Producing Spin

In the days of yore when Rod Laver's topspin forehand was the most powerful shot in tennis, the general understanding of just how he managed to develop such wicked topspin on the ball was that he used his monstrous, lobsteroid left arm to "roll over the top of the ball" thereby imparting spin. Many of us tried to replicate that famous stroke, but most of us found ourselves driving the ball repeatedly into the base of the net. Some smart teaching pro noticed that successful practitioners of this art also typically started the racket's forward motion from a point well below the ball and finished well above the ball. They surmised that the apparent "rolling over the ball" was an illusion, and that the actual physics involved was friction up the back of the ball causing a torque on the ball that we know occurs whenever a racket is moving in one direction and pointing in another (See Figure below if that is unclear). The application of this technique to producing topspin was called "brushing up the back of the ball" and in those days the brushing technique was applied every time one wanted to add spin to a tennis ball. The brushing action seemed to correlate pretty well with appropriate grip selection; the western and semi-western forehand grips seemed to enhance topspin production, and we attributed the improved spin to having the hand more under the racket to achieve more upward racket head speed. The continental grip seemed a natural for applying underspin to groundstrokes and topspin and sidespin to the serve because the hand was on top of the racket. The physics seemed sound and many talented players who learned and practiced this technique (like Bjorn Borg) developed serviceable topspin groundstrokes, at least as long as rackets were made of wood. The topspin backhand was still a bit of a problem even for the most talented people on the planet but, heck, one could always resort to the two-handed backhand which put plenty of top on the ball, so no one worried about the one-handed topspin backhand.

As rackets improved allowing everyone to hit with more and more pace, the brush technique for adding spin proved to be problematic. Achieving topspin by brushing up the back of the ball is the natural enemy of pace. The greater the angle of attack, the more spin but less pace you get on the ball. Control was also an issue. When our groundstrokes would start missing the baseline, we would accentuate the upward brushing motion in an attempt to get more topspin, but dragging up on the ball drags it up and out of play. (see Push Syndrome). We try to compensate by tipping the racket face down a little, so it is actually pointing at the base of the net when we make contact with the ball, and that seems to work...sometimes. Its a rather delicate dance though - drag up for topspin, point down to counter the dragging up. Just another one of those magical things that only very talented people can do consistently, or so I thought.

The brushing method is unattractive for another reason: In order to generate heavy topspin, the "angle of attack" of the racket must be steep. The steeper the angle of attack the greater the spin, but the smaller the effective surface area of the racket face and therefore the harder it is to address the ball. That is why one tends to mis-hit or "shank" the ball more when going for heavier topspin by brushing. This issue became progressively more acute for playing pros as the speed of serves and groundstrokes increased into the 100+ mph range we now enjoy. Pros discovered they needed a new way to generate topspin that did not interfere with addressing the ball or producing pace. Enter the era of rotational impulse.

Rotational Impulse

So.... how else does one apply topspin to the ball? Answer: By rolling the racket face over the ball immediately after the moment of contact. You read me right - the "rolling" happens long after the ball has left the racket. How does this rolling motion affect the ball if it happens after the ball is already gone? Well, it doesn't affect it, of course. What affects the ball is the torque or rotational force that is present (but completely invisible) at the moment of contact, and that causes the racket to roll over in the follow through. When we watched Laver punish the ball in the '70s, it was the impressive wrist roll we were looking at, but when we tried rolling the racket over the ball, we only succeeded in driving the ball into the net. A racket face rotating at a constant rate will not deliver spin to the ball, and if the orientation of the racket face changes before contact, it will disrupt the flight path of the ball. The rotation of the racket face has to be accelerating under force on contact with the ball but not actually rotating. The physics is analogous to the linear "impulse" force that transmits directional control to the ball. The tricky part is understanding that the rotational force on the racket at moment of contact will initially express itself in rotational acceleration, not rotation. An object can be accelerating without moving. Read that sentence again because it is important. It means that many of the necessary forces used by the talented to make their magic with racket and ball are completely invisible, even on super-slow-motion video.

Rotational acceleration is a particularly slippery concept. If a wheel is rotating clockwise - slows down - stops - then starts rotating counter clockwise the maximum rotational acceleration may occur when the wheel is completely stopped and its rotational velocity is zero!

Think of the advantages of being able to apply spin to a ball without moving the racket. If you freeze Federer's forehand at the moment of contact with the ball, the stroke looks like a traditional, flat forehand (see figure below)): The racket face pointing more or less along the intended flight path, straight elbow, arm in front of the shoulder, etc. Immediately after contact Fed's arm coils up like a Chinese noodle. In reality, the arm is not coiling, but re-coiling, since the arm starts the stroke coiled at the end of the backswing, straightens out in the load phase then re-coils after the ball has left the strings.

The point is that the arm uncoils or stretches out during the acceleration or load phase of the stroke because the inertial force of the racket head (about 10 pounds in a pro forehand) overcomes the stored force of the muscles in the forearm and shoulder. That stored force is the result of stretch-shortening of those muscles that occurred during the load. That stored force is opposed and held in place by the acceleration coming from the legs and torso during the lag phase. Just before contact with the ball, the wave of power from the legs starts to dissipate, acceleration ebbs, the inertial force of the racket decreases and the stretch-shortened muscles, now unopposed, release both linear and rotational force into the ball with little or no change in the speed or orientation of the racket until after the ball is gone. The result is precise directional control and wicked spin on the ball that does not interfere with pace-producing racket head speed or with accurately addressing the ball.

Hitting Without Spin

So what about moving the racket head low-to-high on the topspin and high-to-low on the back-spin? These motions are still essential but for two very different reasons. The first reason is simply the old "brush-the-ball" technique which can still contribute to spin directly and can indirectly contribute to control through the process of "modulation". The greater the angle-of-attack of the racket head relative to the intended flight path of the ball, the less pace on the ball and the greater the spin. The reduction in pace can be beneficial particularly on underspin shots which tend "fly" due to the increased aerodynamic lift on the ball caused by the Magnus effect.

The second reason is a bit more complicated and is based on a principal I call "spindirection". Stated simply, it contends that any stroke that ends up with the racket and ball traveling along the same flight path is wrong, wrong, wrong. I understand that this contradicts centuries of common wisdom and common sense, but I have found that the best way to wreck your game and your elbow at the same time is to spank the ball on its bum. A glancing blow is always best.