Aerobic influence on neuromuscular function and tolerance during passive hyperthermia

Area of Science:

• Exercise Physiology
• Environmental Physiology
• Neuromuscular Physiology

Background:

• Aerobic fitness influences physiological responses to heat stress.
• Understanding neuromuscular adaptations during hyperthermia is crucial for performance and safety.

Purpose of the Study:

• To investigate the impact of aerobic fitness on central neuromuscular activation and maximal voluntary contractile force during passive hyperthermia.
• To assess how different levels of aerobic fitness affect tolerance to whole-body hyperthermia.

Main Methods:

• Thirty-seven healthy males were categorized into highly fit (HF), moderately fit (MF), and lower fit (LF) groups.
• Participants underwent passive heating using a liquid conditioning garment in a hot environment (35°C, 50% RH) to induce whole-body hyperthermia.
• Maximal isometric knee extension force and voluntary activation were measured before and during heating, aiming for a core temperature of 39.0°C.

Main Results:

• Passive heating significantly reduced force production and voluntary activation across all groups.
• Cardiovascular strain increased, with higher mean arterial pressure (MAP) in HF and MF groups compared to LF.
• Lower-fit individuals exhibited reduced tolerance to passive heating, terminating the protocol earlier than fitter groups, despite similar thermal sensation ratings.

Conclusions:

• Low aerobic fitness and activity levels are linked to decreased tolerance to passive hyperthermia.
• Despite differences in heat tolerance, maximal force production and voluntary activation were equally impaired at high body temperatures (39.0°C) regardless of aerobic fitness level.