Differing Physiological Adaptations Induced by Dry and Humid Short-Term Heat Acclimation

Area of Science:

• Environmental Physiology
• Exercise Science
• Sports Medicine

Background:

• Short-term heat acclimation (STHA) protocols aim to improve thermoregulation and exercise performance in hot environments.
• The effectiveness of STHA may vary depending on environmental conditions, such as humidity levels.
• Understanding divergent adaptations to dry vs. humid heat exposure is crucial for optimizing training strategies.

Purpose of the Study:

• To compare the effects of a 5-day STHA protocol in dry heat versus humid heat on endurance cycling performance.
• To assess physiological adaptations, including mechanical efficiency, oxygen uptake, energy expenditure, plasma volume, and sweat rate, following dry and humid STHA.
• To determine if STHA in different heat conditions impacts endurance performance in a temperate environment (21°C).

Main Methods:

• A randomized, cross-over design involving 11 cyclists completing two 5-day STHA blocks (dry and humid) matched for heat index and total exposure time.
• Pre- and post-STHA assessments included temperate endurance performance metrics (mean maximal power, lactate thresholds) and a heat stress test.
• Physiological markers such as plasma volume expansion and sweat rate were monitored to evaluate heat adaptation.

Main Results:

• No significant differences in temperate endurance cycling performance were observed between dry and humid STHA conditions.
• Dry STHA likely reduced gross mechanical efficiency and increased oxygen uptake and energy expenditure for a given workload.
• Plasma volume expansion was similar post-acclimation, but retention was longer after humid STHA. Sweat rate increased significantly during dry STHA.

Conclusions:

• STHA protocols, whether in dry or humid conditions, did not enhance temperate endurance cycling performance.
• STHA induced distinct physiological adaptations between dry and humid environments, suggesting condition-specific responses.
• Further research is needed to understand the long-term implications of these divergent adaptations on performance.