HEART RATE VARIABILITY (HRV) IN SOCCER PLAYERS DURING MILD ACUTE RESPIRATORY ILLNESS

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