What is ISR?
Part 1: The Serve

John Yandell

First let me say I dislike technical terminology and especially acronyms. I think tennis can be and should be explained in simple language any coach or player can understand.

But this term ISR has now worked its way into the discussion of tennis technique much the way the term pronation did over a period of years. So what is ISR? How does it work? And what value does understanding it have for players and coaches?

The most common use of the term is in understanding the serve motion. But in reality ISR is present to a greater or lesser extent in all the strokes: forehands, both types of backhands, and even volleys. So let's explore all that in this new series, starting with the serve.

Definition

ISR stands for internal shoulder rotation. But in tennis the term shoulder rotation can be very confusing because we think of shoulder rotation as the turning of the upper body backward in the preparation and forward in the swing phase of the strokes.

That's not what the term ISR means. Technically it means the rotation of the upper arm in the shoulder joint. This rotation is from right to left or counterclockwise.

The term comes from academic biomechanics not tennis coaching. It's also related to so-called external shoulder rotation, or ESR, another bio mechanical term, which is the rotation of the upper arm from right to left or clockwise.

In tennis strokes these two shoulder rotations are paired together. Let's see how this happens on the serve.

Clearer

From a layman's or tennis coaches' point of view, a clearer description would be internal upper arm rotation and external upper arm rotation. But that's not the way the terms have passed into teaching discussions, so let's just accept that and proceed to some explanation.

There is a lot of disagreement about serve technique, starting with the stances. Pinpoint or platform? How wide is the stance and where does the weight start? Then there's the windup. Should the arms go down together and up together? Should the tossing arm reach extension first with the racket behind or delayed?

Should the palm be face down or an edge in the wind up? Does the racket delay or hesitate when it does reach the top of the wind up?

Then there is the toss. How high? And where is it front to back and side to side? And which of all these elements should be paired together? (For a brilliant explanation of how to pair them, Click Here for Doug Eng's 2 articles, Part 1 and Part 2.)

What's Critical

But whatever the combination of elements, ISR and ESR are critical in all.

Let's start with ESR. As the arm and racket move through the wind up to the drop, the upper arm externally rotates backwards in the shoulder joint. For most players the result is that at the bottom of the drop the racket shaft is aligned with the edge of the torso, and a line drawn across the face of the strings is roughly perpendicular to a line drawn across the torso. Federer for example.

But some players take this rotation a little further, pairing the upper arm rotation with a small amount backward rotation of the arm from the elbow joint, so called supination, another biomechanical term.

Now the racket tip ends up pointing not straight down but at a slight angle to the player's right outside the plane of the torso. Isner and Roddick for example.

But whatever the degree of backward rotation, the ESR rotation is critical because it sets up the other rotation, ISR, in the upward swing to the ball. Now the arm and racket rotate in the opposite direction, counterclockwise to the contact point and then continuing that rotation out into the followthrough.

The arm and racket first rotate about 90 degrees to the ball, then about another 90 degrees to the extension of the motion in the followthrough so that the face of the racket is on edge to the court. Again some players like Isner take this rotation somewhat further.

The Role?

There is a lot of discussion in tennis about what the key elements are in creating power on the serve. Is it the extension of the elbow on the way up to the ball? Is it the leg drive? Is it the movement of the wrist from laid back at the drop to neutral at the contact? Is it the amount of body rotation?

You can find different charts that assign different percentages of contribution to racket head speed to all these factors—and they are all worth examining as a player or coach. But no doubt the combination of ESR and ISR is the most important key.

Notice I didn't mention so called "wrist snap" or that other term "pronation." I've written many times about the myth of the wrist.

Yet "wrist snap" is still commonly believed to be a key or even the major key to the serve. As I said the wrist moves from laid back to neutral in the upward swing. If you want to call that snap, fine, though to me it's misleading.

In any case, the "snap" or flex forward after contact doesn't happen in good serves. If it happens it actually cuts off the critical IRS rotation.

You can still see this forward so called snap even in tour players like Kei Nishikori (Click Here), Andy Murray (Click Here), and as we have documented recently, Aryna Sabalenko (Click Here).

In my view this movement not only reduces racket speed, it's a possible source of injury, with Kei being a possible good example.

And what about that other term "pronation?" Let's look at that in conjunction with the other rotation discussed above, "supination."

Supination and Pronation

Supination and pronation are rotations that occur in the forearm coming from the elbow joint. You can see some supination in the racket drop of a player like Isner. And then pronation when the motion reverses to move up to the ball.

But the common use of the term "pronation" in coaching refers to the turning over of the racket after contact—the position in which it is on edge and about 90 degrees to the court.

That is not technically pronation though. It is the rotation of the upper arm in the shoulder joint or ISR. It's what I prefer to call in my own teaching unitary hand, arm and racket rotation.

Since Isner has a little supination, at the start of the upward swing there is also some slight pronation. But as the racket moves further up to the ball and out to extension there isn't significant and probably not any pronation from the elbow in good serves as Brian Gordon's quantitative research has demonstrated. So again if you want to call it pronation as a teaching description go for it, but just don't try to make it happen from the elbow on top of ISR.

As with the all the strokes, verbal terminology while nice to understand if you are so inclined, is not the key to developing technique. The real key is to physically and mentally master the key positions, best done with the use of on court video. I am a broken record on that point obviously.

So that's it for the serve. For another complimentary perspective on ISR checkout Chas Stumpfel's previous Tennisplayer article. (Click Here.)

Next question: how do ESR and ISR work on the forehand?