Passive Heat Stress, Like Exercise Training, Leads to Improved Vascular Function

Epidemiological data indicate that repeated heat stress improves cardiovascular health, making passive heat therapy (PHT) a potential alternative for those unable to exercise. Few studies to date have examined the potential exercise mimetic effects in humans, and it is unclear how adaptations compare in magnitude to exercise training. OBJECTIVE: To examine the effects of 6 weeks of localized, muscle-focused PHT on resistance artery vascular function, exercise hemodynamics and performance, molecular regulators of vascular tone, and skeletal muscle capillarization relative to the adaptations observed following high-intensity aerobic exercise training focused on the same muscles. HYPOTHESIS: 6 weeks of PHT, applied to the knee extensor muscles through pulsed shortwave diathermy (2 hr, 3 days/week), would improve both resistance artery function and exercise hemodynamics, as well as increase vascular tone regulators more than a sham treatment, but less than single-leg knee extension (KE) exercise training (EX; 40 min, 3 days/week). Methods: We randomized 34 sedentary but otherwise healthy, young adults (ages 18–36; n = 17 female, 17 male) to receive PHT, EX, or sham heating sessions (SHAM; 2 hr, 3 days/week) over 6 weeks. Vascular function was determined through the blood flow response during both a passive leg movement (PLM) assessment and a knee extension graded exercise test (GXTmax). Muscle biopsies were taken from the vastus lateralis at baseline and after 6 weeks of intervention. Results: Peak blood flow during PLM increased to the same extent in both the EX (~10.5% increase, P=0.009) and PHT groups (~8.5% increase, P=0.044). KE peak flow increased in EX (~19%, P=0.005), but did not change in PHT (P=0.523) and decreased in SHAM (~7%, P=0.020). Peak vascular conductance during KE increased by ~25% in EX (P=0.030) and PHT (P=0.012). KE peak power increased in EX by ~27% (P=0.001) but did not significantly change in PHT (P=0.175) and SHAM groups (P=0.111). EX, but not PHT or SHAM, increased muscle capillary-to-fiber ratio (P = 0.0003), capillary density (P = 0.0428), and the Capillary to Fiber Perimeter Exchange Index (P = 0.0089). Protein expression of endothelial nitric oxide synthase (eNOS) increased significantly in both EX (P=0.0028) and PHT (P=0.0095), but only EX resulted in increased angiogenesis. CONCLUSIONS: 6 weeks of localized PHT, when applied to young healthy individuals, improved resistance artery function at rest and during exercise to the same extent as exercise training. Moreover, our data suggest that these PHT-induced improvements can be primarily attributed to increased expression of the regulators of vascular tone and not neo-angiogenesis. In conclusion, passive heat therapy mimics many, but not all the vascular benefits of exercise training. Funding for this study was provided by the Department of Exercise Sciences at Brigham Young University. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.