Serving with Power
There are a few fundamental secrets to increase your serve by 10-15 mph.
Perhaps the most important source of power in the serve (and every other shot in tennis) is the contact of the racquet with the ball. The center of the strings is called the sweetspot for a reason. It acts like a trampoline in that it causes the ball to explode off the racquet. To get the right contact, you must see the ball when the racquet contacts the ball. To see it properly, you must keep your head still as you make contact and keep it still until you finish your swing.
Another source of power is the use of the legs. Bending the knees as you toss allows you to throw your body up into the ball as reach up to hit it. Many players actually jump up as they hit the ball Jumping can be rough on the back however so be careful. A more common use of the legs is that used when throwing a ball. Bend your front knees as you toss the ball, then bend your back knee as you swing at the ball with the racquet.
Most of us would like to have more power in our serve. Although control and accuracy are the most important, if your serve is too slow most players will put you on the defensive or even hit winners off your serve. There are several sources of power for the serve. One is the wrist. Engaging the wrist causes the racquet head to speed up at the contact point. Pronating your wrist allows you to flatten Another is the elbow. Bending the arm at the elbow engages the biceps and triceps in the serve. It also lengthens the distance the racquet head travels just prior to contact with the ball.
Another source of power for the serve is the shoulder. The shoulder muscles triggers the arm, so timing is very important. But so is the position. If you tilt your body so that the shoulder is in a low position when the toss is at its peak, you will get power from the shoulder moving up.
If you turn your body away from the court as you toss the ball, you will get power from your body turning back to hit the ball. This is often called body torque.
A good example is how Andy Roddick used his shoulder and knees to hit 150 mph first serves and 110 mph second serve kick serves consistently.