From the book :Long Walk to Freedom .
I attempted to follow my old boxing routine of doing roadwork and muscle-building from Monday through Thursday and then resting for the next three days. On Monday through Thursday, I would do stationary running in my cell in the morning for up to forty-five minutes. I would also perform one hundred fingertip push-ups, two hundred sit-ups, fifty deep knee-bends, and various other calisthenics.
I have always believed exercise is a key not only to physical health but to peace of mind. Many times in the old days I unleashed my anger and frustration on a punchbag rather than taking it out on a comrade or even a policeman. Exercise dissipates tension, and tension is the enemy of serenity. I found that I worked better and thought more clearly when I was in good physical condition, and so training became one of the inflexible disciplines of my life. In prison, having an outlet for my frustrations was absolutely essential.