|Racket Technology & Tennis Strokes|
When you watch tennis being played today, you immediately notice that the tennis rackets are quite different from the wooden rackets that were in use 30 or more years ago. The modern racket is larger in both head length and head width, considerably lighter, less flexible, made of a reinforced plastic material, and it is possibly an inch or so longer. When you watch the more proficient young players hitting, you also see that the style of tennis strokes has also changed over that period of time. The classic, smooth, flowing ground stokes that were displayed by the tennis champions of the first 2/3 of the 20th century have been replaced by a game that emphasizes power from the baseline and the ability to end the point with a single swing anytime the opponent is slightly out of position or hits a shot that bounces short. Is it possible that the changes in stroke mechanics are a direct result of the changes in racket technology, or did the strokes evolve independently of the racket changes? What this article will explore, is how the new technology in tennis rackets has allowed players to modify the way they hit the ball, and get away with it.
The old standard wood rackets that were used by most players up until the 1970 era, weighed at least 14 to 15 ounces, had a neutral balance, and had a small head in both length and width. Because of the structural limitations of wood, in order to fabricate a racket light enough for a player to comfortably swing, the frame could not be made very thick, which resulted in some degree of flexibility, particularly near the tip. In addition, the strength to weight ratio of wood dictated that the head could not be too large if you wanted to string it at a reasonable tension. The modern rackets are molded from a graphite-reinforced material (plus, on occasion, other exotic reinforcing materials), weigh 9 to 11 ounces, and are often handle light. It is even possible to construct rackets with a weight as low as 7 ounces, yet have a head that is quite large and produce a frame that is exceedingly stiff and durable.
The classic groundstroke used by most players up until the middle of this century was a long flowing swing with good, early preparation and a long follow through. The players stood sideways and smoothly accelerated the racket through to the hitting region as the body weight was transferred forward (you stepped into the ball). The shot was often hit flat or with some topspin, but occasionally players preferred to use a small amount of slice (underspin) on the backhand. The modern forehand groundstroke that many players have adopted has an open stance with a semi-western or western grip. The players coil the body, and then uncoil it as the shot is hit, bringing the big trunk muscles into action. This uncoiling, rotational motion often causes the player to leave the ground during or at the end of the shot. The result is that the ball is hit with excessive topspin and usually very hard.
In today's tennis, even the youngest juniors seem to be whipping their rackets around with abandon. Years ago, probably only the exceptional player would have had the physical ability to have done that with the old, heavy wood frames. The average player and the recreational player needed the long, flowing swing to accelerate the old, heavy racket up to hitting speed. In addition, this gradual acceleration gave the player much more control of the racket head and allowed the player to hit the ball at approximately the same location on the racket face each time it was swung. This was essential, since various impact locations on the old rackets often responded quite differently (the "Sweet Spot" was small). A ball hitting an inch or so away from the desired location on the head might easily end up in the net or go long. With these old rackets, to get the ball to land consistently in the court required a consistent impact location on the strings, which could only be accomplished with a controlled swing.
With the quick, whipping swing that many players use today, it is more difficult to hit the ball at exactly the same location on the head each time the racket is swung. However, due to the characteristics of the modern racket and the heavy topspin strokes used, the resulting ball trajectory is much less sensitive to the exact location of the ball impact on the strings. (The racket is more "forgiving" or it has a large "Sweet Spot".) If there is a preferred location to hit the ball (such as the center of the strung area), the new rackets give you more latitude for error of impact location both in a direction across the racket face, and along the main axis of the frame.
Because the new frames are much wider than the old wood rackets, they are much more stable against twisting when the ball impact point is not along the principal axis. The physical property of the racket that produces this stability is called the polar or roll moment of inertia. The larger this moment, the less the racket will twist on off-center hits and the less the power degrades as the ball impact point moves off of the axis. This moment of inertia is proportional the weight of the racket and to the square of the width of the racket head. A 10-inch wide head (the size of a typical oversize racket) is 25% wider than the old 8-inch wide wooden frame, so it has a moment of inertia that is over 50% greater. This more than makes up for the 25% reduction in weight that comes with the newer rackets. This increase in polar moment reduces the racket twist on off-axis impact (hence reduces the ball's errant angle due to the twist), and keeps the rebound ball speed from changing too much on such off-center hits. Both of these effects give the player a larger margin for impact location error in the striking of the ball. In addition, the use of topspin gives the player a much larger "window" of acceptable angles to hit into, if the shot is to land in the court.
When the power of a racket is measured in the laboratory, balls are fired at the frame and the ball rebound speed is measured for various impact locations. For a typical racket, the ball rebound speed is a maximum for impacts near the throat and the ball rebound speed falls off as the impact location moves toward the tip. As a general rule, the further the impact location is from the balance point, the lower is the ball rebound speed in the lab. In addition, stiffer rackets tend to have more power than flexible rackets, particularly closer to the tip. Many of the new rackets are head heavy, which means their balance point is further up in the head. Because of this, the maximum power point moves up, away from the throat. As the racket head is made stiffer, the power degradation near the tip is reduced. These results are for a situation where the racket is not being swung, but is at rest in the laboratory.
When the racket is swung, the tip is moving somewhat faster than the throat and the ratio of these two speeds depends upon the exact nature of the swing. A very wristy, whipping motion will have a greater tip to throat speed ratio than the old classic swing. Because the tip is moving faster than the throat, the maximum power point moves up higher in the head. When the physics of all of these factors (the racket response and player swing) is combined to predict the actual playing characteristics of the racket, the new frames with the modern style of swing show a uniformity of power response over a large area of the head. This is contrasted by the old frames, coupled with the classic swing, where the ball had to be hit at exactly the same place every time to get a uniformity of response.
For the classic racket and classic swing, if the ball impacted beyond the center of the head, there was a loss of power and the ball might not clear the net. For those old wooden rackets, if the ball impacted closer to the throat than the center of the head, the result was more power, and it was likely that the shot would go over the baseline. With the modern racket and modern swing, these sorts of variations in ball impact location will result in very little variation in resultant ball speed, compared to the ball speed when the impact is at the center of the head. This means that if the ball is struck an inch or two from the center of the head, the result will still be a good shot.
If a player had the physical prowess to swing a heavy, classic racket in the modern, wristy manner, any small resulting miss-hits might end up spraying the ball all over the court. The modern rackets, due to their forgiving nature and their lighter weight, have allowed players to adopt a new style of swing and still get the ball to land in the court where they want it.
There is a second reason why strokes using the western and semi-western grips are seen more frequently these days than 30 years ago. From the beginning of tennis, the game was played on grass, which is a low bouncing, fast surface. Three of the four "Grand Slam" tournaments were on grass, as were many of the other tournaments. (For example, the USTA was known as the USLTA, where the L stood for lawn.) The western and semi-western grips are not optimized for fast, low bounces but are ideal for medium or slow paced, high bouncing surfaces. Today the professional grass court circuit lasts at most a month, so most players learn, practice, and play on either hard courts or clay courts. Consequently the new grips and the new types of strokes have evolved along with the new racket technology to match the predominant court surfaces.