The role of the kinetic chain Increasingly, the role of force generation by other body components is being assessed. For instance, the power generated by the shoulder in the tennis serve has been preceded by power generated by the legs, trunk and back. The muscle mass of the shoulder is relatively small, and if inadequate power is generated by the preceding links in the kinetic chain the shoulder has to play 'catch-up' and generate power rather than acting as a force regulator. Improving the server's leg action, lumbar strength and trunk rotation during the serve will reduce the incidence of rotator-cuff injury. Such biomechanical analysis is difficult but, in skilled hands, is a crucial and effective element in injury prevention.
It also connects with kyle's post on the kinetic chain. I think this is how a lot of people continue to screw up their shoulders, and I realize I'm still doing it even as I try to rehab. You need to use your shoulder muscles just to stabilize the joint, not to muscle a throw. So if you just stand in place and don't use your leg power and try to throw a ball hard, you can really destroy your shoulder, but if you do a correct throw and get leg and hip drive, your shoulder should just be a transfer to your arm and not really generating the power.