Physiological response to exercise in the heat: Implications for risk mitigation and adaptation

Area of Science:

• Exercise Physiology
• Environmental Health
• Human Performance

Background:

• Recent studies show a disconnect between core body temperature (Tc) and task completion during physical exertion in hot conditions.
• Understanding the physiological factors influencing individual responses to exertional heat stress is critical for safety and performance.

Purpose of the Study:

• To investigate physiological differences between individuals who successfully complete exercise in the heat versus those who become hyperthermic or symptomatic.
• To test the hypothesis that central and peripheral physiological variables differ systematically between successful completers and those experiencing heat strain.

Main Methods:

• Thirty-eight healthy participants performed a 120-minute walk (5 km/h, 2% grade) in a 40°C, 50% relative humidity chamber.
• Physiological heat strain was assessed at rest and during exercise.
• Participants were categorized as Completers (n=27), Hyperthermics (Tc > 39°C, n=7), or Symptomatics (n=4).

Main Results:

• Hyperthermic individuals had significantly higher visceral adipose tissue compared to Completers (p=0.034).
• Hyperthermic participants exhibited higher heart rate (p=0.009) and lower end-diastolic volume and stroke volume (p=0.031) early in the walk.
• Symptomatic individuals did not reach Tc > 39°C, and Hyperthermics reported no symptoms.

Conclusions:

• During exertional heat exposure, increased adiposity and early hemodynamic alterations are linked to elevated core body temperature.
• Hyperthermia itself was not associated with symptom development in this cohort.
• Baseline body composition and fitness parameters did not predict symptom development.