Hi, This is about a basic fact which has never been clear to me. As per the equation Work = Force x Displacement, if there is no displacement there is no work. On the other hand, one of the exercises I do involves pushing a wall. I stand in front of a wall and try to push the wall as if I am moving a big cupboard. The idea is to push as hard as possible until one feels exhausted. The excecise is supposed to be good for arm and calf muscles. Technically, however, no work is being done. Now my question is: If there is no work being done, why do I fee exhausted (loss of energy) after doing that action? Thanks, MG.