Heart Rate Variability for Longitudinal Monitoring in Professional Soccer Players: A Systematic Review of Prospective Observational Studies

Background/Objectives: Heart rate variability (HRV) is widely used to monitor autonomic responses to training and match load in professional soccer, but the evidence is heterogeneous and its added value remains debated. This systematic review aimed to synthesize and critically appraise prospective longitudinal observational studies on HRV monitoring in male professional and semi-professional soccer players. Methods: The review followed PRISMA 2020 and was registered in PROSPERO (CRD420261533316). PubMed, Scopus and Web of Science were searched from inception to September 2026. Two reviewers independently selected studies and extracted data; methodological quality was assessed with the Newcastle–Ottawa Scale (NOS). Owing to heterogeneity, a narrative synthesis was conducted. Results: After duplicate removal, 489 records were screened and eight studies (109 players; mean ages 18.7–26.6 years) were included: six during the competitive season and two during pre-season. NOS scores ranged from 6 to 8 out of 9, with confounder control as the main weakness. Time-domain vagal indices predominated (RMSSD in three studies, Ln-RMSSD in three). Reductions in RMSSD/Ln-RMSSD were associated with accumulated fatigue and high training loads, whereas increases in RMSSD and SD1 accompanied fitness gains during pre-season. RMSSD also related to match performance. One study linked an elevated LF/HF ratio to fatigue and muscle injury, although respiratory control was rarely reported. Conclusions: Short-term morning RMSSD/Ln-RMSSD, interpreted against individual baselines, appears the most consistent HRV marker, whereas spectral indices require cautious interpretation. Heterogeneity, small samples and limited confounder control prevent firm conclusions. Standardized protocols and multidimensional monitoring frameworks are needed.