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0 May 2, 2002
Volume 32 Number 15
  A publication for the faculty, staff, administrators, and friends of California State University, Chico
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Biomechanics Study Shows Oversize Tennis Balls Safe


The new tennis ball is 6 percent larger than the old one—2.79 in. diameter versus 2.62 in.

Photo by Duane Knudson

Is it just the players’ imagination, or does the new oversize tennis ball have an adverse effect on the body? That’s what Duane Knudson, associate professor of biomechanics and associate chair of the Department of Physical Education and Exercise Science, wanted to find out. He is addressing that question with the help of three research grants from the United States Tennis Association (USTA). The 6 percent larger ball—2.79 inches in diameter versus the standard 2.62 inches—was introduced to the public in the summer of 2000. It was designed to slow down the high-speed serves—up to 140 mph in men’s singles—that are beginning to dominate play and turn pro tennis into a “who serves, wins” game. The International Tennis Federation, in a move to maintain spectator appeal, approved the oversize ball for a two-year test period beginning in 2000. The bigger ball experiences more air resistance, so it slows down more than a regular ball—slowing pro serves by 5 percent and lengthening rallies. But pro players weren’t so eager to lose their advantage, and regular players have been slow to adopt the oversize ball. They complained that the new ball felt heavier, even though the Penn Championship OS (for oversize) is the same weight as the standard ball, and the oversize Wilson Rally Ball is actually 2 grams lighter.

Players also worried that using the larger ball might cause injury over time, and that’s where Knudson’s research came in. His primary area of interest is in the biomechanics of tennis, so he set about evaluating the differences in upper-extremity muscle activation and impact shock using both the oversize and standard balls in match play.

His first experiment was conducted in summer 2000 at the San Francisco Tennis Club with Dr. John Blackwell of the University of San Francisco. They studied the effect of the oversize ball on 14 intermediate to advanced players (10 men, four women) aged 21 to 66. Knudson thought that players might have to swing harder to return the slower ball. To test this hypothesis, he put accelerometers on players’ rackets and attached electrodes to their upper bodies to measure the intensity of muscle activation while they played actual matches with both oversize and standard balls.

He found no difference in muscle activation, but half the players did, indeed, show significant increases in impact shock when using the oversize ball. This sounds more important than it is, however, Knudson said, because players have large variability in loading in tennis play anyway, depending on how hard they hit, where the ball hits the racket, and many other factors. The differential in his study did not indicate that the larger ball carried a higher risk of injury, he stated. Knudson presented his findings in June 2001 at the International Symposium on Biomechanics in Sports in San Francisco.


Duane knudson

Photo by Jeff Teeter

The first study did not quiet the concerns of players or the USTA, however, so Knudson undertook a second study in October of 2000, this time testing only the serves of advanced male players. His data would more closely match the “power-serve” player for whom the oversize ball had been designed. Results showed no significant effect on muscles and a difference in impact shock in only one-third of the subjects, which was “really nothing to worry about,” according to Knudson. “A change in the breeze would have as much effect as this,” Knudson claimed.

Knudson is now working on a volley study, with Dr. John Chow of the University of Florida, using high-speed video to measure players’ reactions to three key strokes. He wants to “put to rest any worry” about injury from oversize balls. The 10­20 percent increase in impact shock sometimes seen is less than the normal stroke-to-stroke variation in normal tennis play.

Knudson would like to see a comeback in tennis popularity. Twenty million people have given up tennis since the boom of the ’70s, he said, but he hopes players like the Wimbledon-winning Williams sisters—Venus and Serena—will repopularize tennis. The oversize ball may make it easier. Rallies last longer, serves are easier to return, there’s more time to prepare for strokes, and it’s easier to keep the ball in the court—with no added risk of injury. The oversize ball is popular on European clay courts.

Knudson received his B.S. from the University of Wisconsin-Oshkosh, an M.S. from Baylor University, and a Ph.D. from the University of Wisconsin. He has 46 peer-reviewed articles to his credit, as well as a book, Qualitative Analysis of Human Movement, with Dr. Craig Morrison. He has taught biomechanics at CSU, Chico State since 1997.

Francine Gair

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