The concept of a power wave in tennis is designed to counter the widely held belief that, as athletes, we are nothing but a bunch of rods and levers motivated into action by the sequential contraction of our muscles. There is truth in that belief, but it cannot begin to explain the beautiful miracle of a well-hit topspin forehand, nor the horrific spectacle of a shoulder-high volley dumped into the base of the net. For one thing, the mechanical model of tennis stroking does not account for the expression of the stroke over time. We don't push off with our feet at the same moment we strike the ball, luckily, since our brains could never handle performing that many tasks at once. Indeed, the only way to accomplish something as complex as a tennis stroke is to generate the energy in our feet early in the stroke then transmit or propagate it through time and space via the medium of the body and ultimately to the racket and ball. Along the way, various parts of the body harvest energy and momentum from the wave, converting them into ball control, spin and pace.

The Shape of the Wave

When we conceptualize a wave we like to take a tracing over time as in the diagram above. The horizontal axis represents time, while the vertical axis represents directional power. (Physicists will balk at this notion as their concept of power has no direction. Tennis players will understand this concept. Power in the positive direction is power directed towards the target. Negative power is in the opposite direction, i.e., in the direction of the backswing.) Imagine a sensor on the player's wrist that measures the forces trying to displace it. At first, the force is backward, dragging the wrist and racket away from the ball. This move is the lock, an essential component of every stroke from the drop shot to the serve and typical of wave motion which is always too and fro, forward and back or up and down. Suddenly the power changes direction towards the ball starting the acceleration forward.

At this point the racket flips, loading stretch shortening forces into the forearm which will ultimately be injected into the ball as impulse for spin and control. As the power wave continues to crest, the racket is dragged behind the wrist - accelerating and building racket head speed for pace - the lag phase. Suddenly, at the peak of the wave, the power reverses, and the shoulders start decelerating. The racket head's inertia allows it to catch up with the wrist, releasing the stored forces into the ball. At the moment of contact with the ball, all power neutralized, and there is no force on the racket coming from the shoulders. Eventually, the wave becomes completely negative, keeping the racket from wrapping totally around the player's neck. It is important to remember that the lower body generates the power wave - a conspiracy of the feet, legs, and hips. Although you can add power to the wave using your upper body, that can be a dangerous procedure. In particular, at the moment of contact power delivered through the shoulders should be zero, not positive, lest one misdirect the ball away from its target by injecting impulse in the wrong direction (see Push Syndrome).

Back and Forth

Every shot in tennis starts with the racket moving away from the oncoming ball. I call this move the lock or "backswing proper" to differentiate it from the unit turn - the larger and more ostentatious component of the backswing.

Every shot in tennis starts with the racket moving away from the oncoming ball.

The lock, like every component of every stroke, requires a source of power. That power comes from an initial reverse-rotation of the hips as you transfer bodyweight from the front foot to the back foot.

This early weight transfer id portrayed as the initial dip of the power wave. In the diagram of the power wave, the initial fall below the zero-line indicates that the direction of momentum of this segment of the wave is away from the oncoming ball, driving the hips, shoulders and eventually the racket into the backswing proper. As the body weight accumulates on the other, hitting foot, we get solid contact with the earth. The main forward thrust of the power wave follows as the hitting leg pushes off and the hips change direction and begin rotating the body in the direction of the oncoming ball with all the force the hitter can muster.

Pace = Power X Time

The amount of power that is ultimately transferred to the ball by the power wave depends on a raft of factors, importantly the loss of energy as the wave makes its way through the body to the racket. But the total amount of power that can be delivered by a given power wave is proportional to the area under the power curve. That means that the width of the portion of the power wave that is above the zero-line is just as important as the maximum height of the wave, the "max power" and is why short strokes, i.e., strokes without a lag phase, tend to result in less pace than longer ones. The storage of control and spin forces requires less power than generating pace and is harvested earlier, so a properly hit short stroke is replete with directional control. This surfeit of control over pace in short strokes is why talented short strokers will often complain about their lack of pace as they are stuffing trophies into their trophy cases.

Maximizing Control

The production of control and spin forces requires a rapidly increasing force early in the stroke to stretch-shorten the forearm muscles followed by a rapidly decreasing force at the end to release them. The short stroke power wave is ideally shaped to satisfy those two criteria which is why short strokers seem to have so much control over their balls. They are often referred to as " placement players" rather than "pushers", at least by those whom they have beaten. The two-handed topspin backhand is a short stroke and is well known for the amount of control it gives one over the ball. To achieve such a shape one must generate a very short, intense power wave. Both the upstroke and the downstroke must be steep to maintain control over the ball. If you cheat on the downslope, i.e., try to pour more power into the stroke late instead of backing off and letting the racket do the work, you are essentially "pushing" the ball, robbing it of spin and control.

Pushing Back Against the Push

How could Federer flub his forehand? Just dump an easy, short ball into the center of the net or hit it ten feet behind the baseline? What can we learn from his mistakes? That makes the most natural of all tennis strokes, the forehand, so glitchy, even for the best of the best? What is the universal forehand fail?

The culprit lurks in the second half of the power wave. Just after the power wave peaks, it takes a perilous nose-dive, indicating a sudden reversal of momentum from forward, in the direction of the oncoming ball, to back away from the ball. This reversal happens before the moment of contact. Ponder that for a second. Just before you lay into the ball you have to pull back on the reigns, stomp on the brakes, pull your punch. Could there be anything less natural than that? Does that even make sense?

In fact it makes perfect sense. To take control of the ball's flight path one must store and later deliver muscular forces to the ball. There are alternative ways of delivering spin and pace, but there is only one reliable source of control, and that is impulse. The forces one uses to gain hegemony over the ball are stored during the lock and load phases of the stroke, then held in place by inertia during the acceleration or lag phase. During the lag phase, forces are driving the racket towards the ball, but they are not appropriate control forces. Their direction is determined by the need to address the ball and the need to add spin to the ball. The acceleration forces are not pointing at the tiny window over the net through which one must direct the ball for it to find its target. If you continue to accelerate the racket through the moment of contact, those mighty acceleration forces will overwhelm any stored control or spin forces in the forearm and send your ball God knows where.

Thus, to deliver the stored control forces to the ball at the moment of contact, all acceleration forces must be gone. Imagine that for a second and be amazed. For a few tenths of a second, you are pouring as much power into the racket as you can muster, then 'poof' - power is gone, and the racket glides into the ball under its own steam. The timing of this loss of forward acceleration is pretty critical. A few milliseconds late and you drive through the ball and release the stored control and spin forces in the follow through with no effect on the ball. Generally, this fail results in a long ball. A few milliseconds too early and the stored forces will begin to contort the arm and racket into pretzel position before one makes contact with the ball. The pretzel position is best reserved for the follow through where it can't do any harm to the ball. Generally, when the forces are released early, you drive the ball into the center of the net, mishit it or flag it all together.

The Power Wave Shuffle

The video above is a demonstration of how foot-to-foot weight transfers allow you to harvest directional momentum from the earth to feed the power wave for the topspin forehand. Remember that each weight transfers occurs hundreds of milliseconds before the portion of the wave that it creates reaches the racket. Waves must travel from the source to the sink, and that takes time. This delay between action and result messes with our heads as players. It syncopates the rhythms of the stroke and has made it hard in the past to understand the relationship, say, between "stepping into the ball" and stopping the forward swing. I always thought that stepping in somehow added power to the shot just by the translation velocity of my body. If you take the 3 miles per hour that I can move and add it directly to the pace of the ball, well, obviously that is not a valid reason for "stepping-in". Stepping-In has two functions; stepping-off of the back foot, which starts the upstroke of the power wave and motivates the forward portion of the swing, and stepping on to the front foot, which secures the front foot for pushing back against and reversing the direction of the power wave. So, just like a bullwhip, the footwork of the forehand groundstroke is back - forward - back because the stroke is back (backswing proper) - forward (load and lag) - back (explode). If you fail on the last portion, say because you don't plant your front foot sufficiently to push back against the stroke, you lose all of your control and most of your spin. You still have plenty of pace, though, to carry the ball into the back fence.

From the preceding discussion we can derive a fundamental rewrite of the old "Step into the ball!" tip. It is; "Step early into the ball!" The 'early' part is vital because stepping-off (of the back foot) has to finish well before the load phase begins, and stepping-on (the front foot) must finish well before the moment of contact. Only by completing both of these steps before the moment of contact can you use your front foot as an anchor to reverse-rotate your hips against the forward rotational inertia of the body. This action puts the brakes on the stroke and releases the stored forces.

Step into the ball early!

So what do you do if you can't get your front foot down? How do you stop the forward rotation of the shoulders? The pros instinctively counter-kick out forward with the front foot. That can provide just enough momentum to quell forward acceleration and allow control forces to be released.