In various sports athletes aim for lightness and muscular power, so they can maintain speed over a long time (in running or cycling), or can be « explosive » (boxing) or lift one’s weight (climbing). This is called the power-to-weight ratio, with a low fat mass and a high muscular mass (within personal limits). To lose fat mass, fasted running could be performed. How does it work and how to do it safely ? Here are some advices and recommendations.