Introduction: Heart rate (HR) variability (HRV) describes the temporal change (variation) of the heart period or beat-to-beat variability of the HR over a defined measurement period and reflects the dynamic end-organ response of the heart to physiologic and/or pathologic perturbations. The time series of successive R-R intervals is called "tachogram" from which various metrics with different durations (e.g., from ultra-short-term (~1 min), short-term (~5min) up to long-term (~24 h) depending on the context) are derived (2, 9, 20, 57, 60, 61, 68). HRV is primarily created by the non-linear interaction of the efferent positive chronotropic influence of the sympathetic (SNS, "fight or flight" response), the negative chronotropic influence of the parasympathetic (PNS, "rest or digest" response) branch of the autonomic nervous system (ANS), and the intrinsic activity of the hearts pacemaker cells and reflects the context-dependent psychoneuroendocrinological modulation of cardiovascular control (8, 42, 48, 71). This complex modulation integrates several feedforward and feedback mechanisms that act on different time scales (e.g., central command, arterial baroreflex, respiratory control, circulating catecholamines, muscle metabo-/mechanoreflex, renin-angiotensin-aldosteron-system) and is therefore related to the dynamic regulation of circulatory control of arterial blood pressure and HR to meet the context-dependent needs of the whole organism. Thus, different evidence-based HRV metrics are used as markers of human cardiovascular health and risk stratification (10, 11, 19, 20, 62, 69) or as measures of load quantification, exercise response and performance, respectively (6, 7, 27, 28). However, for a valid use of HRV in the fields of sports medicine and exercise science careful considerations on standardized assessment, preprocessing, analysis, and context-sensitive interpretation are mandatory.
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