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Heart Rate Variability During Exercise-Heat Stress Following Seven Days of Passive Heat Acclimation in Older Males: A
Andres E Carrillo1, Rachel A Luomala1, Kristina-Marie T Janetos2
1Department of Exercise Science, College of Health Sciences, Chatham University, Pittsburgh, PA.
Purpose:
The extent to which heart rate variability (HRV), a non-invasive marker of cardiac autonomic modulation, reflects thermoregulatory adaptations to heat acclimation in heat-sensitive older adults remains unclear. We examined the effects of seven consecutive days of passive heat acclimation on HRV during intermittent exercise-heat stress in older males.
Methods:
Eleven older males (mean (SD): 68.0 (5.6) years) completed seven days of warm-water immersion (~40°C; ~90 min/day). Before and the day after the intervention, HRV, rectal temperature, and cumulative body heat storage were assessed during three, 30-min exercise bouts at fixed metabolic heat production rates (150, 200, and 250 W·m⁻²), each followed by 15 min of rest, inside an air calorimeter regulated at 40°C and ~13% relative humidity.
Results:
Rectal temperature and body heat storage were significantly higher during light-to-vigorous exercise before acclimation compared with after (p ≤ 0.007). Heart rate was lower post-acclimation (p = 0.012), while high-frequency (HF) band, reflecting greater cardiac parasympathetic modulation, tended to be higher (p = 0.07). A greater increase in HF following acclimation was associated with corresponding reductions in body heat storage during light-intensity exercise-heat stress (r = -0.72, p = 0.01). RMSSD, another index of parasympathetic activity, increased following acclimation, particularly during light-intensity exercise (p = 0.012).
Conclusions:
Seven days of passive heat acclimation was accompanied by changes in HRV responses during light-to-vigorous exercise-heat stress in older males, consistent with greater parasympathetic modulation. These responses provide insight into cardiac autonomic regulation beyond changes in heart rate. These changes corresponded with reductions in heat storage and likely reflect, at least in part, reduced physiological strain, which may contribute to improved heat resilience and physiological stability.
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