climate. Armband monitors, (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA), were used to continuously measure core temperature (°C) during training and baseline heart rate (HR), HR recovery and HR variability (HRV) 24 hours post training. Two HRV time-domain indices were estimated: root mean square of the standard deviation of the NN interval (rMSSD) and the standard deviation of the NN interval (SDNN). These data (2023) were then compared to data collected throughout the previous season (2022) during which players were not provided a cold-air intervention. As such, the 2022 cohort served as a "control" group.

Results: Linear regression models no statistically significant associations at the following core temperature thresholds: ≥ 37.0 °C (baseline HR: p = 0.74; HR recovery: p = 0.71, rMSSD: p = 0.74; SDNN: p = 0.78), ≥ 38.0 °C baseline HR: p = 0.74; HR Recovery: p = 0.74; rMSSD: p = 0.68 and SDNN: p = 0.89) and ≥ 39.0 °C: baseline HR: p = 0.75; HR Recovery: p = 0.82; rMSSD: p = 0.78 and SDNN: p = 0.83). Compared to the 2022 season, the athletes spent considerably less time training under the higher core temperature thresholds (≥ 38 °C: 19.5 min vs 9.7 min; ≥ 39 °C: 10.5 min vs 0.9 min), respectively, despite longer overall training duration, (161.1 min vs 187.4 min).

Conclusions: Our findings suggest that incorporating bouts of cold air exposure when training in high heat and humidity may mitigate the established heat-related deterioration in ANS function.