At any moment in physical labor, a group of muscles are working toward some end. What that end is, and what the process is that proceeds towards that end, dictates what groups of muscles are most intensely involved.
It is a joy for the muscles and the body as a whole to merely exercise itself, to grow and produce as it is used up. But then if one is placing too much of a load on a given group of muscles, those muscles wear out from sustained usage. They have become overexploited, in which they are used beyond their capacity to regenerate themselves or even maintain themselves.
And so success in this regard has to do with being mindful of one's body, by shifting weight constantly, economizing one's exertions, and doing even work on the various parts of one's body, making use of structural strengths to manage weight and things of the like.
And then one gives the body what it wants: enough nutrients to regenerate, and the sweet rest that makes the exertion just as sweet.
