Background: Increased endurance exercise at high intensities may cause an acute reduction in cardiac function, causing a physiological cascade that releases cardiac biomarkers. Heart rate variability (HRV) is a physiological measurement used to evaluate the autonomic nervous system (ANS) state. This study set out to determine the changes in the ANS by participating in a three-day endurance mountain bike cycling event using HRV as an outcome measure.
Methods: Sixteen healthy participants (male and female) participating in a three-day endurance mountain bike cycling event underwent five-minute resting electrocardiography (ECG) recordings in a supine position. In addition, heart rate variability measurements were recorded two days before the race (baseline testing), after each race day, and at 24-hour post-event (recovery).
Results: Time-domain and frequency domain measures showed significant changes from baseline HRV parameters after each race day (p≤0.05). These changes reflected an increase in sympathetic activity and parasympathetic withdrawal after each day of the event. In addition, our data revealed that the mean heart rate (HR) and R-R variability variables did not return to baseline values after 24-hours of recovery, reflecting autonomic nervous system (ANS) dysfunction and that changes persisted for at least up to 24-hours post-event.
Conclusions: Our study demonstrated that competing in an endurance mountain bike cycling event led to diminished vagal activity and decreased HRV throughout the event, which persisted for at least 24-hours post-event. The body was under continuous sympathetic dominance and parasympathetic withdrawal at rest and each racing day, implying that each race day inflicts significant physiological stress. This physiological stress causes a disturbance in homeostasis and an increase in ANS dysfunction. This dysfunction has implications for further research, including dysrhythmia risk and monitoring of athletes in advising participation and returning to strenuous activity post-endurance events.