Every belief, substantiated by research, is that exercise is good for one. The evidence is that the impact of exercise leads to healthy physiological adaptations and that most endurance athletes benefit from these adaptations.1 The healthy heart of an endurance athlete can respond effectively to acute exercise and can delay fatigue during prolonged exercise.1 George et al.1 studied the acute and chronic adaptation of endurance athletes’ hearts and showed that increased endurance exercise might cause an acute reduction in cardiac function, causing a physiological cascade that leads to the release of cardiac biomarkers. This evidence indicates that some endurance athletes may develop a pathophysiological cascade. Athletes and clinicians should be mindful of this and react adequately to this pathophysiological phenomenon.1 Considering the potential for exercise to cause significant cardiac conduction variations due to stimulation from the autonomic nervous system (ANS), this study set out to determine the effect of participation in a three-day endurance mountain bike cycling event on athletes’ hearts using heart rate variability (HRV) as an outcome measure.
Allostasis and allostatic loads are physiological responses due to stress (positive or negative). Allostasis represents the adaptive process of maintaining homeostasis through complex physiological changes and therefore achieving a stable environment in the body.2,3
A deteriorated and cumulative allostatic process refers to an allostatic load that the physiological system cannot adapt to.2,4 In addition, enduring challenges cause impairment of physiological regulating systems, which influences the sympathetic nervous system and the immune system.5
Therefore, exercise over time has implications, which can be either good stress (eustress) or bad stress (distress), but the individual’s reaction to the stressor causes a physiological change.2
The ANS regulates the body’s internal functions; the central nervous system (CNS) transmits impulses to peripheral organs, one of which is the cardiovascular system.6 The ANS controls heart rate (HR), heart contraction force, vasoconstriction, vasodilatation of the blood vessels, and contracting and relaxation of smooth muscles in various organs.6 The ANS is divided into two systems. The sympathetic and parasympathetic (vagal) systems transmit automatic signals to the organs. The sympathetic system increases metabolic function to cope with challenges outside the body, while the parasympathetic (vagal) system increases functions associated with growth and repair in the body.7–10
Heart rate variability is measured using the time between the R-R intervals of the QRS complex on an electrocardiogram (ECG) recording and determining the variability between the consecutive R waves.8,11,12 In addition, heart rate variability is a physiological measurement used to evaluate the autonomic nervous system (ANS).8,11,12 Heart rate variability monitoring is a valuable indicator in the diagnosis and prevention strategies of overreaching in athletes.13 Meeusen et al.13 defined over-reaching as an ‘accumulation of training and/or non-training stress resulting in a short-term decrement in performance capacity with or without related physical and psychological signs and symptoms of maladaptation in which restoration of performance capacity may take from several days to several weeks’.
The sympathetic and parasympathetic nervous system stimulates the heart variably and influences heart rate.8 When the consistency of the R-R intervals has maintained an equilibrium, the sympathetic and parasympathetic nervous systems are in homeostasis.8 A high HRV indicates an adaptation to exercise, which improves the function of the ANS. A lower HRV indicates a lack of adaptation or inadequate adaptation to exercise, which will impair the ANS.
Often, overreaching is related to several warning signs, one of which includes ANS dysfunction and imbalances.8 In addition, due to increased exercise, the physiological system is compromised, either from the increased intensity or the duration of exertion.7 Thus, this study aimed to determine the effect of participation in a three-day endurance mountain bike cycling event on the heart of athletes using the outcome measure of HRV as a marker of autonomic function. The hypothesis was that there would be significant variations in HRV during and after the three days of competing in an endurance mountain bike cycling event, and HRV would not fully recover to baseline values within 24 hours.