How to Directly Video Internal Shoulder Rotation on the Tennis Serve

The shoulder joint with humerus doing internal shoulder rotation (ISR). I don't see any pronation.
Slow playback. Go full screen. For single frame on Youtube use the period & comma keys.

The humerus is rotated by ISR during the high level serve. ISR is the joint motion that provides the most racket head speed at impact.

At the elbow, there are boney parts of the humerus and also ulna and radius boney parts. Tendons may attach close to the elbow. These bones and tendons rotate with the humerus and are an indication of when ISR starts and how many degrees the humerus rotates from start to impact. If the boney parts at the elbow can be observed in high speed videos, then we have a direct observation of ISR and can analyze how effective the serve is likely to be.

Joint motions are based on the joints and bones, not on the fleshy parts of the arm. The bone motions can be difficult to see because the fleshy parts of the arm lag behind rapid motions and flop around. But near the elbow there are bones that stick up and tendon structures that form hollows. The high and low points produce shadows in direct sunlight. Often these shadows may be seen as the arm rotates from ISR.

To do single frame on Youtube use the period & comma keys. The video pauses for seconds, hold down the period key to get past those pauses and single frame back & forth between 0 and 29 milliseconds. An arrow at the elbow indicates the elbow shadow that indicates ISR.

Many serve videos have been posted but observing ISR is mostly not possible because of the lack of direct sunlight and poor image quality of the elbow area. This thread discusses how to set up the camera and the lighting to often directly see ISR. This only requires one video of a representative serve and effective ISR or not can be seen. Otherwise, many other serve issues get attention, toss, footwork, follow through, etc and the main issue for serve technique, ISR, is missed.

1) Frame rate - 240 fps or more.
2) Shutter Speed - Nearly all cameras with high speed video mode, automatically select shutter speed based on available light level. Video in direct sunlight (forget artificial lighting and cloudy skies). Many smartphone cameras can produce high speed videos with small motion blur in direct sunlight.
3) Pick a time of day and court side plus ad or deuce court so that the shadows of your elbow area show well.
4) A behind camera view, as in the video above, shows the elbow, important angles and the ball bounce. The elbow also is going away from the camera and that helps reduce motion blur.
5) The camera should be close to the server, but not distracting. Get a larger image of the elbow area, for example waist up to above racket.
6) You should have a tripod. A taller tripod is often useful. Adapters for smartphones are about $10.
7) You might develop a few hand signals to video after each serve - Thumbs up for a very good serve. Finger down for 'in the net', finger up for 'beyond the service line', other hand signals, etc.

Please post any high speed video close ups of the elbow area in this thread.

Good stuff, and a nice idea. Having said that, I am in disagreement with the following:

Chas Tennis said:

Otherwise, many other serve issues get attention, toss, footwork, etc and the main issue for serve technique, ISR, is missed.

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The vast majority of posters on this forum looking for serve advice have other technical issues that should be addressed long before focusing on their ability to maximise ISR. It is true that ISR is present in all high level serves, but getting to something resembling a high level serve doesn't initially require extreme ISR; whereas a good toss location, proper tossing arm extension, adequate shoulder tilt, passing through some kind of trophy or power position, avoiding the dreaded waiter's tray and allowing for a proper "set to launch", which in turn should result in correct timing of the leg drive and an adequate racquet drop, are all priorities long before ISR deserves a mention and in fact possibly contribute to it.

Just my opinion and maybe I'm wrong, but put another way, how many elite coaches of the online world - Salzenstein, the Serve Doctor, Macci for instance - mention ISR when working with clients that already possess decent servers? Surely this could be a clue and leads me to believe it is entirely possible ISR happens to be, at least in part, the result of other correctly executed serve mechanics and should not be a primary focus.

I did find this, which appears useful for learning the proper execution:

8) Film yourself NAKED so clothing doesn't obscure Chas' view of your bones.

Digital Atheist said:

Good stuff, and a nice idea. Having said that, I am in disagreement with the following:



The vast majority of posters on this forum looking for serve advice have other technical issues that should be addressed long before focusing on their ability to maximise ISR. It is true that ISR is present in all high level serves, but getting to something resembling a high level serve doesn't initially require extreme ISR; whereas a good toss location, proper tossing arm extension, adequate shoulder tilt, passing through some kind of trophy or power position, avoiding the dreaded waiter's tray and allowing for a proper "set to launch", which in turn should result in correct timing of the leg drive and an adequate racquet drop, are all priorities long before ISR deserves a mention and in fact possibly contribute to it.

Just my opinion and maybe I'm wrong, but put another way, how many elite coaches of the online world - Salzenstein, the Serve Doctor, Macci for instance - mention ISR when working with clients that already possess decent servers? Surely this could be a clue and leads me to believe it is entirely possible ISR happens to be, at least in part, the result of other correctly executed serve mechanics and should not be a primary focus.

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There is a lot to understand on the high level serve and observing ISR is pivotal for understanding the high level serve. There are many instructional videos and other materials for learning serves but the biomechanics is often not accurately described. ? In addition, even the part played by ISR was not confirmed by tennis researchers until 1995. Their discovery started with the bare bones ISR joint motion shown in the OP video.

The tennis serve lasts one second and all its sub-motions are coordinated and sequenced and show certain body positions in high speed videos. High speed video cameras at modest cost are available to everyone. There is free expert advice on videoing ISR in the OP.

Why would anyone want to hide the truth at the center of the high level serving technique? ............

You are mistaken about the vast majority of the posters on the forum. Some do ISR because they read about it, but do most ISR after impact. Some use pronation but not much ISR. Some have little forearm-to-racket shaft angle when ISR is occurring. In the last year or two, I think that there have been fewer serve posts with the Waiter's Tray technique. Maybe that's progress! ? I've been posting analyses of serve videos for years now and believe that misunderstanding the serve is a very important thing. Many readers are new and some are serious about their serves but need to observe the 25 milliseconds leading to impact.

I missed the truth about the serve and ISR for 35 years and wish someone had told me about it in 1975. This thread and a high speed camera would have done the trick.

Players in the ATP and especially in the WTA should do what is described in the OP.

[Note- 3D 'Motion Capture' systems are the gold standard for studies of human motion. Motion Capture attaches small retro-reflective balls to the body, mostly it works well and has been used for years for tennis strokes. But for ISR with its axial rotation of the humerus, there is some lag between the bones and the skin motion and the reflective balls have some errors. This was discussed in detail in a thread. The bone shadows technique has some issues as well that affect accuracy but it can adequately show ISR and its timing often if set up correctly.]

RogueFLIP said:

8) Film yourself NAKED so clothing doesn't obscure Chas' view of your bones.

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put a circular sticky dot over each palpable bony prominence (coccyx included) so Chas can see every joint angle

For reference here is the defined joint motion pronation and supination.

You can see that that the prominent boney high points at the elbow don't move with pronation.

(The tennis usage of 'pronation' for the serve is not correct.)

Chas Tennis said:

For reference here is the defined joint motion pronation and supination.

You can see that that the prominent boney high points at the elbow don't move with pronation.

(The tennis usage of 'pronation' for the serve is not correct.)

Click to expand...

Which is why you use landmarks at the distal forearm to measure supination and pronation

do you also realize that pronation and internal rotation of the shoulder can occur at the same time?

yossarian said:

put a circular sticky dot over each palpable bony prominence (coccyx included) so Chas can see every joint angle

Click to expand...

I don't know if I can trust any skin points around the elbow. Arm straight, put your finger on the coccyx and other landmarks and see how far you can move the skin (with sticky dot). I get maybe a minimum of 1/2 " skin movement over high boney points.

But a boney high point on the humerus - by definition - will not lag. Interpreting the elbow shadows is a bit of an art for now and the accuracy has to be estimated. I believe that some tendons on some servers may move with the humerus if the attachment is close to the elbow joint (proper name?)

The elbow is a hinge joint that bends in only one direction. Sometimes that fact helps in interpreting the upper arm orientation.

The shadow itself is tricky since shadows depend on both the direction of the light on the rotating high point or low point and the bone's rotation. Low body fat would work better.

If you have control of the camera & server, an angular scale could be videoed before or after the serve. A big advantage of the elbow shadows is that they work on videos where you have no control of the videoing and serving, so that you can't calibrate the humerus angle for ISR.

There is a technique used for a baseball pitcher where styrofoam cylinders, looking like short pencils, were applied near the elbow. But that would have to be checked out for the same moving skin issue.

Chas Tennis said:

You are mistaken about the vast majority of the posters on the forum.

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OK. You didn't mention it, so where specifically was my mistake? I am interested in any correction with supporting evidence, since I'm always up for removing any false beliefs I may hold.

Chas Tennis said:

Some do ISR because they read about it, but do most ISR after impact. Some use pronation but not much ISR. Some have little forearm-to-racket shaft angle when ISR is occurring.

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This may well all be true (I don't know the actual details on "some" versus "most") but none if it affects what I said about addressing other more obvious and glaring flaws


Maybe that video isn't overly representative of servers on this forum, but I am curious as to how many of those servers you would recommend focusing on ISR as an immediate area of improvement?

The OP is about how to video ISR on the serve. I have some information on that for interested readers.

Chas Tennis said:

A considerable percentage of posters are doing ISR, but doing it incorrectly as I listed and you quoted.

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I'm not disputing the above, but rather that there are often other things more important that should be addressed first. You may disagree but that alone doesn't make me wrong or mistaken, and it also doesn't make me right. I linked a video of amateur servers to try and make the point - almost none of them need to worry about ISR until they fix the fundamentally and horribly broken mechanics, starting from the very first moment they begin their motions (there are a couple of exceptions in that video, obviously).

Edit: I assume the OP you refer to is from another thread, and not you!

Chas Tennis said:

The OP focuses on ISR cookbook style.

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Here is a video that shows a shadow in the forearm that ends near the elbow. The shadow probably involves muscles and tendons in the forearm. Shadows at the elbow itself, can show ISR, but the shadows in the forearm are questionable because the muscles and tendons away from attachments may not move with the bones (lag or flop). The wrist orientation is shown by the wrist shape determined by wrist bones. I believe that wristbands with logos usually move with the wrist during ISR. There might be a small black logo on this wrist band. ? I believe that these elbow shadows show that ISR stops but that pronation may continue in the follow through. This video is low resolution, 488 x 336. A higher resolution smartphone camera should do much better if the shutter speed is fast.

To single frame on Vimeo, go full screen and hold down the SHIFT KEY and use the ARROW KEYS. Wait a few seconds on a frame and the time scale will disappear so that you can see the elbow.

The shadows of the net ribbon and opponent's foot and body indicate the position of the sun.

You can place a marker from blue painter's tape very near your elbow. But be aware that the shadows from bones can be the most accurate indication of ISR provided that the shadows can be interpreted.

There is an additional issue, Jello Effect distortion, an artifact of CMOS cameras. I can explain later when there are some posts of clear arm videos.

Digital Atheist said:

I did find this, which appears useful for learning the proper execution:

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It appears that focusing on a high elbow finish with the strings facing the outside of the net can accomplish the proper pronation. The shoulder should almost feel kind of like a shrug against you on the finish as well.

Power Player said:

It appears that focusing on a high elbow finish with the strings facing the outside of the net can accomplish the proper pronation. The shoulder should almost feel kind of like a shrug against you on the finish as well.

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I think that the racket head to the side is a good sign and many ATP serves display that, but not all.

There are forum posters that do pronation or ISR late or after impact and get to the 'fully pronated" position. Fully
pronated position alone does not prove the serve used ISR effectively.

You need to video ISR as described in the OP and see the quick movement of shadows at the elbow. Compare.

Chas Tennis said:

I think that the racket head to the side is a good sign and many ATP serves display that, but not all.

There are forum posters that do pronation or ISR late or after impact and get to the 'fully pronated" position. Fully
pronated position alone does not prove the serve used ISR effectively.

You need to video ISR as described in the OP and see the quick movement of shadows at the elbow. Compare.

Click to expand...

I can tell when it's done right because the pop off the racquet is substantial and the ball goes a lot faster with less effort.

Its absolutely a huge part of having a powerful serve, but Its something I practice with and engrain, but don't obsess over.

Not sure how you think its been missed....Do you mean that you missed it before or that Cross missed it in his work you often site. I've often and for many yrs on TT,,, discussed the thing most players call pronation and how it can be seen as ISR or long axis rotation. What would be interesting is if you could find who initially called it pronation if that is your issue with the term. I'm pretty sure pronation was on tennisone.com and then often used on tennisplayer.net for years. Did they originate that term or phrasing?

I think it was about 1988 or so that I must have read about 'pronation' on the serve. I have all my old books............ I can remember going up to the ball with the racket and rotating the face of the racket close to impact. In 1988 they were still using high speed film I believe and that is too expensive for the average person. I borrowed a 16 mm camera and filmed my Waiter's Tray in 1980.

In 1985 the badminton researchers had been identifying ISR for both the tennis serve and badminton smash. But I think that it did not take hold in the tennis community. I'd like to know more of that history. I think that probably the baseball researchers had more funded research and understood ISR for baseball pitching in the 1980s.

Thread on the Badminton Tennis Connection

Read the first publication in 1995 and the 2000 publication below by Marshall and Elliott and another author in the 1995 publication. Marshall and Elliott make a point of the missing link in the kinetic chain. In 1995 I had been a big tennis fan since the mid 1970s. I was 35 years late in learning what the missing link was.

Ask yourself what the 'missing link' is?


In the mean time I believe that most joint motions had been named long before. I'll see of I can find internal shoulder rotation and when the term was used.

This video shows the elbow shadow motion - a clear indicator of internal shoulder rotation (ISR). In direct sunlight, many smartphones with high speed video would show this at about 240 fps.

Serve impact is around 2:15. Video starts just before 2:15.
To single frame on Youtube, stop video and use the period & comma keys.

ISR is the most important joint motion at the time of impact - it alone accounts for roughly 50% of racket head speed.

Before videoing, you can simply check out the lighting conditions & elbow shadows by looking at the shadows by eye.

For many years, posters have asked about how the serve is done, and many serve videos have been posted. But serve posters somehow always never seem to post videos with the elbow shadows visible. It's simply a smartphone at 240 fps with the right lighting - just point the camera!

If you don't see those elbow shadows in the Hurkacz video, you are serving with a different technique than the ATP.

Please post some videos showing elbow shadows. Then the forum could finally move forward to more interesting topics.

For those that are more serious, there are ways to calibrate the ISR angle from ISR start to impact. These methods are not difficult, they involve carefully videoing an angle scale with similar geometry to the serve video in the same lighting. The angle calibration process could be done just before or after the video recording. The angle scale could be placed on a piece of cardboard with a hole in the middle for the arm.

The capability of measuring the ISR angle start to impact of the serve should be of interest to anyone interested in optimizing the serve, such as college coaches.

[3D motion capture systems have a problem measuring ISR angle because they use reflective balls strapped to skin. The skin flops around (for long axis rotations) during the tennis serve and that causes inaccuracies in measuring ISR angle. See skin flopping in Hurkacz serve. Joint angles are determined by bones, not skin location. The bone shadows, with care, might be a more accurate indicator of ISR angle that 3D motion capture systems. ? ]

I posted here on a crude Sharapova ISR angle measurement several years ago. (Toly was in that thread.) But it was made from an internet video and did not have a cooperating server, so it was more difficult and less accurate because an angle calibration procedure could not be applied.

Can you see enough shadows here?

onehandbh said:

Can you see enough shadows here?

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Thanks for posting that video. You got it!

Unusual lighting that I've never seen, low light level and indirect sunlight from a high window. But despite the low light level, your camera's motion blur is small and you can see details like the shadows.

That shows some shadows that indicate ISR. 40 degees? seen? Then there is not enough window light to display the full range of the ISR shadows. The lighting on the far side of your arm is low.

If you try that in bright sunlight you may see more degrees of ISR rotation.

What model smartphone are you using? What frame rate?

These are slice serves based on how the racket head moves at and after impact.

Instructions for comparing videos.

ISR means nothing without an angle or angles to create a distance from the arm's ISR rotation axis to the racket head = racket head speed. The elbow is near straight but it is often still straightening more and that is a factor. There is also a forearm-to-racket shaft angle and that varies. Angles are changing in time.

The camera angle that you have shows ISR and all the upper body and racket angles but some angles are hard to estimate. A second camera viewing angle is needed for some other angles. To get around all the angles, what I suggest is to compare to a pro video with very similar camera angles. Look for differences. That is especially true once you have some ISR and it is contributing some to your racket head speed at impact, as yours is.

Chas Tennis said:

For reference here is the defined joint motion pronation and supination.

You can see that that the prominent boney high points at the elbow don't move with pronation.

(The tennis usage of 'pronation' for the serve is not correct.)

Click to expand...

You are right. The term pronation led me to drive this racquet motion with forearm muscles and tennis elbow. Your constant posting about isr, finally got to me 3 months ago: the shoulder muscles should be driving this racquet motion. I really really appreciate that. But the way you insist on videos and jargons can turn off a lot of people.