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

Abstract

Objectives Cooling interventions mitigating heat-related declines in autonomic nervous system (ANS) function are understudied in collegiate American football. Methods This study exposed 50 collegiate male athletes during their 2023 season to two bouts of cold air via air-conditioning before (60 min) and during (5 min) training sessions in a hot and humid climate. Armband monitors (Warfighter Monitor™; Tiger Tech Solutions, Inc., Miami, FL) were used to continuously measure body temperature (°C) during training and baseline heart rate (HR), HR recovery and HR variability (HRV) 24 h 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 with 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 body 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 with the 2022 season, the athletes spent considerably less time training under the higher body temperature thresholds (≥38°C: 19.5 vs 9.7 min; ≥39°C: 10.5 vs 0.9 min), respectively, despite longer overall training duration (161.1 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.