Several weeks ago there was a discussion following a student question
about whether work is done by holding up some object with one's hand but
without moving it, and why does one get tired anyway. I may have missed
it, but I do not remember anyone making reference to an excellent
discussion by Feynman on precisely this subject. There is a long
paragraph on page 14-2 of volume 1 of the Feynman Lectures. He mentions
muscles that do work, and get tired, and others that do not.