Abstract

INTRODUCTION:
Heart rate variability (HRV) measures the variation in time between consecutive R-R intervals and is widely recognized as an indicator of autonomic nervous system function. Reduced HRV has been associated with illness or disease, reflecting the body’s diminished capacity to adapt to stressors. Monitoring HRV can therefore provide valuable insights into health status, early signs of sickness and the progression of medical conditions, making it a useful tool in both clinical and athletic settings.

METHODS:
Eleven professional soccer players (age 17.64 ± 1.03 years, weight 72.73 ± 6.05 kg, height 177.27 ± 5.83 cm) from the same team volunteered to participate in the study. Morning HRV measurements were collected at the end of the season using photoplethysmography via the HRV4 training smartphone application. Players also provided self-reported data, including fatigue levels, sleep quality and morning mental energy. For this study, HRV were compared between sick and healthy days, which served as a secondary purpose of the research.

RESULTS:
Linear mixed-effects modelling revealed a marked suppression of HRV during illness. On sick days, players exhibited an estimated 41% lower rMSSD compared with healthy days (p<0.01). After adjusting for self-reported sleep quality and fatigue, this effect remained the same, with rMSSD averaging 38% lower on sick days (95% CI: 23% to -54%, p<0.01). Similarly, LnRMSSD (log-transformed rMSSD), confirmed a substantial reduction in recovery status during illness. Sleep quality and fatigue did not significantly contribute to rMSSD variation. Approximately 18% of the variance in HRV was attributable to stable differences between players, while the majority (82%) reflected day-to-day within-player fluctuations. Other HRV indices showed consistent, though somewhat weaker patterns, with SDNN decreasing and resting heart rate increasing on sick days.

CONCLUSIONS:
These findings demonstrate a suppression of cardiac sympathetic activity during mild acute respiratory illness or flu episodes in professional soccer players. HRV appears sensitive to acute illnesses and may serve as a practical tool for monitoring recovery and health status in athletes, independent of sleep quality and perceived fatigue.