The ability to move is a prize. There are more than 600 muscles in the body, and they do sometimes have limitations on them. But it is an accepted way of life that one should be able to be “up and at 'em” — jump out of bed in the morning, exercise, go through the day, and still have stamina by evening without flopping down, exhausted, powerless, or drained of strength. Dragging around arms that feel like bowling balls or legs that feel like rooted tree trunks — these are not happy experiences.