Resting Heart Rate Variability in Relation to Body Mass Index and Physical Activity in Young Adults: A Cross-Sectional Study

Abstract

Background: Heart rate variability (HRV) provides a non-invasive window into cardiac autonomic modulation. Excess body weight and low habitual physical activity may alter autonomic balance even in otherwise healthy young adults. This study examined the relationship of resting HRV with body mass index (BMI) and self-reported physical activity. Methods: A cross-sectional observational study was conducted among 140 apparently healthy young adults aged 18–25 years from March 2025 to February 2026. Height and weight were measured using standard anthropometric methods, and body mass index (BMI) was calculated as weight in kilograms divided by height in metres squared. Habitual physical activity was assessed using the International Physical Activity Questionnaire–Short Form (IPAQ-SF). After 10 minutes of supine rest, a 5-minute electrocardiogram was recorded under standardized laboratory conditions. RMSSD and SDNN were derived as time-domain indices of heart rate variability. Group differences were assessed using one-way analysis of variance, and linear associations were examined using Pearson correlation. Statistical significance was set at p<0.05. Results: Resting HRV decreased progressively across increasing BMI categories and increased with greater physical activity. RMSSD declined from 42.3 ± 5.8 ms in the underweight category to 28.4 ± 6.5 ms in the obese category, while SDNN declined from 55.2 ± 7.1 to 39.9 ± 8.0 ms. BMI-group differences were significant for RMSSD (F=23.90, p<0.001, η²=0.345) and SDNN (F=20.31, p<0.001, η²=0.309). RMSSD and SDNN also increased significantly across low, moderate and high physical-activity groups (F=42.54 and 37.82, respectively; both p<0.001). BMI correlated inversely with RMSSD (r=-0.51) and SDNN (r=-0.47), whereas physical activity correlated positively with RMSSD (r=0.48) and SDNN (r=0.53); all correlations were statistically significant (p<0.001). Conclusion: Within the limits of a cross-sectional design, higher BMI was associated with lower resting HRV, while greater physical activity was associated with more favorable autonomic indices. These findings support early lifestyle-based cardiovascular risk reduction in young adults, although causal inference requires longitudinal or interventional research.

Keywords: heart rate variability; body mass index; physical activity; autonomic nervous system; RMSSD; SDNN; young adults