A Single Combined Active and Passive Isothermic Heating Protocol Results in a Similar Core Temperature Response as Exercise Alone in Hot Conditions

Heat acclimatization can be induced by targeting a core temperature 38.5°C for at least 60 minutes per day lasting 5 to 21 days, complementary to normal exercise activity. However, consistently meeting this threshold on consecutive days may be difficult for athletes. The objective of this study was to evaluate the efficacy of four single-bout heating protocols to reach a core temperature 38.5°C. The study was set up as a non-randomized field study, factoring in the September-October outdoor desert conditions, Tempe, AZ, USA. Environmental conditions were measured using a Kestrel heat stress tracker. Protocols were constituted out of 3 elements: PAS – passive heat exposure in a tent (54±1°C), EH - exercise in hot condition with high intensity interval training (HIIT) outdoors in the heat in a tent with a ventilator (43±1°C), EM – exercise in moderate conditions with HIIT indoors (22±0.4°C). All participants performed protocols in the following order: 1) PAS 60-min; 2) EH-PAS (EH 30-min + PAS 30-min); 3) EH 60-min, and 4) EM 60-min. A cycle ergometer was used for HIIT (2-min warm-up followed by 7x2-min sprints with 2-min relative rest between sprints during the first 30 min and stationary cycling for the second 30 min), with a self-selected workload at 80-100 rpm and similar heart rate (HR) response during exercise testing for EH: 146±10, EM: 142±13, and EH-PAS: 142±13 (P>0.05). A total of 10 active male students (25±3 years old) reported no difference between protocols for baseline Tc (P=0.37) and HR (P=0.28). During the first 30-min, Tc was significantly different between protocols (average ranging from 37.3-37.6°C, P=0.01), but from a practical perspective, differences were limited. During the second 30-min session, the Tc for EH (38.5±0.4°C) and EH-PAS (38.6±0.4°C) were significantly higher from EM (38.1±0.4°C) and PAS alone (37.8±0.4°C), P<0.001. The average HR (bpm) was significantly lower in PAS (110±17) and EH (136±13) during the second half of the protocols compared to the EH (151±10) and EM (149±16), P<0.001. In conclusion, exercise alone vs. a combination of exercise and passive heating in hot conditions resulted both in a body temperature 38.5°C, but the combination was more efficient since participants exercised for only 30-minutes.