Connecting arm movement and activation of the glutes.
Connecting the movements of the limbs increases the probability of them complementing each other. If the periodic movements of the arms and legs while running are constructively reinforcing each other, the instances of maximum force coincide. It is easy to see an example of this in a vertical jump. If the movement of the arms and legs are timed correctly, a higher jump will result. Another benefit of connecting the movements is simplifying the mechanics of running. If two or more movements are connected, they can be seen and executed as one movement.
It is beneficial to connect the movements of the arm and the same-side leg. If the right arm is passively brought forward, the right leg’s backward movement may be diminished in some way. The leg may float before being pulled backwards with the glutes. This delay will make the foot slower at ground contact and may prevent it from contacting the ground under the hips.
Even if the right arm is active, it may not move back fast enough to create a stretch reflex in the shoulder against which the right glute can contract. If the arm is tight and only pulled back with the shoulder, it probably will not be able to accelerate during the second half of the pull after the arm passes the torso. The arm will be slowing down in the second half because as the muscles contract, there is less further movement in the arm that the muscles can generate. Due to this, the arm will probably not be able to stretch the shoulder muscles by the time the glutes are ready to contract.
One way to accelerate the arm in the second half of its backwards swing is to keep the lower arm loose in the front half of the swing. The loose arm will extend and accelerate as the shoulder pulls down and back. This extension and acceleration creates a force against which the biceps can contract. Contracting the biceps suddenly shortens the overall arm radius and accelerates the arm in the back half of the swing to its full rotation. The shoulder muscles can be stretched to create a force that both lifts the body and provides force against which the glutes can contract. This shoulder position is achieved quickly so that the stretch and forward arm drive coincides with the glutes contracting and pulling the thigh down and back. The early activation in the arm and glutes allows the arm and leg to align when the foot contacts the ground under the hip. By connecting these movements, and training the nerves to execute them automatically, both movements can coincide and be initiated just by contracting the biceps.