"Blessed Be the Physically Fit" by Pete Able in Drunk Monkeys
My muscles burn and pull on me to such an extent I feel I will spontaneously combust. I say nothing. I am the lowly client. I am like a sort of pet - like a tired dog for whom my master just keeps on throwing the damn ball. And I must chase it, of course. To do otherwise would be out of the question. Read story here.