the left side of the brain controls the right side of the body, I remember seeing a neurologist who had a test where you sat and went through a motion with your left leg, when the right leg joined in doing the same motion in the opposite direction the left leg followed. it is said impossible for the legs to do the two opposite motions at he same time, he concluded that the right side of the brain controlled the right leg, there must be a crossover point somewhere.
Guess I was asking in an abstract way which muscle groups in the upper body were thought to have the most effect on forward movement, the chest or back.