Everybody experiments fatigue holding a weight, and almost everybody knows that points of applications of the involved forces don't move. We also know that we cannot use the standard equation of the conservation of energy ( ΔK + ΔU = Wext ) because the system (Body+weight) is composed by objects that have an internal structure. (Here K stays for Kinetic Energy, and U is potential energy of the inner forces, supposing they're conservatives) According to this observation we write, more generally : ΔK + ΔU + ΔEint = Wext Let's see what happens to the system (Body+Weight) ΔK = 0, obviously. Wext = 0 too, according to the fact that the the external forces (weights and reactions of the soil) don't move their point of a. ΔEbody<0 ; we burn calories to avoid fatigue, decreasing our internal energy. ΔU=0 because the system does not change configuration. I'm ok with the microscopic behaviour (sarcomeres contract and strecht continuously to achieve the 'rigid' stand, so they actually do work) but how it can be explained with the conservation of energy?