Author(s): OERTEL, M., SCHUSTER, J., SCHNEIDER, C., HOBEIN, E., WIEWELHOVE, T. , Institution: IST UNIVERSITY OF APPLIED SCIENCE , Country: GERMANY, Abstract-ID: 1666

INTRODUCTION:
Heart rate variability (HRV) has emerged as a key metric for assessing autonomic regulation and training adaptation in endurance sports. While short-term HRV changes following acute exercise are well documented, longitudinal HRV dynamics in response to varying training loads in daily life remain underexplored. This study investigated whether nocturnal HRV reflects individual fatigue and recovery patterns in trained endurance athletes when analyzed in relation to weekly training load.
METHODS:
Ten healthy, trained endurance athletes (8 men: 39.4 +/- 9.4 years, 73.7 +/- 5.9 kg, 9.7 +/- 5.0 h weekly training, 47.0 +/- 3.2 bpm resting heart rate, 57.7 +/- 8.8 ml/min/kg VO2max; 2 women: 29.0 +/- 3.0 years, 71.4 +/- 7.5 kg, 7.9 +/- 5.2 h weekly training, 52.0 +/- 10.3 bpm resting heart rate, 45.9 +/- 5.9 ml/min/kg VO2max) were continuously monitored over 26 months using Garmin wearable devices. Nocturnal HRV was quantified using the natural logarithm of RMSSD (lnRMSSD), averaged over weekly intervals. Internal training load was assessed via weekly averaged Edwards TRIMP (eTRIMP) scores. To examine the association between eTRIMP and HRV, individual linear regressions were performed per participant. Additionally, a linear mixed-effects model (LMM), including random intercepts and random slopes for weekly eTRIMP, was fitted to account for repeated measures and interindividual differences in HRV responses to weekly training load at the group level.
RESULTS:
Significant associations between eTRIMP and lnRMSSD were observed in three of ten athletes (p < 0.05). One participant exhibited a negative association (Estimate = -0.245, p = 0.001), while two showed positive relationships (Estimate = 0.228, p = 0.046; Estimate = 0.314, p = 0.001). The remaining seven participants demonstrated no significant relationship (p > 0.05). At the group level, the LMM revealed no significant fixed effect of eTRIMP on HRV (Estimate = 0.044 per 1000 AU, 95% CI = -0.057 to 0.144, p = 0.392). However, the high conditional R2 (0.886) indicated pronounced interindividual differences in how weekly training load was associated with lnRMSSD.
CONCLUSION:
This 26-month longitudinal analysis revealed marked interindividual variability in nocturnal HRV responses to weekly training load in trained endurance athletes. While significant associations between eTRIMP and lnRMSSD were observed in a subset of individuals, no systematic relationship emerged at the group level. These findings suggest that weekly aggregated HRV metrics may capture general individual trends but may be insufficient to fully capture short-term autonomic responses to training stimuli. Consequently, HRV appears to be a highly individualized marker whose practical utility depends on athlete-specific interpretation and appropriate temporal resolution. Future research should focus on higher-resolution analyses and integrative approaches combining HRV with contextual and subjective measures to improve its utility for day-to-day training monitoring.