NEVER hit flat!

I think we are done here.

I could stop right there, and anyone who takes that admonition seriously will markedly improve their consistency and power. Sadly, some people (like me) need to know the whys and wherefores or they will misinterpret and misapply such an admonition.

First the 'wherefores'; When I say 'never' hit flat, I mean NEVER HIT FLAT! Not on first or second serve, not on the volley, the overhead, the lob or drop shot. Watch the pros. They put spin on everything. Even the "squash shot" - the shot that all of the pros use when they track down a wide ball - is all slice.

The 'why' requires a bit more elaboration. Fundamentally we are all trying to strike the ball in such a way that it lands on a target in our opponent's court. It makes intuitive sense to guide the ball along a straight line path to the target, but there is a net in the way. There is also, thankfully, gravity so we can hit the ball up to clear the net and it will then dive into the court hopefully onto the target. Therefore controlling the depth of the ball means aiming for a target above the net; not too high, not too low. Avoiding the alleys is different - that is more or less a straight line deal, but let us put that aside and concentrate on controlling depth because that is way more complex and difficult.

So as beginners we were taught to swing low-to-high, guiding the racket head along a straight line path that connects the point of contact (POC) to the target hovering over the net strap then follow-through the ball on the same straight line path towards the target, in essence guiding the ball along. This 'push' stroke is taught to every kid that picks up a tennis racket. We were assured that, should we mistime the ball a little, keeping the racket on the ball's intended flight path will give us the best chance of a successful strike. Eureka!

If this is the be-all, end-all of hitting a tennis stroke, why do all of the pros end up their strokes with their rackets wrapped around their necks? Or behind their ears? Why does the pro volley follow through go across the body? And what is with that corkscrew hitting motion that typifies the pro serve? Why not just slap that sucker? The answer is that as endearing as the old follow-through-the-ball admonition is, it just doesn't work. Period.

This is counterintuitive even to me. It seems very satisfying to spank the ball on its little behind and send it on its way. That is how I was taught. It feels so right to hit it that way that even when I am trying not to hit the ball flat, I revert to it over and over again. Notwithstanding the constant haranguing of their first teachers, the talented quickly evolve beyond the "spank". They somehow develop these twisty, contorted strokes that deliver amazing power, pinpoint precision and wicked spin and, in doing so, violate every rule that has been beaten into our brains about how not to hit a tennis ball. Yet for the "rest of us", the admonition to "hit through the ball" - this blackest of black pearls - haunts us throughout our tennis lives.

Why Not Flat?

It isn't that hitting a ball flat is bad, it is just that it is impossible. Not impossible to hit, mind you. I hit flat balls all the time and some of them, the small minority, actually find their way into my opponent's court. Most end up in the net or behind the baseline. Flat balls are inconsistent. The absolute requirement to hit with spin is NOT; I repeat NOT primarily about the aerodynamics of the ball. Sure, topspin tends to pull the ball down allowing you to hit the ball with blistering pace, ample net clearance and still, the ball dives into the court. What if your net clearance is not all that ample? Topspin will pull the ball right into the net. What if it is too ample? The ball will still fly out. There is still a narrow window over the net through which you have to hit if you want the ball to find your opponent's court. The real purpose of topspin aerodynamics is to change the shape of the ball's flight path. It is not, as I believed for many years, a magic spell that creates consistency. It simply makes impossible balls possible. You still have the same degree of responsability for controlling the ball into play.

What about underspin? We hit most volleys and approach shots with underspin. The aerodynamics of underspin are adverse - they tend to make the ball fly long - so why hit spin at all, especially if you are hitting a volley with your tongue hanging over the net? Why not just spank the heck out of that bad ball?

Spin is necessary because whenever we use a racket to hit a tennis ball, we are required to accomplish four synchronous objectives; address the ball, add pace, add spin and take control, i.e., steer the ball towards a target, all in the .05 seconds during which the ball is in contact with the strings. Since we only get one swing at the ball, you must accomplish all of these things with one swipe of the racket.

Consider trying to accomplish just two things at once; addressing the ball - getting the ball and the center of the racket face to meet - and controlling the ball - directing it towards your target. To exercise control of the ball, i.e. to counteract thed ball's tendency to do what it wants after contact, you must inject impulse into the ball. You create impulse by accelerating the racket head along the desired flight path of the ball at the instant of contact. To clear the net you need to accelerate your racket face along a precise, imaginary line drawn from the ball through a narrow window over the net. Imagine a line drawen from that window over the net, through the point of contact and then extend it back behind you. To hit the ball flat, you must first place the sweet spot of your racket behind you on that line, then swing through the ball on that line, constantly accelerating. That would be easy if you make adjustments along the swing path to assure that the sweet spot makes solid contact with the ball, bu you can't! Any deviation of the path of the racket to address the ball requires a change in the vector of acceleration and thereby send the ball off in trhe wrong direction. What if the ball bounces higher or lower than you expect? If you are hitting flat, you cannot reliably address the ball and take control of it at the same time no matter how adroitly you spank it. Try adding spin or pace, and it is a real mess.

Part of the solution to this tangle is to start by adding spin to everything. When you hit a ball with spin the racket head is moving in one direction while facing another (usually at the target) and neither of these directions coincides with the desired flight path of the ball. The motion of the racket is partly forward and partly across the ball; either up, down, sideways or more often some combination of these. Now your stroke is less of a spank than a glancing blow - like a moving airplane propeller hitting a juicy watermelon. All of a sudden the position of the racket in the backswing is less critical, and you do not need to predict the exact position that the racket and ball meet. Unlike most chores in tennis, this one is easier than it sounds. Just hit the ball with a slap instead of a jab. Which one of those are you more likely to miss? Addressing the ball is simply a matter of timing as you move the "propeller" into the ball; the racket and ball need only coincide somewhere in time and space. Gaining control over the ball's flight path is achieved in the backswing, long before the moment of contact (see "Impulse"). That frees up control over the speed and direction of the racket face to determine how much pace and spin you inject into the ball.

Pace and spin a mostly determined by racket head speed and the angle of attack of the racket relative to the flight path of the ball. Since racket head timing, speed, and flight path are all independent parameters, addressing the ball and addition of spin and pace are all independent of one another. You can, for example, add more pace without loosing spin or mishitting the ball. More importantly, by first 'sketching' control in the backswing, you are free to free to utilize the full palate of spin and power whilst still weeping your game between the lines.

Isolating the act of getting control of the ball is a bit trickier; it involves separating racket head velocity (e.g., speed and direction which are responsible for pace and spin) from racket head acceleration. The acceleration is the change in velocity of the racket head, and like velocity, it also has both a magnitude and a direction. Importantly it is completely independent of racket head velocity. This independence of velocity and acceleration is key. You can have the same amount of acceleration in a racket that is going really fast, really slow or not moving at all. Unlike velocity, acceleration can be invisible, especially if it only lasts a few milliseconds. Acceleration of the racket only happens during the period when you are applying an unbalanced force to it. If this is happening during the .05 seconds that the racket face and ball are in contact, it results in an impulse that changes the speed and direction of the ball. The larger the impulse injected into the ball, the more the ball redirects itself in the same direction as the impulse. It doesn't matter what direction the racket head is moving or how fast - the ball is turned in the direction of the impulse, not the velocity of the racket or where the face is pointing. Impulse is how you deliver control to the ball. Once you have the racket moving, you stop pushing on it. The racket head's velocity will remain constant, but it will stop accelerating just before contact, and that is when your secret weapon - stored force - is released into the ball delivering impulse and giving you control over its ultimate flight path.

This separation of the sources of addressing the ball, pace, spin, and control allows you to optimize each one independently of the others. When hitting with spin the racket face, the velocity of the racket head and the control forces are all going in different directions which makes the stroke much easier to maintain. Otherwise, the mind gets confused. It helps the feeble brain of the untalented if we keep our objectives, control, pace, spin and solid ball contact, very separate from one another. If you don't put some spin on the ball, you tend to push through it sending it God knows where in a big hurry.

Please note that this is not a "more is better" situation. Extreme spin is not the objective. Sometimes just a skoach is the right amount and all you need to help your mind parse out the levers of pace, control, spin and solid contact with the ball.

The Puppet Problem

Another important reason never to try to hit the ball flat is that we are built like marionettes and can't our joints-and-levers-style bodies to move the racket in a straight line through the ball towards its target. Think of hammering in a nail. It makes sense to think about guiding the hammer's head towards its tiny target in a straight line that coincides with the nail itself, but that is not what anyone does.

It is impossible to develop any power by "pushing" the hammer towards the nail. Instead, we swing the racket head in an arc made up of a sequential rotation of the shoulder, elbow and just a little wrist. The result is excellent hammer-head speed and accuracy. The linkage from the body to a tennis racket handle comprises a bunch of rotating levers of varying lengths. Any efficient stroke we hit will end up describing a series of rotations that carry the racket through curves and arcs. Remembering that our four goals are to address the ball and add spin, pace and control we can create pace through racket head speed, control through the application of impulse and some spin to alter the shape of the flight path of the ball - more curved with topspin or more straight with slice. We would also like to have at most two forehands (topspin and slice), two backhands (topspin and slice) and one serve with which to deal. We want the volleys, lob, drop shot, squash shot, overhead, high and low balls all to be slight variations of our basic strokes. That is a particularly tall order since addressing a low bouncing groundstroke at knee level seems, at least on the surface, to be very different from a volley over one's head, but if you always hit with spin, you will find that the differences are purely cosmetic.

The time-honored principle I discredited above - that the racket should be pointing at and traveling towards the target for as long as possible before and after the point of contact - is still partly sound. The racket face at least needs to be pointing more or less in the direction of the opponent's court at the moment of contact. So over the years players developed stroking styles designed to keep the racket face pointing at the target for as long as feasible. Guiding the racket 'along a rail' is not a trivial task when what you have to work with is a series of jointed, rotating rods flailing in 3D space. We must also compensate for our human inability to perfectly time the point in spacetime that the ball meets the racket face. We need to increase the 'happy time' - the duration of the interval of time during which contact between racket and ball will result in a happy outcome, namely the ball leaving the racket face on a beeline to our intended target.

The solution they found to maximizing this "happy time" is to apply spin, because when you hit across the ball instead of directly at it, your racket face is pointing at your target longer! Just look at the video of the "Borg-style" forehand (aka "WTA-style forehand") above. Note that the racket face is brushing across the back of the ball but is pointing into the opponent's court for an absurdly long time. That means that if you are a bit early or late contacting the ball, it will still go where you want. Although this stroke has been largely supplanted by the Federer-style forehand in the pro ranks, it is still widely used especially by intermediate players. The semi-western grip tends to force one to hit with spin, and the extreme difference between the racket and ball flight paths teaches the value of that separation on a fundamental level.

Underspin, sidespin, spin on the serve and volley all follow the same principle. You establish a rotation of the racket that is orthogonal (at right angles more or less) to the desired direction of flight of the ball like a propeller then push that propeller through the ball, relying on timing to achieve solid contact. The head of the racket will describe a helix - like the tip of the racket is following the turns of a screw - counter-clockwise (from the player's perspective) for a topspin forehand and clockwise for an underspin forehand. A "looser screw" - meaning the threads are farther apart and the follow through ends up more out in front - means less spin and more power (forward racket speed). A "tighter screw" - for example, the "buggy whip" forehand with the follow through ending up behind the right ear - means more spin and less pace. This allows one to modulate the stroke for the tactical situation. To handle a low short ball, for instance, more spin and less pace are necessary if the ball is going to end up in the opponent's court.

The real elegance of this approach is how one handles balls of differing heights. Since the racket is pointing at the court for such a long time, you can use the same swing to hit a low, medium or high ball. Having one swing to handle many different balls is useful since once you learn a stroke, you have to maintain it. The fewer basic strokes you have, the easier that is. The millennium forehand demonstrated above works equally well on an ankle height ball and one over your head. The same is true of the slice. It is the shape of these strokes which makes them so versatile, and such versatility is impossible with a classic flat stroke.

The Evolution of Spin

All of this is true to an extent - to the extent that one is achieving spin by "brushing across the back of the ball." It gets a bit more complex when one is trying to deal with blistering pace on the incoming ball. To make solid contact with a ball that is coming in at 60-100 mph, one must reduce the vertical and cross-wise motion of the racket head or risk mistiming and failing to address the ball. The less "angle of attack" of the racket relative to the flight path of the ball, the greater the window in time to get the ball and racket face together. The magnitude of spin is significant in A-level tennis, but it must be accomplished with a much looser helix and much less "brushing" up the back of the ball.

Although we try to separate the pace, spin, control and addressing functions, they can interact in ugly ways especially when dealing with pace. Now, this is a little tricky, so let us boil it down using the "Borg-style" forehand as an example. In that stroke, the racket head above is pointing at its target, but it is moving in two distinct ways. First, it is describing an orbital ellipse around the elbow that is oriented at right angles (orthogonal) to the flight path of the incoming ball like a propeller. At the same time, the entire ellipse is itself moving in the direction of the incoming ball to bring the ball and racket face together. Often the racket speed across the ball is greater than the speed into the ball. This indirect application of racket's head speed is an example of indirection in the application of spin or simply spindirection. The greater the spindirection, i.e., the greater the angle of attack and the tighter the threads of the screw, the greater the topspin but, the harder it is to time the moment of contact.

Enter the "Federer-style" forehand. This stroke evolved from the "Borg-style" first popularized by clay players like Bjorn Borg. The Federer-style forehand relies on two kinds of acceleration (linear and rotational) and two kinds of constant velocity (forward linear and transverse rotational) simultaneously. The velocities are similar to the Borg-style forehand but the orthogonal velocity - the velocity across the path of the incoming ball - is much less and the linear velocity - the velocity in the direction of the incoming ball - is much more in the Federer-style. The spindirection is still present - the stroke is still low-to-high - but is a considerably more linear, straight-through-the-middle-style stroke. A swing path that is nearly parallel to the path of the incoming ball makes it much easier to time the balls that come in with tons of pace, and as a side benefit, optimizes the pace delivered by the stroke. Additional spin is added using stretch-shortening and rotational impulse which, since it is invisible and does not affect the speed or direction of the racket head, does not interfere with the address of the ball or the addition of pace and control.