The more and more I see club level players, including myself, I notice that on groundstrokes they are using the force of their arm to rotate their body. By this I mean the force pulling their body is their arm - their arm is pulling their body. There isn't much buildup of hte legs and core. When I see pros hit groundstrokes, it seems to me like they build up power from their legs and lower body in an overall effort to bring power to their core to rotate their body, and the arm just follows with it. Is this true? Are you supposed to build up your core to use for rotation, and have the arm just follow with the rotation on a groundstroke or should you be using the force of the arm on the groundstroek to pull your body? According to your answer, what muscle groups should be worked in order to increase the speed of this rotation and what are some effective exercises to do so. Note: I am only 14 so I guess I am less prone to injury and am more focused at doing exercises to improve my game rather than prevent injury.