|What Happens at Impact and Why It Can Hurt|
Where the shot goes in tennis is largely determined by that very brief instant that the ball is in contact with the strings. Many pros and players get hooked on the game because of the excitement and feel of a well hit ball. Bill Tilden opened his classic book Match Play and the Spin of the Ball by saying, "There is no sensation in the sporting world so thoroughly enjoyable to me as that when I meet a tennis ball just right in the very middle of my racquet and smack it, just right, where my opponent should be but is not." Unfortunately, most people do not appreciate the intensity and severity of these collisions on the body until there is an injury. High-speed imaging and biomechanical instruments have painted a pretty clear picture of what happens at impact in tennis. First, let me illustrate what happens at the moment of impact between a tennis ball and the strings.
A Lot Happens in 4 to 8 Milliseconds
Many pros know that the impact of the ball with the strings is a very short event, usually lasting between 4 and 8 one one-thousandths of a second (0.008 sec.). One to two milliseconds (ms) later the shock wave of the collision has reached the player's hand. This shock wave dramatically increases forces on portions of the players hand. Where the impact occurs on the racquet face, how the racquet was gripped prior to impact, ball speed and the speed of the racquet all affect how the hand is loaded. Figure 1 illustrates the typical pattern of loading at the base of the index finger in a forehand drive. Several studies have shown skilled players increase grip force just prior (about 50 ms) to impact and the peak loading on the hand after impact can vary widely.
Figure 1. Typical pattern of force applied to the base of the index finger
in a tennis forehand using an eastern grip. Peak forces after impact on a
very small area (<0.7 square inches) can vary widely from 1 lb to over 70lbs.
Although much of the energy of the collision is stored and recovered because of the elastic nature of the string bed, there is a considerable shock wave that moves down the racquet and slows the forward motion of the racquet and hand. In some cases (like a reflex volley), the motion of the racquet can be stopped and even reversed. Especially in off-center impacts, the frame is bent by the force of impact. Very high speed imaging (over 1000 images per second) is required to get a glimpse of how tennis racquets are bent during and after impact, as well as the violent shock the arm experiences. The ball leaves the strings before even the stiffest racquets can recoil, so most all the energy in the shock wave stresses the racquet and arm and does not return to the ball. Later in this article, I will discuss the controversial issue of the grip forces and ball rebound speed that is related to this phenomena.
To give you some understanding of this invisible world, lets look at the motion of the wrist joint in typical forehands and backhands. Schematic results of high-speed imaging and direct measurements of wrist joint flexion/extension I have made in the forehand and backhand are illustrated in Figure 2. The rapid reversal of wrist flexion (forehand) and wrist extension (one-handed backhand) created by the impulsive forces impact create large loads on the forearm muscles. This forced lengthening of active muscles (eccentric actions) is known to create large tissue forces and is the primary mechanism of most muscle strain injuries and delayed onset muscle soreness.
After the ball has left the strings the player's arm is beginning to experience not only the forces of the shock wave, but also forces of racquet vibrations. Small vibrations in the force on parts of the hand after impact are related to the racquet vibrations. Forces on the hand as a result of vibration (not illustrated in Figure 1) are much smaller and have less energy than the impulsive forces of impact. Hand forces almost always dampen out the racquet vibrations within the first 1/10th of a second after impact. Research has also shown that string damping devices do not affect loading on the body. String dampers are just effective in stopping the high-frequency and lower energy vibrations of strings that create the "ping" or "pop" sounds of impact.
One of the common results of this repetitive impulsive and vibration loading from tennis impacts is the development of tennis elbow. Next, I will define tennis elbow, review the likely causes of tennis elbow and discuss what can be done to reduce the risk of injury.
What is Tennis Elbow?
Tennis elbow is one of those catch-all terms for elbow pain from repetitive motions that is localized on the inside or outside of the elbow. Pain on the inside of the elbow results from irritation of the common wrist flexor (several muscles) attachment on the medial epicondyle that is usually associated with the vigorous wrist flexion actions in serving or forehand drives. Pain on the outside of the elbow is from irritation of the common wrist extensor attachment (lateral epicondyle) that is usually associated with the one-handed backhand drive. A mild (short term: less than a month) case of tennis elbow may be correctly called "tendonitis", meaning tendon inflammation. A more chronic bout of tennis elbow is usually a serious tissue degeneration problem (tendinosis) of the tendinous attachment of the forearm muscles. Players with chronic tennis elbow should seek medical attention and carefully follow the rest, treatment and rehabilitative prescriptions of their physician.
Biomechanical Causes of Tennis Elbow
Sports medicine professionals have known for years that many kinds of repetitive movements can result in tennis elbow. Equipment companies would have you believe that their latest designs, composite materials, grips or damping devices are the breakthroughs that will minimize your risk of developing tennis elbow. While biomechanical research has used a variety of tools to look at the problem of the potential mechanical cause of tennis elbow, there is less research on the myriad of racquet designs and devices promoted to reduce tennis elbow. Let's take a quick look at the research, remembering that the forces of tennis impacts can be divided into impulsive (shock wave) forces and vibration forces. The vibrations of the racquet frame after impact are of interest here, not the minute vibrations of the strings mentioned earlier.
Figure 3 illustrates the force transmitted to the hand below the index finger (IF) and the little finger (G) for an off-center impact in a forehand drive. The IF force is representative of the pivot point of the racquet in the hand, while the G force is representative of the gripping forces created by the player and racquet motion. This schematic also zooms in on the 2/10ths of a second near impact, while Figure 1 shows the IF force for nearly the full stroke. While the peak forces on the hand after impact are highly variable from stroke to stroke, note that the impulsive forces at impact tend to be larger than the vibration forces superimposed on the general pattern of loading. The small size of the frame vibration forces, their limited energy and their quick damping by hand forces suggests that vibration is not a major cause of tennis elbow. Impulsive loading in tennis is the likely mechanism of tennis elbow. Only these large impulsive forces can create the recoil of the racquet that rapidly stretch the muscles of the forearm. Recall that the forced lengthening of muscles creates large stresses in the tissue that are related to several muscular injuries. These two mechanisms (shock wave and eccentric stretch of the muscles) are the likely source of the overload and overuse of the common muscle attachments seen in tennis elbow.
Figure 3. Typical force at the base of index finger (IF) and on
little finger side (G) of the hand in an off-center impact
of a tennis forehand with an eastern grip. Note that the
forces of frame vibrations are smaller than the pattern of
impulsive loading and are damped out in less than 1/10th of a second.
There is no denying that off-center impacts create large and annoying impulsive and vibration forces on the hand. Unfortunately, a player's perceptions of the severity and annoyance of tennis impacts are not very consistent. For example, a player switching to a demo racquet might think there was a "softer feel" of the second frame, but this effect could be due more to differences in ball speeds at impact, string tension and impact locations on the strings than the racquet's vibration damping properties. A recent study found no differences in player perception of impact vibration between two racquets of vastly different design (traditional vs. vibration-isolation design) when there was no visual or auditory information available. Tennis players and coaches often talk about the importance of a "feel" for impact, but it is not likely that the average recreational player will have the ability to accurately perceive subtle differences in the severity of impact across various combinations of equipment (racquet, stringing, grips, etc.). Most recreational players should base their racquet, stringing and playing technique on recommendations from tennis professionals (who presumably are up on the latest sport science research) more than their subjective feel or the claims of tennis equipment advertisements. Players do need to listen to their bodies, but they should not let these subjective impressions totally guide their equipment selection.
The most important factors, in order of influence, related the size of the impulsive force acting on the hand in tennis, appear to be the impact location on the racquet face, the speed of ball/racquet impact, racquet stiffness and pre-impact grip force. Note that the tennis player has some control over all these factors. The next section will outline how tennis players can minimize the impulsive loads transmitted to the forearm musculature and potentially limit their risk of tennis elbow.
First, remember that pain is a sign that rest is needed for the body to recover. If elbow pain is persistent after playing, be sure to consult a knowledgeable sports medicine physician for treatment. Second, tennis professionals are in an excellent position to help players adjust stroke techniques that may help reduce impulsive loading and the risk of tennis elbow.
The most important factor in minimizing the forces transmitted to the body is to reduce the number of off-center impacts. Players can dramatically reduce force on the body by using a racquet with a larger head or using moderate spin rather than excessive spin on groundstrokes. Using moderate spin will likely result in fewer off-center impacts and a lower ball/racquet impact speed for shots of similar depth.
Other changes a pro can suggest are using a racquet with greater mass and lower stiffness. Another very effective approach is stringing the racquet with a lower tension. This gives players both greater ball speed and lower impulsive loads. The other technique factors that can be employed are to use a two-handed backhand and not to use excessive grip forces. The force of the grip should only be as tight as to control the racquet. Skilled players only increase grip forces to significant levels just before impact (Figure 3:G). Another strategy for prevention and treatment involves strengthening the forearm and arm muscles that ultimately absorb the shock of impact. Finally, if a person develops tennis elbow, there is a variety of compressive braces that appear to be effective in distributing some of the impulsive and vibration forces away from the irritated muscle attachment. This may assist in healing and an earlier return to play.
The impact of the ball on the racquet face occurs over a very short time (4 to 8 ms). The results of these high-energy impacts are shock waves and vibrations transmitted to the player's hand and arm. What is essentially invisible to players and coaches does stress the upper extremity and contribute to many kinds of overuse injuries. The impact of the ball on the racquet face creates large impulsive forces and smaller vibration forces on the hand. Tennis elbow is a degeneration of the common muscle attachment of the forearm muscles, most commonly seen in tennis on the outside of the elbow because of muscular overuse from one-handed backhands. Biomechanical research suggests that the impulsive loading (initial shock) of impact may be the cause of overuse of the muscle-tendon attachment that leads to tennis elbow. Chronic tennis elbow should receive quick medical attention, but there are several equipment and technique changes tennis pros can recommend that can decrease the impulsive loading in tennis. Adopting these changes may reduce the risk of tennis elbow and get players back in the game more quickly.
Adapted from a paper initially appearing in Sept/Oct 2001 issue of TennisPro magazine.