i have asked many lecturers and students this question and have never received a logical sounding answer. Why is it that if you hold a weight in a position (lets just say at the midpoint of a bicep curl) that you are using energy yet there is no net work being done as there is no distance that the force is going through. i have heard answers such as pertubations around this point account for the energy loss. it seems as though the energy is lost as heat (this is why the muscle heats up) the question is more directed to the governing equation behind this. The muscle is technically acting as a structure. i was thinking something along the lines of a Qh(heat input) is the input and Wout(work output) is zero this makes this system a zero efficiency system.