Objective: A meta-analysis was conducted on the effect of overload training on resting HR, submaximal and maximal exercise HR (HR), and heart rate variability (HRV), to determine whether these measures can be used as valid markers of over-reaching. Methods: Six databases were searched using relevant terms and strategies. Criteria for study inclusion were: participants had to be competitive athletes, an increased training load intervention had to be used, and all necessary data to calculate effect sizes had to be available. An arbitrary limit of 2 weeks was chosen to make the distinction between short-term and long-term interventions. Dependent variables were HR and HRV (during supine rest). Standardised mean differences (SMD) in HR or HRV before and after interventions were calculated, and weighted according to the within-group heterogeneity to develop an overall effect. Results: In these competitive athletes, short-term interventions resulted in a moderate increase in both resting HR (SMD = 0.55; p = 0.01) and low frequency/ high frequency ratio (SMD = 0.52; p = 0.02), and a moderate decrease in maximal HR (SMD = 20.75; p = 0.01). Long-term interventions resulted in a small decrease in HR during submaximal (SMD = 20.38; p = 0.006) and maximal exercise (SMD = 20.33; p = 0.007), without alteration of resting values. Conclusion: The small to moderate amplitude of these alterations limits their clinical usefulness, as expected differences may fall within the day-to-day variability of these markers. Consequently, correct interpretation of HR or HRV fluctuations during the training process requires the comparison with other signs and symptoms of overreaching to be meaningful.Peak performance in sport requires training loads that will occasionally push the adaptation capabilities of the human body to their limits. If significant improvements are expected when sufficient recovery is allowed before competition, 1 2 athletes may also experience unexplained underperformance.3 4 Various terms have been used in the scientific literature over the past decades to describe this disruption in the training adaptation process. In an effort to clarify the terminology, the Task Force of the European College of Sports Science distinguished functional over-reaching (FOR), non-functional over-reaching (NFOR) and overtraining syndrome (OTS) to mark out the continuum that leads to long-term underperformance. Over-reaching refers to an accumulation of training and/or non-training stress resulting in short-term decrease in performance capacity with or without related physiological and psychological signs and symptoms of overtraining, in which restoration of performance capacity may take several days to several weeks.5-7 FOR is occasionally used by athletes during a typical training cycle. An improvement in performance capacity above what would be achievable after a normal training cycle is expected if the athlete is allowed sufficient recovery to permit the occurrence of the supercompensation effect. As suggested by Meeusen et al...