Environment Shape Of Where Individuals Train

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.