How can shoulder stretch contribute to running speed?
Any athletic movement is improved as more of the components of the movement are connected. As they are connected, the acceleration of one component is the base acceleration of the another component that accelerates on top of the base. An example is throwing a ball. Using the triceps to extend the arm and throw the ball accelerates the ball, but this movement can’t accelerate the ball very much. Using the shoulder to accelerate the arm while extending the arm using the triceps is better, but not optimal. The speed of the ball can be increased by using the leg to drive the hip forward stretching the abdominal muscles. This gives an acceleration to the shoulder on which the shoulder and arm acceleration can be added. With these mechanics, the acceleration of the ball is connected to the ground and more muscle groups can contribute to a greater acceleration and higher velocity at the time of release.
In running, if the movement of the limbs can be connected to the ground, they will have a more direct contribution to running speed. If the arm movement has a soft connection to the hip, the arm may have no effect on the eventual speed of the foot. If the backward arm movement is able to stretch the front deltoid and pectoralis, a force is created that pulls the shoulder slightly upward and back. This stretches the muscles of the torso pulling the hip slightly back. This backward movement provides a backward force against which the glutes can contract. The glutes and hamstrings can accelerate the foot down and backward to contact the ground completing the connection of the shoulder movement to the ground. The continued forward arm drive maintains the connection of the shoulder to the hip through the torso as the glutes and hamstrings complete the backward pull on the ground. Like in the mechanics of throwing a ball, here more muscles groups are connected and are able to make direct contributions to the foot accelerating before contacting the ground as well as pulling with more force while in contact with the ground.