After watching one of the episode of "Enter The Dojo" it got me to think about the environment(shape) an individual train in and train for.
Boxers train in a boxing ring. (square)
Cage fighters train in an octagon cage
Martial artists train in a circle. And that circle could be planet Earth. Or that circle to me represent the whole universe, as it is used in a Yin-Yang symbol, symbolizing opposite coexist, balance, unity, peace and harmony of the universe, which is the circle of life.