Physiological responses during acute, high intensity functional training: normal vs. hot environmental conditions.

Introduction: Compelling evidence confirms that regular exercise improves health and reduces the risk of chronic diseases. Only 1 in 4 adults meet exercise guidelines. The common reported barriers include a “lack of time or enjoyment”. High intensity functional training (HIFT) is a time efficient modality combining resistance training and high intensity cardiorespiratory exercise. Additionally, substantial evidence shows heat acclimation can improve thermoregulation, attenuate physiological strain, reduce heat illness and enhance endurance and other cardiometabolic outcomes.  Therefore, the purpose of this study was to determine the physiological, metabolic, and perceptual responses during HIFT with and without artificial heat exposure. Methods: 19 healthy adults (age=19-43 years) completed an acute bout of HIFT in both a normal (NORM) and hot (HOT) environment with physiological, metabolic, and perceptual measures obtained for each session. Results: Paired t-tests revealed multiple HIFT session physiological and metabolic responses were significantly higher (p<0.05) in HOT compared to NORM conditions, respectively: overall session HR (151 ± 25.9 bpm vs. 144.1 ± 27.2 bpm), % HR reserve (65.9 ± 6.6% vs. 60.7 ± 7.5%), % oxygen uptake reserve (63.2 ± 5.2% vs. 59.1 ± 5.9%), metabolic equivalents (8.8 ± 1.4 METs vs. 8.4 ± 1.4 METs), energy expenditure per minute (10.4 ± 1.9 kcal/min vs. 9.8 ± 1.9 kcal/min), overall HIFT session energy expenditure (311.6 ± 59.5 kcal vs. 294.1 ± 57.1 kcal). Overall, core temperature in HOT was 0.4% greater than in NORM (100.2 ± 0.9° vs. 99.9 ± 0.6°, p<0.05). Paired t-tests revealed both HIFT rating of perceived exertion (7.1 ± 1.4 vs. 6.5 ± 1.4) and HIFT thermal comfort rating (3.3 ± 1.0 vs. 2.3 ± 0.7) scores were significantly higher (p<0.05) in HOT vs NORM conditions. Conclusion: Quantifying the acute physiological and metabolic responses to HIFT provides health and exercise professionals insight into of the safety and effectiveness of this type of exercise. An acute bout of HIFT in HOT conditions can elicit greater physiological and metabolic responses relative to NORM. Therefore, adding heat to a HIFT session represents an additional variable, beyond volume and/or intensity that can modify the training stimulus. In summary, HIFT may be an effective, safe, and time-efficient exercise modality for many populations.