Usage patterns
肉体を鍛えるto train/build up one's body
He trains his body every day at the gym.
肉体が衰えるfor the body to weaken/deteriorate
It's natural for the body to weaken as one ages.
肉体を酷使するto push one's body to the limit; to overwork the body
Long hours of factory labor push the body to its limits.