Objective: A meta-analysis is conducted on the effect of overload training on resting heart rate, submaximal and maximal exercise heart rates and on heart rate variability, to determine whether these measures can be used as valid markers of overreaching. <br> Methods: Six databases were searched using relevant terms and strategies. Criteria for study inclusion were: participants must be competitive athletes; an increased training load intervention must be employed; all necessary data to calculate effect sizes must be available. An arbitrary limit of two weeks was chosen to make the distinction between short term and long term interventions. Dependant variables were heart rate and heart rate variability (during supine rest). Pre–post intervention standardized mean differences (SMD) in heart rate or heart rate variability were calculated, and weighted according to the within– group heterogeneity to develop an overall effect.<br> Results: In these competitive athletes, short term interventions resulted in a moderate increase in both resting heart rate (SMD=0.55; p=0.01) and LF/HF (SMD=0.52; p=0.02), and a moderate decrease in maximal heart rate (SMD=-0.75; p=0.01). Long term interventions resulted in a small decrease of heart rate during submaximal (SMD=-0.38; p=0.006) and maximal exercise (SMD=-0.33; p=0.007), without alteration of resting values. <br> Conclusion: The small to moderate amplitude of these alterations limits their clinical usefulness, since expected differences may fall within day-to-day variability of these markers. Consequently, the correct interpretation of heart rate or heart rate variability fluctuations during the training process requires the comparison with other signs and symptoms of overreaching to be meaningful.
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