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Ask the High Perfomance Lab -- Mar. 28

May 25, 2008 12:25 PM

PLEASE NOTE: The medical opinions in USTA.com's Ask the High Performance Lab are responses intended for the average player. Please consult with your primary physician before beginning any new exercise program.

At the 2005 Competition Training Center Workshop (CTC) held in January 2005, USTA Sport Science Committee members Ben Kibler, MD, Howard Brody, PhD, and Duane Knudson, PhD gave brief presentations on "Technique, Technology and Tennis Injuries." This is the topic of a white paper these three committee members worked on outlining the key point coaches and players should know about injuries and technique.

Following the presentations they held a question and answer period for the CTC coaches. The responses to some of the questions that were asked are included below.

Additional information on the panelists:

  • W. Ben Kibler, MD - Medical Director for Lexington Clinic Sports Medicine Center in Lexington, KY.
  • Howard Brody, PhD - Emeritus professor of physics at the University of Pennsylvania.
  • Duane Knudson, PhD - Associate Professor, Department of Physical Education and Exercise Science, California State University, Chico.

This is Part II of a two-part series on this topic.

For additional information on technique, technology and injuries in tennis, download the USTA white paper on this topic.

For injury screening tests, and injury prevention ideas, download the USTA High Performance Profile.

Q: Is there any evidence to suggest that one grip style puts a player at a greater risk for injury compared to others?

Dr. Kibler: It appears that the extreme western forehand, because of the positioning of the wrist/hand, places a greater load on the wrist and you do tend to run a slightly higher risk for injury. Every single junior player I have seen with wrist injuries has used, on the forehand side, a big western grip. I think, once again, it's almost an adaptation - that wrap-around motion (of the western grip) - because that is the only way they can get the arm into a position where they can pronate. That position is not necessarily going to hurt the wrist, but I think it's an indicator to the body's attempt to compensate for something else that is wrong.

As for the backhand, the problem with the 2-handed backhand now is that everyone is going to the backhand style where the left arm becomes the power arm. It's kind of a guided one-handed backhand. Well by definition, that left arm is weaker - it's supposed to be non-dominant - and to try to get that extra power you change the wrist position and place added loads on the wrist. Once again it's not necessarily how you hold the racket, it's what the arm and racket are doing when the arm is in that position; it is the combination of the wrist position and the load the wrist experiences.

I wouldn't worry so much about the grip, except for the extreme western forehand, but I would worry more about how the racket is positioned as the player comes forward into the shot.

Q: Strings and string tensions... Are the any differences in injuries or performance that come from using different strings or having rackets strung at different tensions?

Dr. Brody: It is generally accepted that looser strings (lower tension) give you more power. How much more power? You wouldn't believe it - it's one percent, or so, for a 10 pound change in tension. Does that mean very much? Well, for identical strokes, that 1% of added ball velocity will cause the ball to land 8 inches further out, or 8 inches further from where it would have landed using tighter strings. What does change with looser strings is the dwell time; the time the ball spends on the strings increases. A lot of people feel that gives you better control. There is absolutely no data indicating that looser strings give you better control. And it is generally accepted, anecdotally, that tighter strings give you better control. Is there any data that suggests tighter strings give you better control? There is only one paper showing a plausible reason why tighter strings would give you better control.

As for injuries, when you hit a ball, there is a certain change in the ball's momentum. That change in momentum is caused by an impulse (the amount of force multiplied by the length of time over which the force is applied). If you go to looser strings the dwell time gets longer, and therefore for the same change in ball momentum, the peak force is reduced. If peak force is causing injury, going to looser strings will reduce the peak force. Does peak force cause injury? You probably think it would, but we have no good data on it.

Q: If you had to identify one area of the body that you would want to make sure had sufficient strength, what would it be and how would you strengthen that body part to reduce injury risk?

Dr. Knudson: From a biomechanical perspective, there is a lot of trunk muscle activity in the tennis strokes, all tennis strokes. There is a lot of abdominal muscle activity. Many people call this the core and it is a popular theme in strength and conditioning these days. There basically is a channeling of energy from the lower body to the upper body through the core. The core musculature also protects the spine from getting out of position where it would get hurt. What tends to happen is that players have strong abdominal muscles, naturally, from hitting their strokes and tight and over-stressed lower back muscles - although these show a lot of activity in tennis strokes as well. I would encourage a player to develop strength and flexibility for the trunk musculature.

Dr. Kibler: Hip and trunk is by far the #1 area. If you have a player who comes to you and says "I want to improve my serve velocity, what rotator cuff exercises can I do?" You would give them an "F". You would give them an "A" if they asked you "What exercises should I do for my hip and trunk to allow me to have good stability and generate the needed force?"

If there is one single area you want to have flexibility, it would be at the shoulder - in internal rotation. This is one area where we know that decreased flexibility has a direct correlation to injuries in the shoulder and decreased performance. The High Performance Profile does a good job at assessing this. You have to have good internal rotation to get effective long axis rotation. You have to make sure that is maximized.

Q: If there is one take home message you would want coaches to leave here with, what would it be?

Dr. Brody: As far as I know, there are no studies linking racket parameters with injury. I've seen all kinds of ads saying "our racket has less shock, less vibration, etc." I know of no studies linking injuries to racket properties.

Dr. Kibler: As coaches, you are all so good at observing these things (related to technique). Try to put together the observations you make with the mechanical problems that technique flaws can cause. Also, remember that there may very well be an anatomical reason (for the observations you make related to technique). The High Performance Profile will catch a lot of those things that we feel cause these biomechanical abnormalities.

Dr. Knudson: Trust your instincts and focus on good technique. Most injuries are not acute (happen suddenly) but occur over time. If you trust the good technique knowledge you have developed, you will do your athletes a real service. Think holistically as much as you can; sometimes an injury is symptomatic of other things. Treat the athlete and the whole stroke and you will do a good job.


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