Applications of Heart Rate Variability in Monitoring Training Load and Recovery: Theoretical Foundations and Practical Implications

Training load–recovery management aims to dynamically align external stimuli with an athlete’s adaptive capacity, yet conventional monitoring approaches remain constrained by inter-individual variability, contextual confounding, and limited comparability across time scales. Heart rate variability (HRV), conceptualized as a time-series phenotype of sinus RR-interval fluctuations, can provide a longitudinal reference of cardiac autonomic modulation—particularly vagally mediated modulation—under standardized resting conditions, thereby complementing the internal load–recovery monitoring chain. This review synthesizes the theoretical foundations, interpretive frameworks, and applied evidence of HRV in sport sciences. Mechanistically, we outline how autonomic regulation contributes to exercise stress and post-exercise re-equilibration, and we delineate the physiological meaning and inferential boundaries of time-domain, frequency-domain, and non-linear HRV indices across relevant time scales. From an applied perspective, we summarize characteristic HRV responses to acute training stimuli and to accumulated load, and we discuss sources of consistency and heterogeneity across sports and populations in fatigue identification and training adaptation assessment. We further appraise HRV-informed prescription strategies that operationalize threshold-based decision rules (e.g., rolling estimates and smallest worthwhile change logic), suggesting that their practical value may lie more in managing unnecessary high-intensity exposure than in producing uniform performance gains, which often appear modest and context-dependent. Finally, we emphasize that the interpretability of HRV is contingent on measurement windows, posture/respiration control, and artifact processing, and we caution against mechanistic overreach of spectral ratios (notably LF/HF). We propose an implementation-oriented framework centered on standardized acquisition, individualized baseline modeling, and multimodal integration with external load and subjective recovery measures to enhance auditability and translational utility in training governance.