Tennis is a complex sport that places high physical and cognitive demands on the athlete. When a coach fails to adjust training loads, the athlete runs the risk of suffering injuries, syndromes and quitting the sport. Having a tool that can monitor these demands can be a differentiator in the sports world, as it will be possible to check the right moment when the athlete is getting fatigued and adjust the training load according to the psychophysiological responses obtained. This would reduce the risk of injury and increase performance during a match or even a championship. This book outlines a study that observed heart rate variability responses during a cognitive test in tennis players before and after physical exertion. The idea is to see if there is cognitive decline after exposure to intense activity.