Fitness represents the direct ability of the Body to Control itself, its position, its movements, etc. It is used to jump great distances, for endurance while chasing an enemy, and for dexterity to balance a speeder bike on a narrow ledge at 400 miles per hour. Any check which requires your body to be well adjusted is a Fitness check. If the check changes the world, such as moving a heavy object, rather than yourself (such as jumping to a different location), it is a Strength check instead of a Fitness check.