Combined active and passive isothermic heating leads to similar core temperature compared to exercise alone

Context: The training stress of heat acclimatization optimizing exercise performance in a hot environment can be demanding.

Objectives: This study evaluated the efficiency of different single heating protocols to elevate core temperature.

Design: Not randomized controlled trial.

Setting: Passive heating (PAS), 30-min active heating using a high-intensity bike protocol (HIBP) in a hot environment with 30-min passive heating (EH-PAS), 60-min HIBP in a hot environment (EH), or 60-min HIBP at room temperature (EM).

Participants: Ten male participants (25±3 years) performed four different 60-min heating strategies at least one week apart.

Main outcome measures: Body core temperature and heart rate.

Results: The highest peak gastrointestinal temperature occurred in EH-PAS (39.1±0.4°C), followed by EH (38.9±0.3°C), EM (38.4±0.3°C), and PAS (38.1±0.5°C). The average HR, measured as a control for intensity, was not different between exercise strategies (EH-PAS: 142±12.3, EH: 146±9.7, and EM: 142±13.3, p>0.05), while different from PAS: 98±15.2 bpm, p<0.05.

Conclusions: Adding passive heating to a shorter exercise protocol can be just as effective in keeping core temperature elevated as exercise in the heat alone during a 60-minute session. Therefore, a single-bout combination of exercise and passive heating may result in a similar body temperature induction compared to exercise heat stress alone.