Associations Between Physical Activity, Body Mass Index, and Cardiac Autonomic Function in Adolescent Men

Physical activity (PA) is positively associated with well-balanced regulation of the autonomic nervous system (ANS), measured as heart rate variability (HRV), while obesity is linked more towards ANS dysfunction. However, the interplay between PA, obesity, and HRV during adolescence remains less understood. This study examined the relationship between PA, body mass index (BMI), and HRV in a population-based sample of adolescent men. The study included 3,389 adolescent men (mean age 18 ± 0.2 years, mean BMI 23 ± 4) attending military call-ups. Short-term vagal related HRV index root mean square of successive differences (rMSSD) was measured using a Polar (RS800) heart rate monitor. Body composition was assessed via bioimpedance (Inbody 720), and anthropometric data were collected using standard methods. PA was self-reported via standardized questionnaire and categorized into low, moderate, or high PA based on intensity and volume. Vagal modulation associated rMSSD significantly (p < 0.001) increased according to PA level both in normal weight (BMI 18.5–24.9), overweight (BMI 25–29.9), and individuals with obesity groups (BMI ≥30). In multivariable models, PA alone explained 4% of rMSSD variance in individuals with normal weight (p < 0.001). Among participants with overweight PA together with waist circumference explained 5.9% (p < 0.001) of the variance in rMSSD and 7.4% (p < 0.001) of the variance among obese participants, respectively PA was positively associated with vagal modulation of HRV in adolescent men, regardless of BMI. The association was strongest in those with obesity, suggesting that regular physical activity may partially mitigate obesity-related reductions in vagal autonomic function.