Since the work of the Scandinavians, Hermansen and Saltin, we have known that water loss by perspiration leads to a significant loss in muscle power. Habitual long-distance athletes are no longer unaware that the intake of water during effort, thanks to refreshment stations along the course, enables them to struggle effectively against the effects of dehydration, especially if it is hot, there is little wind, the hygrometer reading is high and the pace particularly sustained.