"Physical fitness is not the only one of the most important keys to a healthy body, it is the basis of dynamic and creative intellectual activity. The relationship between the soundness of the body and the activities of the mind is subtle and complex. Much is not yet understood. But we do know what the Greeks knew: that intelligence and skill can only function at the peak of their capacity when the body is healthy and strong."
     ~President John F. Kennedy

What this statement is trying to tell you is that in order for you to be intellectually active all the times, physical fitness can be a good tip to help out your intellectualism in all kind of ways. Physical fitness can help you and your mind process and organize things complex and subtle your mind. An example of this would be how back then the Greeks knew that the capacity of a healthy strong body can lead to intelligence and skill at the same time.