Hair Hormones and Heart Rate Variability as Chronic Stress Biomarkers in a Female Long-Distance Runner: A 22-Month Longitudinal Case Study

Although heart rate variability (HRV) and hair hormone assessments have been proposed as biomarkers to evaluate chronic stress and mental health in athletes, whether combining these biomarkers can predict future mental health decline and performance deterioration remains unclear. In this study, we sought to establish this by evaluating waking HRV and hair hormone concentrations (cortisol, oxytocin, and cortisol/oxytocin ratio) in one elite female long-distance runner over 22 months. We assessed mental health once monthly using the six-item Kessler Psychological Distress Scale and Profile of Mood States Second Edition (POMS2). In addition, self-reported training load, training volume, and athletic performance were assessed as physical and performance indices. Heart rate and HRV were each measured three days per week in both resting (supine) and standing (upright) positions upon waking. Hair samples were collected monthly for hormone analysis. Cross-correlation analysis identified 10 significant time-lagged relationships, and Granger causality testing revealed three significant predictive relationships. Waking standing root mean square of successive differences (RMSSD) predicted future POMS2 vigor-activity scores, hair oxytocin concentration predicted future POMS2 fatigue-inertia scores, and hair cortisol concentration predicted future athletic performance. The findings suggest that combining HRV and hair hormone assessments provides complementary information across multiple timescales, potentially serving as an early warning system for preventing mental health decline and performance deterioration in athletes.