The influence of heat acclimation on the relation and agreement between perceptual and physiological strain in older males during exercise-heat stress.

To increase exercise adherence, there is a growing shift toward telehealth exercise-based programs. Alongside, passive heat therapy is gaining attention as an alternative approach to induce physiological adaptations, especially for individuals unable to exercise. However, heat therapy may reduce perceived effort, complicating actual assessment of physiological strain in telehealth settings. We assessed the relationship and agreement between physiological and perceptual strain in twelve older men (median age: 68 years) during intermittent exercise (three, 30-minute cycling bouts at increasing rates of metabolic heat production of 150, 200, and 250 W·m-2, each separated by 15 minutes of rest) in the heat (40°C) before and after 7 consecutive days of warm-water immersion (~40°C), with core (rectal) temperature clamped at ~38.5°C for 60 minutes. Physiological strain (PSI) was indexed from core temperature and heart rate and adjusted for skin temperature (aPSI) while perceptual strain (PeSI) used rating of perceived exertion and thermal sensation. Linear mixed models assessed the relationship, and Bland-Altman analysis quantified the agreement between measures (mean bias (95% [LoA]). PSI and aPSI increased with perceptual strain (P<0.001) and were not influenced following acclimation (P>0.610). Similarly, the agreement between physiological and perceptual strain was not affected by passive heating (PSI: -0.4 AU; aPSI: -0.8 AU), but the 95% LoA were wide (PSI: [-4.1 to 3.0 AU]; aPSI: [-4.5 to 2.9 AU]). Although physiological and perceptual strain are linearly related, individual variability may limit the utility of perceptual indices as surrogate markers of physiological strain in older adults during exercise. ClinicalTrials.gov identifier: NCT05838612.