Power on the serve comes from three major sources. The primary source is the forward and upward coordinated swing of the shoulder, arm, and hand. This provides about 70 percent of the serve's total power. The second source is the angular momentum gained by rotating your shoulders and hips, first away from the net as you toss the ball and then back toward the net as you swing (accounting for about 20 percent of power). The third source is the vertical momentum achieved by pushing off the ground with your feet as you swing into the ball (contributing about 10 percent).

Understanding the power sources of the serve allows you to concentrate on increasing the power generated from each of these sources.  Maximum power will come from harnessing all three of these power sources together in a smooth linkage, what is called a bio-kinetic chain. In other words, all the power you are generating in your service motion is finally concentrated in the racquet head at the point of ball contact.

Before beginning this discussion, picture yourself standing on the baseline with your feet and shoulders more-or-less perpendicular to the net. Your hitting arm should be up and bent at the elbow in a ninety degree angle, with your racquet pointing directly to the sky; your tossing hand is extended straight up in the air, as if you just tossed the ball for a
serve. This position is how most good players look at the end of their back swing on the serve.

Now, imagine how you would throw your racquet over the net from this position. To clear the net you would have to loft your racquet into the air in an arcing trajectory; and to accomplish this efficiently, you would probably let your racquet drop down behind your back a little and bend your knees somewhat. This is the image you want when actually swinging your racquet to strike the ball on a serve. With this racquet-throwing image in mind, we can now examine the sources of power on the serve.

Let's start with the concept of dropping your racquet down behind your back before you start forward on your swing. This action mainly increases the distance your racquet will travel, allowing your racquet head to build up great speed before it strikes the ball. An easy way to accomplish this is to loosen the grip of your bottom three fingers, holding the racquet lightly between your thumb and index finger. Some players (e.g., Sampras), actually take their little pinky finger off of the handle altogether. Do this at the top of your back swing (when the racquet is pointed up toward the sky), just as you start to transfer your body weight up and into the forward swing toward the ball. Keeping the grip relaxed in this fashion allows the racquet to fall freely and easily behind the back. This adjustment alone can contribute up to 10 percent additional power to your serve.

Most tournament and all professional players pronate their forearms as they strike the ball. Forearm pronation is the outward or external rotation of the forearm. As the arm swings forward to strike the ball the hand is more-or-less on edge with the thumb facing the back fence. As the ball is struck, the forearm turns outward such that the palm begins to turn forward and outward toward the side fence. It's as if you were going to strike the ball with a karate chop, but at the last second gave it a "high-five" instead.  Sports biologists estimate that at least 30 percent of the power on the serve comes from this pronation of the forearm.

The second source of power is the angular momentum generated by rotating your shoulders and hips. Turning your shoulders loads the big muscles in your torso, so when you turn them back toward the net as you swing upward and forward you will increase the power of your serve, just as rotating your shoulders adds power to your ground strokes. Without the shoulder turn, you are only generating power with your arm, a potentially harmful way of serving. The shoulder turn is bio-mechanically more efficient.

You can practice this shoulder turn by standing at the baseline with the toes of both feet on the baseline, and then serving only by rotating the upper body. Do this several times. Now assume your regular sideways service position at the baseline and serve a few balls using a more pronounced shoulder turn, just as you practiced. (Your belly button can provide a nice cue here: it faces the side fence at the end of the back swing and then turns back to face the net as the ball is struck).

Pushing off the ground as your weight shifts and you reach up to hit the ball is the third major way to add power to your serve. Bending the knees loads the quadriceps and other leg muscles, increasing the forcefulness of the thrust upward and forward into the ball. Timing is critical here. You want to finish bending your knees just as you release the ball and the racquet is still pointing toward the sky before dropping down behind your back. Your knees begin to straighten as you start your upward and forward movement toward the ball. This upward thrust also forces your racquet to drop behind your back and will maximize racquet head speed.

Once you understand the sources of power on the serve, you can begin experimenting in your practice sessions. Continue your current service motion, but gradually add one of these sources to your serve. For example, try to relax your grip a little on your serve, so your racquet can fall lower behind your back. Also, as you strike the ball pronate your forearm to generate additional snap to your serve (think "karate chop to high-five"). Try to turn your shoulders away from the net a little more, to increase your angular momentum. You can also try adding a little knee bend to your current service motion, to put more force in your upward thrust. Once you have successfully added one of these power sources to your serve, try adding another. Finally, see if you can incorporate all of these power sources into your serve.

A final note: dropping the racquet behind your back, twisting your shoulders, and bending your knees may take a little more time to accomplish with your current service motion. You'll have to toss the ball higher in order to allow yourself to complete these moves smoothly without rushing your serve.  Coaches who advise you to use a low ball toss are misguided.  You only have to watch a few minutes of pro tennis on TV to see that all good players toss the ball quite high on their serve (a foot or more above the tip of the outstretched racquet).