Age-Related Differences in Heart Rate Variability and Physiological Load Among Professional Soccer Players Using a Wrist-Worn Biosensor

Background: Wearable sensing technologies enable continuous monitoring of physiological states in athletes, yet age-related differences in autonomic and mechanical load among professional soccer players remain insufficiently characterized. This study investigated whether heart rate variability (HRV) indices and accelerometry-derived physical load differ between younger and older professional players during routine morning training sessions. Methods: Fourteen male professional soccer players were monitored using a wrist-worn biosensor. Participants were divided into two age groups: Group A (≤23 years, n = 7) and Group B (≥24 years, n = 7). The device was attached at 08:30 and recorded tri-axial acceleration and cardiac signals continuously until 11:30 (≈3 h). Extracted metrics included root mean square of successive differences (RMSSD), percentage of NN intervals differing by more than 50 ms (pNN50), heart rate recovery (HRR), maximum heart rate, and total beat count. Group differences and position-based differences were evaluated. Results: Accelerometry-based physical load showed no significant difference between age groups. In contrast, HRV indices demonstrated age-related divergence: RMSSD was significantly higher in Group A compared to Group B (p = 0.031), suggesting greater parasympathetic activity in younger players. Total beat count was significantly higher in Group B (p = 0.015). pNN50 showed no group difference (p = 0.918). Pre- and post-session HRR did not differ significantly between groups, whereas maximum heart rate was significantly higher in Group A (p = 0.0017). Position-based comparisons revealed no consistent differences across metrics. Conclusions: Although mechanical load was comparable between age groups, autonomic markers—particularly RMSSD—indicated higher parasympathetic modulation in younger professional players. These findings align with established sports physiology literature but provide preliminary evidence derived from real-world data collected in professional soccer athletes using a compact wrist-worn biosensor. The results support the utility of wearable HRV monitoring for age-related physiological profiling in elite sports environments.