String Tension, Excerpts from High Tech Tennis by Jack L. Groppel, PhD

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Excerpts from High Tech Tennis by Jack L. Groppel, PhD, 1992, Leisure Press
Out of print, but still available used in paperback from Amazon.com via the link above.

    High Tech Tennis describes tennis equipment, strokes, and tactics with an approach that is halfway between that of the more technical Tennis Science for Tennis Players by Howard Brody, and that of typical non-technical tennis instruction books. Although the High Tech Tennis does not cover all the technical ground that Tennis Science for Tennis Players does, it teaches tennis in general better, and is for the most part more clearly written.
    The author, Jack L. Groppel, is the Director of Tennis and the Director of Sports and Health Development at the Saddlebrook Resort at Wesley Chapel, Florida, where Jennifer Capriati, Martina Hingis and Marlene Weingartner have residences (although Martina Hingis reportedly spends most of her non-traveling time at her new house in Switzerland since her retirement). Groppel has written or edited 10 books including this one.
    One complaint that might be made about the book is that there are frequent references to photo sequences intended to explain points, but the sequences are usually of only 2 or 3 photos, and this is not enough to see what is going on in a tennis stroke. Nevertheless, the book is strong overall, and very worthwhile to read. High Tech Tennis is indexed, and also includes an interesting list of references, many from technical journals.

    In 1987 Groppel and other researchers studied the effect of string tension, string type, and racket type on ball impacts, and published the results in 2 papers in the International Journal of Sports Biomechanics. Balls were fired at the same velocity at rackets clamped in place, and the pre-impact and post-impact ball speeds were compared. In the first study, only one racket was used, strung at 4 different tensions with nylon string. The results are summarized in this table:

string
tension

40 lbs
50 lbs
60 lbs
70 lbs
post-impact (bounce) speed
as % of pre-impact (incoming) speed
50.3%
48.2%
46.7%
46.3%
duration of ball contacting strings
in thousandths of a second
4.08
4.07
4.05
3.82

    Until the early 1980s, tennis players were erroneously told that tighter strings produced more power and looser strings more control (you will see this stated in older books, such as the 1982 1st edition of The Handbook of Tennis). Then it was found that the opposite was true. What is surprising about the data above is that racket strung at 40 pounds tension produced only 4 percent more ball speed than the same racket strung at 70 pounds. That is to say, if you hit the ball at 70 mph with your racket strung at 70 pounds, then reduce the tension in order to gain more "power," an identical stroke with the racket strung at 40 pounds will travel at 72.8 mph, less than 3 mph faster.
    The other issue is control. One factor in ball control is that the longer the ball is on the strings, the greater the likelihood of control errors caused by the racket twisting due to off-center hits. Thus the shorter contact duration that results from higher string tensions is thought to reduce the amount of error. At 40 pounds tension, the ball was in contact with the strings of the test racket 0.26 microseconds longer than at 70 pounds tension, which is about 6.8 percent longer. This could be enough time for a significant increase in errors due to the racket twisting from off-center hits. (It is also thought by some that additional control at higher string tensions results from the strings biting more deeply into the ball cover during impact, allowing more spin to be imparted to the ball by an experienced player.)

    Even more surprising results were obtained when Groppel and his coresearchers decided to run impact tests comparing nylon and gut strings, on both oversize and midsize rackets.

oversize racket
string
tension

40 lbs
50 lbs
60 lbs
70 lbs
80 lbs
nylon string
post-impact (bounce) speed

as % of pre-impact (incoming) speed
51.8%
49.8%
48.1%
47.8%
50.0%
gut string
post-impact (bounce) speed

as % of pre-impact (incoming) speed
50.5%
48.8%
48.8%
45.9%
46.5%
midsize racket
string
tension

40 lbs
50 lbs
60 lbs
70 lbs
80 lbs
nylon string
post-impact (bounce) speed

as % of pre-impact (incoming) speed
38.7%
40.4%
37.5%
35.9%
36.2%
gut string
post-impact (bounce) speed

as % of pre-impact (incoming) speed
40.7%
37.9%
43.1%
35.9%
33.4%

    The numbers in the last 2 tables were obtained by reading graphs that were printed in a very small size in High Tech Tennis, so they are approximate (within about 0.2%).

    Notice that:
    1. The oversize racket produced around 13% more "power" (ball speed) than the midsize racket.
    2. Although gut strings were once claimed to be more "lively" than nylon, the nylon strings produced higher ball velocities at most string tensions on the more "powerful" oversize racket, and tied with gut on the midsize racket.
    3. The results are non-linear. Unlike the first study, lower tension did not always result in higher post-impact ball speed. Apparently the racket used in the first study was not used in the second (although Groppel does not say, and he does not say what models of rackets were used).
    The reason or reasons for the non-linear results are obscure. Groppel says: "It basically seems that each racket has vibration (or harmonic action) that is specific to its design." The non-linear results may be related to a mismatch between the time of ball contact with the time of string deflection and repositioning. If the ball is not in contact with the strings throughout the period of string deflection and repositioning, it cannot obtain all of the available force from the strings.

    High Tech Tennis, among other things, also contains a good explanation of how spin works to curve a ball's path in flight, and a list of tennis facts and commonly held fallacies, part of which is exerpted below.

Chapter 10: Facts and Fallacies of the Game (pages 189-):

    ...I have compiled these facts and fallacies after years of examining tennis performances with high-speed film, which can be stopped frame-by-frame and studied.

Facts

    Fact # 1. The fewer body segments involved in a movement (within reason), the less chance for error. Using enough of your body to generate an optimal amount of force is necessary, but maintaining control over a specific stroke is also important. Players at times exaggerate their movements, using many segmental actions in attempts to develop more force or disguise a shot. Let's examine the ball toss while serving, for example. By using all three joints of the upper limb (shoulder, elbow, and wrist) when tossing, you increase the number of body parts that must be used accurately. Having three joints in action decreases your chances of placing the ball in the exact position to hit the best serve, because a movement error could occur in any of the three joints. Instead you should toss the ball with your upper limb moving only at the shoulder, thereby maintaining fairly rigid elbow and wrist joints and increasing the likelihood of an accurate toss...

    Fact # 2. Force in tennis comes from both linear and angular momentum. Regardless of the type of shot you attempt, your body uses both linear and angular momentum to produce force. When you step into a shot, you create momentum in the linear direction of the step. Angular momentum occurs from the rotation of the hips and trunk to bring the racket head into position...

    Fact # 3. Most of the force in tennis comes from the ground. We know that for every action there is an equal and opposite reaction. The initiation of force to effectively swing a racket must come from the ground reaction force and be transferred through your body's linked system of legs, hips, trunk, and upper limb...

    Fact # 6. The racket face must be near vertical to achive optimum impact. In almost every shot attempted, the racket face, at contact, must be nearly perpendicular (within 5 degrees) to the flight of the ball. The only time the racket can sway farther from the vertical is when you attempt underspin. Then the racket can be more than 5 degrees (but not much more), because the racket head's action is from high to low. The high-to-low swing accomodates a slightly open racket face and usually keeps the ball in the court.


Fallacies

    Fallacy # 1. You should keep your eye on the ball. The fact is that you should track the ball when competing, which is as close as you can come to keeping your eyes on it. It has been scientifically shown that as the ball travels toward the player it becomes a blur 5 to 6 feet away, and vision usually remains focused at that point 5 feet away. The better the player, the easier it is for him or her to utilize a mental motor program (or image) of a stroke, anticipating where the ball will be based on projectile motion. This procedure is more difficult for intermediates, who usually have not yet developed proper motor programs. The risk in telling intermediates to keep their eyes on the ball is that they actually try to do it. Some rotate their heads to maintain eye contact as the ball approaches. If the ball becomes a blur near you, head rotation creates a useless imbalance. Also, if you develop a bad habit of head rotation, you will have a more difficult time constructing a proper motor program for stroke production. It is better to use the cue "concentrate on the ball," which will not prompt you to turn your head as you swing.

    Fallacy # 4. Hit down on your serve. ...computer simulation shows that you must have a reach of about 11 feet with the racket and hit the ball at approximately 120 miles and hour to be able to hit downward 3 degrees from the horizontal. As few tennis players meet these physical requirements, it is important to let gravity work for you. It is much better to hit the ball straight out from the racket or even to hit up when serving. From another perspective, it is much easier to develop your serve by hitting the ball beyond the service line and then working on bringing it back in than it is to hit serves into the net and then work on getting them over.

    Fallacy # 6. Intentionally jumping when hitting ground strokes or serves automatically increases power. ...a great player never intentionally jumps when hitting a ball; rather his or her body is actually pulled off the ground by the leg action and body rotation...

    Fallacy # 11. You should roll the racket head to produce topspin. Although some players seem to actually roll over the ball to hit topspin (like everyone used to think about Bjorn Borg), it has been repeatedly demonstrated that this does not occur. The only way to produce topspin is to initiate the forward swing below the contact point and brush the backside of the ball in an upward manner, completing the stroke with a high follow-through. The more vertical the swing, the more topspin can be applied. A rollover appears to occur because of the internal structure of the shoulder joint. As your arm comes across your trunk in the follow-through your shoulder rotates, giving the appearance of a rollover that is meant to produce spin, but this rollover does not even begin until the ball is about 1 to 2 feet off your racket.

Find more books by Jack Groppel at Amazon.com
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