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November 2003 Article

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Biomechanics In Tennis - Part 3

Jani Macari Pallis, 
Ph.D. Photo
Jani Macari Pallis, Ph.D.

In the previous column we began a discussion of serve biomechanics (with thanks to Pete Sampras). Several graphics of racquet head patterns were included within the article and I asked if you could determine which patterns were flat, kick or slice serves. I also noted that we'd include the velocity of the tip of the racquet head in this month's column.

Again my fellow Tennis Server writer, Ron Waite, has two great articles on the first and second serve which review the flat, slice and kick serves from grip to finish. (Ron includes some nice photos of professionals performing these strokes as well.)

There are a couple of things that we'd like to point out about the diagrams.

  • The footage was taken from the sideline at the baseline.
  • The ball is in yellow. You can see the ball falling from the toss and its path once it has been hit.
  • The racquet head pattern is in blue and is composed of about 200 points.
  • When the points are close together that means the velocity is low. When the points are farther apart, the velocity is higher. Speed is calculated by dividing the distance between two points by the time.
  • You may note that most of the diagrams have two "empty spots" which correspond to the pre-toss. Those points are where Sampras' legs were. The racquet head tip was out sight when it passed in front of his legs.

Serves Were Captured From The Sideline Near The Baseline Facing Sampras From The Left Side Of His Body

Could you pick out the patterns? Take your racquet; go through the motions and visualize the pattern of the racquet tip. You or a friend can also perform the stroke in front of a wall where you can make a shadow. Watch the shadow of the racquet head tip and see which graphic pattern it matches as you go through the motion of a flat, slice or kick serve. If that is still too difficult to see you can tape a ping-pong or little Styrofoam ball to the racquet tip which makes it easier to pick the racquet head tip out.

Sampras Flat Serve

Sampras Flat Serve

Sampras Slice Serve

Sampras Slice Serve

Sampras Kick Serve

Sampras Kick Serve

Now we've labeled the serves. Intuitively this makes sense. The flat serve has little spin. As contact with the ball is made, the racquet head moves forward. If you are right-handed, the racquet crosses the body and finishes on your left side.

In the slice, the finish is similar in that again the racquet finishes on the opposite side of your body. In the kick serve the racquet has brushed the ball moving from lower left to upper right and the racquet finishes on the dominant hand side of your body (right-handers finish on the right, left-handers finish on the left).

Sampras was the first player on which this analysis had been conducted. After seeing this beautiful fluid circular motion I assumed every pro player must have a similar motion flow.

WRONG! Here's the pattern of a first and second serve of a young Venus Williams at her first US Open in 1997. Aside from a very different serve pattern, look at the first and second serves. The patterns are quite similar. Instead of the distinct serves Sampras used, Williams basically used the same serve every time. (This wasn't an isolated case -- comparing a series of William's serves produced the same results.)

Venus Williams First Serve

Venus Williams Second Serve

So these patterns are like a fingerprint or a signature; a player's patterns are extremely distinct.

Now look at the velocities of this sample of Sampras' flat and slice serves -- very consistent. It's just amazing to me how similar the velocities are at each phase of a particular type of serve.

Velocity Of Sampras Flat Serve

Velocity Of Sampras Flat Serve

Velocity Of Sampras Slice Serve

Velocity Of Sampras Slice Serve

Another clue to differentiate the first from the second serves is that you would anticipate the first serve to be faster. In the diagrams, the closer the dots are, the slower the racquet head or ball is moving; the farther apart the dots are the faster the object. Look at the trajectory of the ball after it is struck on the first 6 diagrams above. The ball is faster on the first two, which indeed are the flat serves.

The velocity for the racquet head tip is also very consistent from one serve to the next. Sampras not only has a consistent pattern of motion but a very consistent velocity of motion.

Interesting and fun, isn't it! Next month we'll look at a particular software package tennis professionals are using to analyze player performance.

Until Next Month ... Jani

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This column is copyrighted by Jani Macari Pallis, Ph.D., all rights reserved.

Dr. Jani Macari Pallis is the founder and CEO of Cislunar Aerospace, Inc., an engineering and research firm in San Francisco. In addition to her engineering practice, she has led two collaborations between NASA and Cislunar, creating educational materials on the aerodynamics of sports for pre-college students and educators. As the head of NASA's "Aerodynamics in Sports" project, she has led a team of researchers investigating the aerodynamics, physics and biomechanics of tennis. The group has conducted high speed video data capture at the US Open and research of ball/court interaction, footwork, serve speeds, trajectories and ball aerodynamics. Pallis received a BS and MS from the Georgia Institute of Technology, an MS in mechanical engineering from the University of California, Berkeley and a Ph.D. in mechanical and aeronautical engineering from the University of California, Davis. She is a member of the Executive Committee of The International Sports Engineering Association.

Questions and comments about these columns can be directed to Jani by using this form.


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