how is work measured in a muscle?

Answer 1

If this is about intramuscular measurement, I have no idea. Small spring tensiometers and some way of measuring distance, I'd think.
If measuring muscle work on a non-dissected living being, you'd use any of several simple apparatuses including treadmill, weights, stationary bicycle, wind tunnel, water stream. You'd want to estiamte the mass of the muscle groups involved if you want any 'specific' or mass-normalized figures, such as specific work in joules/kg.