Heart Rate Variability (HRV) as a Marker for Overtraining Syndrome Prevention in Endurance Athletes: A Systematic Review

Abstract

Background. The autonomic nervous system (ANS) regulates physiological recovery. In sports medicine, distinguishing Functional Overreaching (FOR) from Overtraining Syndrome (OTS) remains difficult due to the absence of a gold-standard biomarker. Heart Rate Variability (HRV) has emerged as a non-invasive indicator of vagal withdrawal and sympathetic activation. Aim. To critically evaluate daily HRV monitoring as an early-warning biomarker for Non-Functional Overreaching (NFO) and OTS, and to assess the reliability of consumer-grade wearables versus clinical ECG. Materials and Methods. A systematic review of clinical trials and longitudinal studies (2020–2026) was conducted using PubMed/MEDLINE, Scopus, and the Cochrane Library. Following PRISMA and PICO criteria, 14 high-quality studies involving competitive endurance and elite athletes were included. Study quality was assessed with the Cochrane RoB 2 tool and the Newcastle-Ottawa Scale. Results. RMSSD and the non-linear index DFA alpha 1 were the most sensitive markers of acute autonomic fatigue and organismic stress. Evidence indicates that HRV-guided training (HRV-GT), using the Smallest Worthwhile Change (SWC) threshold, significantly improves VO₂max and peak power output compared with fixed periodization. High-end chest straps (e.g., Polar H10) demonstrate strong agreement with ECG, whereas wrist-based PPG sensors are more prone to motion artifacts. Conclusions. HRV is a sensitive leading indicator of autonomic maladaptation but should not serve as a standalone diagnostic tool for OTS. For clinical relevance, HRV monitoring should be integrated with subjective wellness scales and performance metrics. Individualized 7-day rolling averages and gender-specific protocols are recommended to reduce overtraining risk in elite athletes.