Heart rate variability (HRV) represents variations between consecutive heart beats (beat to beat or R-R interval) over time (Figure 1). This beat to beat variation in heart rhythm is considered normal and even desirable. Disappearance of variations between consecutive heart beats is a result of autonomic dysfunction which can be associated with neurological, cardiovascular and psychiatric disease states1. There is a large body of evidence reporting that higher variability of heart rhythm is associated with reduced mortality 2,3, improved quality of life4 and better physical fitness5. The most common use of HRV analysis is in risk prediction and prevention of heart failure6,7. Interestingly, in ancient China, HRV was known to physician Shu-he Wang (265 to 317 AD) who described heart rhythm as an indicator of disease: "if the pattern of the heart beat becomes as regular as the tapping of a woodpecker or the dripping of rain from the roof, the patient will be dead in 4 days"8. The physiological background of HRV is complex and affected by circulating hormones, baroreceptors, chemoreceptors and muscle afferents. An important factor which influences HRV is respiratory sinus arrhythmia - the natural variation in heart rate (HR) that occurs during breathing5. During inspiration, HR increases whereas expiration is characterised by a decrease in HR. The autonomic nervous system (ANS) through sympathetic (SNS) and parasympathetic (PNS) pathways regulates the function of internal organs and the cardiovascular system1,5. Sympathetic activity increases cardiac contractility and HR, whereas parasympathetic (vagal) stimulation reduces HR5,9. Any source of stress (psychological, physical or illness) will provoke disturbance in the ANS and consequently in HRV. The long-term presence of imbalance between sympathetic and parasympathetic tone can impair the performance of athletes. The analysis of HRV in sport has become established and recognised in the past 2 decades as a non-invasive method for evaluation of the body`s reaction to training loads, recovery methods and overtraining syndrome (OTS). In the last 5 years, innovations in wireless technology have significantly increased the number of devices on the market which are using HRV indices to control and manage the training processes of athletes.
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