Strengthen the Arm Itself
The forearm acts as a lever in a tennis stroke. Just as a firm racket can increase the power of a shot, a strong forearm can increase the power as well. Working the flexors and extensor muscles is a great way to increase tennis stroke power. This is achieved easily by squeezing a tennis ball and doing wrist curls with the palms up and palms down.
Maximizing strength means using every possible source including the wrist. A strong forearm gives greater control over the wrist. The faster a player can snap her wrist through the ball, the more velocity she can impart on the ball. This is a double-edge sword, however, as snapping the wrist also increases the likelihood for error. When balls go errant, it is best to go back to a firm wrist. Even when snapping the wrist, it is important to keep in mind that the wrist needs to be straight with the forearm and hand for ideal control at the point of contact.
The power of the tennis player is not in a strained muscle, but rather in a loose muscle the flexes on impact. Overstraining the muscles while trying to impart power will lead only to premature muscle fatigue and possible cramping or injury. The grip on the racket should be kept loose until contact is made. This will keep the forearm relaxed until the moment of contact, allowing the power to surge through the arm into the racket and through the ball.