In the chapter "Practice" of Leanne Shapton's autobiographical Swimming Studies, a striking observation about what happens when a body is driven hard, so hard that it's on the verge of failure:
While watching the French Open one morning on television, I leave my chair and walk toward the kitchen. John McEnroe is commentating, and as I put the kettle on I hear him mention that Rafael Nadal is playing through a sprained ankle. I remember the blunt fact that when I was training, I was in constant pain. Not just the sharp pain in my knees, which was taken seriously, but a dull, steady pain in my arms, back, and shoulders. Pain when I sat down, pain when I got up, pain when I leaned back in a chair, pain when I reached for the salt or sharpened a pencil. Thinking of stoic Nadal, I remember how I ignored, then eventually forgot about, pain when I raced, and even to some degree during practice. It was as though pain on land was there to remind me to get back in the water, where, after a certain threshold, the pain went away. For an athlete pain is not a deterrent, because the only place the pain will be eclipsed is in practice or in competition.