Researching the Serve

By Greg Ryan

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Ground Reaction Force

As the knees extend they produce what is called a "ground reaction force." The ground reaction force is generated when the player "pushes" down against the ground with his feet through the knee bend. The ground in turn "pushes" back, generating the force that propels the player upward and forward into the air toward the ball.

There are actually 3 components to the ground reaction force: upwards (or vertical), forward (or horizontal), and sideways (or lateral).

The vertical component is the highest, with a force equal to about twice the player's body weight. The horizontal component is also significant, generating a force that is two thirds to three fourths of the body weight, while the lateral component is minimal.

The ground reaction forces starts the transfer of energy upward through what is called the kinetic chain. In this way the force is "summed" through the body segments so that a high racket head velocity can occur at ball contact.

The extension of the knees propels the body toward the ball. This in turn aids in the loading of the shoulder, and also increases range of motion of the shoulder joint.

Since the body is moving upward or vertically off the court, there is an opposite force being produced at the shoulder joint due to the influence of gravity on the weight of the arm and racquet complex. This is what is called an "eccentric force" which increases the external rotation of the shoulder and flexion, or bend at the elbow.

At this point in the serve the shoulder joint reaches maximum external rotation, typically having rotated backwards about 170 degrees. The elbow joint also reaches its maximum flexion at this time, flexed or bent at about 115 degrees.

This dynamic movement, created by the leg drive and by gravity, is what produces the loading of the shoulder joint, and at the same time, increases the length of the racquet drop. At this point, the upper-body is in an ideal position to produce the maximum amount of force and accelerate the racquet.

The Acceleration Phase

The acceleration phase of the serve begins with the racquet moving vertically toward the ball and ends when the racquet strings contact the ball. The goal of this phase is to accelerate the racquet so that maximum racket head velocity occurs at ball impact, and to create the ideal racket head path and orientation for the intended ball location.

In this phase of the serve, the action of the kinetic chain resembles other physical motions such as, throwing, kicking, and striking. Movement scientists have identified a general principle to explain force production in these movements. This is called "proximal to distal sequencing."

Basically it means that for each segmental movement the force generated is passed to next segment which is referred to as a kinetic chain. In this way the forces generated earlier in the motion are eventually transferred into the racquet head and finally to the ball.

Research has demonstrated that this movement is initiated by the heavier, slower central body segments. Then, as energy increases, the motion proceeds outward to the smaller, lighter and faster segments.

This transfer of energy has been identified in the tennis serve by numerous researchers. Their work has determined that the sequence of the maximum velocities of the body parts are achieved in this order: the hip, the shoulder, the elbow, the wrist and finally the racquet tip.

Put another way, the hips accelerate and reach their maximum speed first. The force is transferred to the shoulder, which reaches it's maximum speed next. This is followed by the force transfer to the elbow, the wrist and the racket tip. This sequencing of the kinetic chain begins at the start of the loading phase and ends a split second prior to ball contact.

The Movement of the Arm

Recent studies have also focused on how the movement of the upper extremity, or arm, contributed to racquet head velocity. The movement not only happens in a split second, it is very complex. There are actually 7 distinct components in the arm movement toward the ball, which occur in a specific following sequence. These components have technical names that we will explain below.

Researchers have been able to determine the relative contribution of some of these segments. Studies show that the greatest contributor is the internal rotation of the upper arm. This is the forward rotation of the upper arm in the shoulder joint, as the racket starts upward to the ball. Internal rotation is responsible for over half of total racket head speed at 54%.

The next largest contributor is wrist flexion, or the forward motion of the wrist toward the forearm. This is the motion of the wrist from its laid back position at the start of the motion to the ball until the contact. Wrist flexion was found to contribute 30% of total racket head speed.

The third largest contributor to racquet head speed is horizontal abduction of the shoulder joint at 13%. This is a foreword movement of the upper arm at the beginning of the acceleration phase.

Taking a cue from this research, a few exercise physiologists have used weight training to strengthen the muscles in the shoulder that produce internal rotation. Their studies show that this increases serve velocity. It has also been hypothesized that the leg drive is of great importance in producing this effect, because it increases the stretching of the internal rotator muscles.

More on the Arm

Despite these results, there has also been controversy in the literature in describing the movement patterns of the arm or the upper-extremity in the acceleration phase. The complexities of the stroke and performer variability make it difficult to simplify or to create a generic pattern of segmental movements.

This is in part because the arm action has overlapping movements, and because the total pattern has never been analyzed for a large number of professional players.

This very complex sequence works in concert with the "differential relaxation" of the muscles. Differential relaxation refers to the performer's ability to control the relaxation and contraction of a muscle group during a movement.

During the loading phase the leg drive to the ball works in concert with relaxation of the shoulder joint to increase external rotation, or the rotation of the upper arm backwards in the shoulder socket.

This increase in external rotation places the internal rotators of the shoulders on stretch which increases the force production of that muscle group. To maximize racket head speed each segmental movement should be paired with the relaxation of the next distal muscle group. This maximizes the loading as that muscle group is placed on stretch.

Each of the movements in the sequence prior to ball impact has this same action and reaction combination. This is what maximizes force in the kinetic chain and therefore ball velocity.

The Followthrough Phase

The follow through phase of the serve is from ball contact to the end of the stroke. The primary goals during this phase is to decelerate the racquet slowly so that no injury is done to the musculature or joints, and to prepare for the next stroke.

Research is very limited in this phase of the serve and has mainly been conducted in a qualitative manner. The main finding is that the racquet slowly decelerates moving from left to right across the body and toward the ground.

Most of the players that have been analyzed have landed on their non-dominant foot, but there are a few exceptions. The stroke generally ends with the performer's hips and shoulders parallel to the baseline in preparation for the next shot.

What's Next

The future of quantitative research lies in doing more studies of high level players both non-invasively and in live competition. A larger data base of studies on the serve will lead to an understanding of questions such as what variations in the motion creat different spins and ball locations. The quantitative study of all the strokes, not only the serve, offer an exciting new avenue for developing information that can help players and coaches not only at the elite, but at all levels.