Investigating the relationship between cardiac autonomic modulation and resting heart rate in youth across physical activity levels

Objective: This study aimed to explore the relationship between cardiac autonomic modulation (CAM), assessed via heart rate variability (HRV) indices (e.g., RMSSD, SDNN, SD1, SD2), and resting heart rate (HRR) in children and adolescents, focusing on how physical activity levels influence autonomic regulation and cardiovascular health. Methods: A total of 134 children and adolescents participated. CAM and HRR were assessed using Polar heart rate monitors, and physical activity levels were evaluated with the Rider questionnaire. The relationship between CAM and HRR, stratified by physical activity levels, was analysed using linear regression adjusted for sex, age, and somatic maturation. Results: Insufficiently active individuals showed a significant inverse relationship between high HRR and lower HRV indices, System: including root mean square of successive differences (RMSSD; b=-0.125), standard deviation of normal R-R intervals (SDNN; b=-0.142), Poincaré short-term variability (SD1; b=-0.176), and long-term variability (SD2; b=-0.114). Physically active individuals had a weaker relationship, with only the SD1/SD2 ratio (b=-0.006) significantly associated with HRR, suggesting physical activity preserves autonomic balance. Conclusions: High HRR is linked to reduced CAM in insufficiently active youth, while physical activity mitigates this effect. Promoting physical activity enhances CAM and reduces cardiovascular risk in youth.