Measuring Speed and Spin
in Your Game

By John Yandell

In the last issue we presented a comprehensive table on the levels of spin for groundstrokes  in pro tennis—as well as some averages on ball speed. (Click Here.)  This was all based on data recorded by shot spot.

One question I got repeatedly after that article was how does this apply to my game?  How do I calculate my own speed and spin levels?

The answer is you don't need shot spot.  Previously we published an article on how to use a pocket radar gun to measure ball speed and how improved technique can increase shot velocity. (Click Here.)

And you don't need shot spot for spin measurements either.  All you need is a compact high speed camera or a smart phone with a super slow motion setting that films at 240frames a second and also has a high speed shutter. 

And you need to understand how to use a simple equation.  I got a B in high school algebra but I did learn how to solve a one variable equation.   That's all it takes. I've used this equation going back to 1997 in our measurements of spin in pro tennis. In this article I will show you how to do it for yourself. 

Another option of course is if you have access to a Play Sight system.   It delivers spin data. But in tests against our cameras we found that it wasn't always accurate and could be off by as much as 50 percent or more on a given ball. So doing it for yourself is really the way to go.

How

   So how?  To help me demonstrate this I asked two friends, Chang and Michael to help. Both are 4.5 NTRP players. We set up the ball machine and first used the radar gun to calculate ball speed on their forehands on a solid ball in the middle of the court. We then measured 10 balls for each player that were near the center of the string bed and at about mid level in the strike zone for each player. 

Michael's average forehand speed was 69mph. Chang's speed was virtually identical at 68mph.

Then we used my Casio high speed camera to record spin.  My camera is an older model, an EH-FX 25.  But if you search around you'll find there are many other models available with similar capabilities. We filmed fairly tight from about the knees up.  This allowed us to see the markings on the ball as it rotated. 

You want to see at least a couple of rotations and how many frames they take.One key to do this is you have to have a high enough shutter speed.  You need at least 1/1000 of a second to be able to see the markings clearly as they rotate. 

As I said you can also do this with a smart phone with a slow motion setting.  Check your phone—that setting is probably 240 frames a second as well. It is on a iphone for sure.One disadvantage to the phone though is that you can't manually control the shutter.  The phone automatically adjusts this depending on the light. 

But if you film in full sun, you should be able to count the rotations.  You don't really need to know the shutter speed so long as you can see the markings on the ball clearly. Don't try it on an indoor court though. And that goes for the Casios as well. 

A second disadvantage of the phones is that it's hard to go accurately frame by frame in viewing the footage.  The best option is to email it to your computer and then do your counting using a frame accurate player like Quick Time 7. 

These are the reasons I still prefer the camera.  You take the memory card out of the camera, put it in the computer, and then open the clips in Quick Time 7 or some other frame accurate player.

The Equation

So what about that equation?  Look at the clips and go frame by frame until you can clearly see the ball marking after contact.  Now count the number of frames to get to one revolution and hopefully 2 or 3 revolutions to verify the count.

Now put the number of frames for 1 revolution into this equation: 

Now cross multiply and divide to solve for X.  If 8 frames equals 1 rotation, you divide 240 by 8.  That equals 30, or the number or rotations per second. Now multiple that by 60 to get rotations per minute or RPMs.  In the example above that comes out to 1800rpm.

Calculate that for about 10 individual shots.  Then average the rpm numbers of the individual shots.  I like to round the average up or down to the nearest hundred.

Where did Michael and Chang come out?  Chang averaged 2000rpm.  Michael was a little higher at 2300rpm. How does that compare to the pros we studied?  Averaging the measurements of all the players we get 2,900rpm. So our 4.5 players were averaging about 2/3s or a little more of the average spin of elite pro players. Speed wise their forehands were at about 85% to 90% of the pros on average.

What Does That Mean?

   So what does all that mean in understanding the levels of the game? Obviously spin and speed are far from the only factors that separate levels. Movement, strategy, mental toughness, and conditioning are equally or more important. 

But it does show that in terms of pure shot quality, the differences between levels are less extreme than you might expect.  That coincides with the filming we did years ago timing the shot cycle.We found that for high level 4.5 and 5.0 players the time between the rackets was only about 2 or 3 tenths of a second longer.  Obviously small increments make a huge difference in tennis! 

More on speed and spin at different levels to come!  Stay Tuned.