thubanconsulting says:

From a physics perspective, on a macroscopic scale, no work is done since there has been no displacement of mass, tehrefore there is no collective increase in energy (either kinetic or potential) to your body (the mass in question).

However, on a microscopic scale, we can view the contraction and expansion/elongation of muscle cells as work since they are accelerating back and forth and there is displacement of mass involved on a cellular level.