Setting Up the Shoulder Rotations
on a World Class Serve
Chas Stumpfel
In part one of this series, (Click Here) we looked in detail at the critical role of upper arm rotation in high performance serving. We saw how internal shoulder rotation, producing forward racket motion, was the largest contributor to racket head speed as the racket moves to impact with the ball.
We also saw that the effectiveness of this internal rotation depended on the external rotation of the upper arm prior to the upward swing. This backwards rotation stretches the muscles used in the upward swing, increasing their explosiveness and top speed.
This pre-stretching of muscles, followed by their rapid contraction, what is known as a stretch shorten cycle.
The first article was devoted simply to describing these two rotations that combine in a stretch shorten cycle. Now in this second article let’s go deeper. Let’s address the specific motions that set it up. Not what the stretch shorten cycle is, but what makes it happen.
A basic characteristic of a muscle, like a rope, is that it can only pull in one direction. A muscle cannot pre-stretch itself. Therefore, to stretch muscles always requires external forces. Here’s how this stretching occurs for the serve.
Six Motions
This article investigates 6 of the motions that set up the most important stretch shorten cycle for the serve. These are: Motion to Trophy Position, the Leg Thrust, the Cartwheel or shoulder-over-shoulder motion, a Direct Stretch, the Trunk Turn (or Twist), and finally, the Somersault or forward tilting of the body. The motions begin in the sequence listed, but also overlap with subsequent motions starting before previous motions are complete.
Exactly how the top servers emphasize each of these motions varies. Still, what a small number of the top servers are doing constitutes the current state of the art. Let’s analyze the listed elements by looking at Tennisplayer high speed videos of the serve of John Isner.
One important note: this analysis is not intended as "instruction" for learning a high level serve. It is intended to help clarify issues and make serving biomechanics more widely understood through video analysis. I have tried to apply much of what I have learned to my own serve but am not a professional instructor or coach.
Motion to Trophy Position
The first factor that sets up the stretch short cycle is the motion of the forearm toward the Trophy Position. Watch how Isner’s forearm and racket simply rotate up and back from the shoulder.
This is accomplished with the external shoulder rotator muscles. For those interested in the anatomy, these muscles are smaller rotator cuff muscles, the Teres Minor and the Infraspinatus.
This is the motion that begins the stretching of the internal shoulder rotator muscles and therefore initiates the start of the stretch shorten cycle. In anatomical terminology, those muscles are the latissimus dorsi, the pectoralis major, the subscapularis, and the teres major.
This motion to the Trophy Position does two things. First, it increases the speed at which the forearm and racket are moving. Second, it positions the forearm and racket so that leg thrust will be used effectively in producing external shoulder rotation.
Note: some high level servers, especially those with high tosses, do not include this Motion to Trophy Position in their service motions.
Leg Thrust
Now let’s look at the Leg Thrust. As the knees extend, the entire body moves upward and forward. This includes the shoulder. As the shoulder accelerates upward, the inertia of the forearm and racket cause them to rotate backward and down, similar to the way that an accelerating car presses you back into the seat.
The forearm and racket accelerate in sync with the upward acceleration of the shoulder and this can be seen in the video. This backwards rotation stretches the internal shoulder rotator muscles. The resistance of these internal rotators then increases. Like springs, as the muscle lengths approach their maximum limits, the resistance becomes very high.
This in turn slows the speed of the backward external shoulder rotation. During the final stages of stretch there is so little joint movement that it is difficult to determine when the maximum stretch has been achieved.
The position of the forearm and racket is important because the goal is the most effective stretch of the internal rotators throughout the Leg Thrust. The right timing optimizes the role of the legs in setting up the stretch shorten cycle.
Cartwheel
There are two other motions, the Cartwheel and the Trunk Turn, that begin at the same time during the Leg Thrust and also contribute to the external rotation and the stretching of the internal shoulder rotators.
The first motion, the Cartwheel, is the "shoulder-over-shoulder" motion. This is the change in the tilt between the front and rear shoulders.
At the Trophy Position, the tossing shoulder is much higher than the hitting shoulder. But from the Trophy Position onward, this tilt reverses. The tossing shoulder drops and the hitting shoulder rises.
How does this contribute to the stretch shorten cycle and the upper arm rotations in the shoulder joint? Like the Leg Thrust, the Cartwheel increases the upward acceleration of the hitting shoulder.
The Cartwheel in turn also increases the external shoulder rotation (back and down), again because of the inertia of the forearm and racket. The net effect is added external shoulder rotation and increased stretching of the internal rotators.
Direct Stretch
In addition, there is an important separate stretch that does not result from external shoulder rotation as happens with the other motions. This stretch motion is closely related to the Cartwheel and has been noted in kinesiology. The Cartwheel and other body motions increase the distance between the upper arm and the spine. This directly stretches the back, particularly the lat muscle, largest of the internal shoulder rotator muscles. This back stretch can be seen in all serve videos as the hitting arm rises out of the Trophy Position to the impact position.
There is a further factor to understand about this Direct Stretch and what happens at the Trophy Position with the so called ‘hip thrust’. At Trophy Position the hip moves forward toward the court. This reduces the distance from the hitting arm to the spine and, at the same time, stretches the muscles on the tossing side of the body. This magnifies the effect of the Cartwheel. How? Because the hitting arm starts closer to the spine, when the Cartwheel kicks in it will now further increase the arm to spine distance. This will create added stretch on the lat muscle and possibly the shoulder joint motion as well, although better understanding is needed for this motion.
Trunk Turn
The next motion is the Trunk Turn that rotates the upper body. Imagine a line drawn between the two shoulders and visualize how that line would rotate as seen from above.
The Trunk Turn also accelerates the hitting shoulder forward on a circular path so that the inertia of the forearm and racket once again results in their backward and downward rotation. For Isner the Trunk Turn appears to be somewhat more than a quarter turn forward to impact.
Somersault
The last motion, the Somersault, is the forward tilting of the body. It first appears in videos as a bend forward at the waist. How much additional stretch the Somersault might add to the internal shoulder rotation muscles, again from forward shoulder acceleration, is not clear, because the stretch may have already maxed out by the time that Somersault begins.
The Somersault extends beyond impact and includes several events: the possible final stretching of the internal shoulder rotators, internal shoulder rotation, impact, and the follow through. The Somersault ends around the time that the front foot lands.
These, in simplified form, are some of the key biomechanical ingredients of the most important stretch shorten cycle in tennis, the one for internal shoulder rotation of the serve! There are still more factors that contribute to racket head speed on the serve. More on that later!
