Comparative Analysis of Autonomic Function Across Endurance Athletes and Sedentary Adults

Thesis Abstract

This study addresses the problem of how autonomic function differs between individuals with high endurance training and sedentary adults, with implications for cardiovascular risk stratification and performance physiology. The aim is to compare autonomic nervous system (ANS) regulation between endurance athletes and sedentary controls, and to identify associations between autonomic indices and physiological markers of fitness, stress, and metabolic health. Specific objectives are (1) to quantify resting and orthostatic heart rate variability (HRV) metrics; (2) to evaluate baroreflex sensitivity (BRS) and sympathetic tone using pupillometry and preejection period measures; (3) to examine autonomic responses to standardized autonomic challenges (deep breathing, Valsalva maneuver, head-up tilt) in both groups; (4) to assess relationships between autonomic indices and cardiorespiratory fitness (maximal oxygen uptake, VO2max), body composition, and lipid profile; and (5) to test whether lifestyle factors modulate autonomic differences independent of fitness level. The methodology employs a cross-sectional, comparative design. A total of 120 participants will be recruited 60 endurance athletes (competitive long-distance runners or cyclists with ?5 years of training and VO2max >60 mL·kg?1·min?1 for men and >55 mL·kg?1·min?1 for women) and 60 sedentary controls (engaging in <1 hour of structured physical activity weekly). Exclusion criteria include cardiovascular disease, hypertension, diabetes, smoking, and use of autonomic-modulating medications. Data collection will involve (1) resting-state HRV (time- and frequency-domain measures), (2) BRS assessment using the sequence method and transfer function analysis, (3) autonomic challenges (deep breathing at six breaths per minute, Valsalva maneuver, head-up tilt 70° for 5 minutes) with continuous electrocardiography and blood pressure monitoring, (4) pupillometry for sympathetic arousal, (5) non-invasive assessment of arterial stiffness (pulse wave velocity) and central hemodynamics, (6) body composition analysis via dual-energy X-ray absorptiometry, and (7) biochemical profiling including lipid panel, fasting glucose, insulin, and inflammatory markers (CRP). VO2max will be obtained from a graded treadmill or cycle ergometer test with respiratory gas analysis. Validated instruments include the Montreal Cognitive Assessment to control for cognitive load during tasks and the Perceived Stress Scale to account for psychological stress. Data analysis will proceed in three stages. First, descriptive statistics will summarize demographic, anthropometric, and autonomic measures. Second, multivariate analysis of covariance (MANCOVA) will compare autonomic indices between groups across resting and challenge conditions, adjusting for age, sex, and body composition. Third, multiple regression analyses will examine the independent contributions of VO2max, body fat percentage, lipid profile, and inflammatory markers to HRV, BRS, and sympathetic indices. Structural equation modeling (SEM) will test a theoretical model linking endurance training, physiological mediators (cardiorespiratory fitness, arterial stiffness, metabolic health), and autonomic outputs. The theoretical framework incorporates the Neurovisceral Integration Model and the Allostatic Load framework to interpret autonomic adaptability as a function of training status and systemic health. Anticipated findings include higher resting HRV, enhanced BRS, and attenuated sympathetic reactivity in endurance athletes compared with sedentary adults, with stronger autonomic reserve during orthostatic and respiratory challenges. It is expected that VO2max and lower adiposity will be positively associated with parasympathetic dominance and more favorable autonomic responses, whereas elevated inflammatory markers may attenuate these advantages. The study contributes to knowledge by delineating which facets of autonomic regulation are most influenced by sustained endurance training and how metabolic health interacts with autonomic control in real-world populations. It also informs clinical risk assessment by identifying autonomic profiles linked to superior fitness and reduced cardiovascular risk. The main conclusion is that long-term endurance training is associated with favorable autonomic regulation characterized by enhanced parasympathetic activity and preserved baroreflex function, with these advantages partially modulated by body composition and metabolic health. Recommendations include incorporating autonomic function assessment into athlete monitoring programs, promoting interventions to optimize metabolic health to maximize autonomic benefits, and exploring longitudinal designs to establish causal pathways between training, autonomic adaptation, and cardiovascular outcomes.