Sports training aims to enhance an athlete's performance (overcompensation). To do that, the athlete must go through periods of fatigue and lower performance (overreaching). When the training plan is balanced, this fatigue is short and reversible.If the training load is too heavy or if recuperation periods are too short, it can lead to persistence fatigue that may only be reversible in the long term. This state of fatigue is part of the broader clinical picture of overtraining, which includes stark changes in performance as well as mood and sleep disorders. Many prediction and characterization methods based on biological markers have been evaluated, but they have not been put into practice in sports training due to obstacles such as reliability, interindividual variability and high costs. This study aims to evaluate a new approach based on the variability of an individual's heart rate (RR variability), which is a way of measuring autonomic nervous system (ASN) activity. It is non-invasive, low-cost, and has already proven useful in athlete health monitoring.