Muscle fatigue due to limited oxygen delivery and lactic acid accumulation is thought to set the speed an endurance athlete can sustain during a competition (e.g. the marathon). This is because above that speed, muscle fatigue would develop too quickly and the athlete would reach exhaustion before the finish line. In this lecture, Professor Samuele Marcora from the University of Kent challenges this physiological model of endurance performance, and proposes an alternative model in which perception of effort and motivation are the main factors determining endurance performance. He also discusses how this psychobiological model can be used to develop innovative interventions aimed at further improving endurance performance.