Photo Study of Andy Roddick's Serve

Here, Roddick's elbow has begun to straighten, but the angle between the racquet and the forearm (via the wrist) has actually increased slightly, due to inertial lag. The mass of the racquet resists being moved upward, so it exerts some downward pull on the wrist, stretching the forward-facing side of the forearm. This stretching of the forearm will contribute to racquet-head speed as the elasticity in the forearm makes it spring back to its normal length while the racquet whips upward.