Work done during equilibrium by any of the external forces on the system is zero.
But consider the case where you are preforming deadlifts. After you pick up the barbell and hold it for a while, your hands start to give in. From the classical definition of work done W = F.s the work done must be zero since, there's no net (considerable) displacement.
But it is clear you are using up energy.
My question is why, where, and how exactly energy is being used up here.
The barbell exerts a torque on your fingers during a deadlift that acts to open them up, dropping the barbell. You maintain the lift by contracting the muscles in your forearms, fingers etc. which is done through an electrical signal. This is where the energy is going; no work is being done through displacement but the human body is a dissipative system (your body feels hot after a workout, doesn't it?). This is in a nutshell, but there are details regarding what happens with your muscle fibers as they twitch in response to this signal.
