Kinetic chain = Connected system in motion. Kinetic means motion and chain refers to a system that is linked together or connected. If one component of that system is not working properly, it will affect the whole system.
1. Think of your body as a system of body parts (feet, legs, hip, trunk, shoulder, arm, wrist). These parts have to be seen as a WHOLE.
2. Think of your shot (contact point) as the consequence of the previous actions that start from the ground-up. That means that there are things that body has to do before the point of hitting the ball (these connected actions are a transfer of energy, the kinetic chain). When you change your perspective about the shot, you will understand how to hit the ball easy and injury free.
Kinetic chain is a flow of energy through the body starting from feet and finishing to the racquet. As a player needs energy to hit the ball, it all begins with the leg drive that generates the ground reaction forces that can be transferred to the other segments of the kinetic chain (hip, trunk, shoulder, arm, wrist) to the racquet.
In groundstrokes the back leg loading (read the previous blog about the loading) and unloading to the hip is a beginning stage of the chain. Legs through hips begin the whole trunk rotation and drive the trunk up and forward and the arm is then following to build the racket speed. In an open stance leg drive starts when “sitting” on the back leg drives the hips and trunk more upwards and sideways while at the closed stance the back leg driving is more forward.
While serving the leg drive is important to create proper vertical velocity. The serving motion is upwards (leg drive) and front (turning hips) toward the hitting point rather then shoulder rotation back to front. The leg drive enables hips to rotate achieving the best possible body position in a contact point- back hip above the front one and right shoulder over left shoulder trunk rotation.
Now I will just explain in simple facts : yes, most of the power does come from the forearm extension, upper arm and shoulder internal rotation, forearm pronation and hand flexion but the great percentage comes from the leg drive. Pressing down the ground the body is getting the ground force needed to jump up and do the hip and trunk rotation.
If there is no leg drive there is increased load on shoulder, medial elbow and abdominal muscles. If there is no hip rotation then the bigger load is on the shoulder and trunk.