Comparative Cardiac Autonomic Function in Endurance vs. Sedentary Adults

Thesis Abstract

Cardiac autonomic function is a key determinant of cardiovascular health, yet comparative data distinguishing endurance-trained from sedentary adults remain limited and fragmented. The study addresses the problem of whether regular endurance training modulates autonomic regulation of heart rate and cardiovascular responses across resting, postural, and stress-provoked conditions, and whether such modulation translates into more favorable autonomic balance and reactivity profiles. The aim is to elucidate differences in autonomic modulation between endurance athletes and sedentary adults and to identify the extent to which training mediates parasympathetic and sympathetic activity, tolerance to orthostatic stress, and autonomic recovery kinetics. Specific objectives are to (i) quantify resting heart rate variability (HRV) indices, including time- and frequency-domain measures; (ii) compare autonomic responses to head-up tilt (HUT) and mental arithmetic stress; (iii) assess baroreflex sensitivity (BRS) and heart rate recovery (HRR) after standardized submaximal exercise; (iv) examine correlations between training history (years of endurance training, weekly volume, and VO2 max) and autonomic indices; and (v) evaluate sex differences in autonomic adaptations to endurance training. A cross-sectional comparative design will be employed. The population comprises adults aged 25–40 years recruited from competitive endurance athletes (n=60; 30 female, 30 male) and age-, sex-, and BMI-matched sedentary controls (n=60; 30 female, 30 male). Data collection will occur in a controlled laboratory setting following a 12-hour fast and abstinence from caffeine and nicotine. Primary instruments include a high-resolution electrocardiogram for HRV analysis (RMSSD, pNN50, LF, HF, LF/HF ratio), noninvasive beat-to-beat blood pressure for BRS estimation (sequence method and transfer function analysis), impedance cardiography for stroke volume and cardiac output, and a standardized isometric and dynamic submaximal exercise protocol to elicit HRR. Additional measures include VO2 at ventilatory threshold via metabolic cart and a validated questionnaire for physical activity history. Data will be analyzed using a two-factor MANOVA with group (endurance vs. sedentary) and sex as between-subjects factors, supplemented by multiple linear regression to examine associations between training variables and autonomic indices. Post-hoc tests (Bonferroni-corrected) will identify specific group differences. Mediation analyses will explore whether VO2 max mediates the relationship between training status and HRV parameters. Assumptions will be checked, and effect sizes will be reported (partial eta-squared, Cohen’s d). A significance threshold of p<0.05 will be applied. Expected findings include greater resting parasympathetic dominance in endurance athletes as indicated by higher RMSSD and HF power and lower LF/HF ratio, enhanced baroreflex sensitivity, and faster HRR following submaximal exercise compared with sedentary controls. Endurance-trained individuals are anticipated to exhibit attenuated heart rate and blood pressure reactivity to orthostatic stress (HUT) and smaller cortisol-related responses during cognitive stress, reflecting improved autonomic flexibility. Training volume and VO2 max are expected to strongly correlate with parasympathetic indices and BRS, while sex may moderate magnitude of autonomic adaptations, with nuanced differences observed in female athletes. The study is expected to contribute to knowledge by providing robust cross-sectional evidence on how long-term endurance training shapes cardiac autonomic regulation in young to middle-aged adults, clarifying whether observed autonomic advantages translate to improved cardiovascular resilience and stress tolerance. The findings could inform guidelines for exercise prescription emphasizing autonomic health, particularly in populations at risk for dysautonomia or orthostatic intolerance. Practical implications may include targeted training programs to optimize HRV and autonomic balance and the identification of individuals who may require tailored interventions based on autonomic profiles. The conclusion anticipates that sustained endurance training is associated with favorable autonomic cardiac function characterized by enhanced parasympathetic activity, improved baroreflex control, and rapid autonomic recovery, with the magnitude of benefits linked to training history and fitness level rather than age within the studied range. Recommendations include longitudinal investigations to confirm causal relationships, exploration of dose–response effects of training intensity and duration on autonomic function, and integration of autonomic assessments into routine athlete monitoring and clinical risk stratification.