Serve mechanics - 90 degree elbow position

[a12345]

Huh? Shoulder extension at contact? This is a puzzling statement. Shoulder extension has the arm down and behind the body (as shown by your posted image). How does this relate to the shoulder at contact?

Not sure what you mean by having the arm in line with your shoulders. Typically, when we say the arm is (more or less) in line with the shoulders, we are talking about the relationship shown in my image above. That is, the arm is not at an extreme angle (like 60° or more), that would result in an impingement of the shoulder. But I suspect that you might be referring to something a bit different.

Anyway, both Kyrgios & Roddick fix their gaze up, with head up, on the ball and contact point for half of their upward swing or more. They are pulling their head down sometime between the inverted L (the Big L) and their contact.

There are some other top servers that do this as well. However there are plenty of images where Sampras, Federer, Murray and other big servers will usually have their head & eyes still up at impact. Their head has moved over somewhat to their left to allow their right shoulder to come up over the top. But they are still looking upward at impact.

Huh? Shoulder extension at contact? This is a puzzling statement. Shoulder extension has the arm down and behind the body (as shown by your posted image). How does this relate to the shoulder at contact?

Not sure what you mean by having the arm in line with your shoulders. Typically, when we say the arm is (more or less) in line with the shoulders, we are talking about the relationship shown in my image above. That is, the arm is not at an extreme angle (like 60° or more), that would result in an impingement of the shoulder. But I suspect that you might be referring to something a bit different.

Anyway, both Kyrgios & Roddick fix their gaze up, with head up, on the ball and contact point for half of their upward swing or more. They are pulling their head down sometime between the inverted L (the Big L) and their contact.

There are some other top servers that do this as well. However there are plenty of images where Sampras, Federer, Murray and other big servers will usually have their head & eyes still up at impact. Their head has moved over somewhat to their left to allow their right shoulder to come up over the top. But they are still looking upward at impact.

What I mean is the arm is at the green circle and not the red one, even if you toss deep into the court.



So Roddicks arm is completely in line with lets call it his back, and in fact at contact was probably slightly behind.



This is for flat fast first serves.

For second serves/spin serves to me it looks like the arm actually does come in front of the line of the body and you can probably watch the ball all the way through to contact.

 

[Raul_SJ]

It looks like he takes his racket back and his arm gets to that 30 degree angle prior to his trophy. However, as he moves thru the trophy phase the elbow goes from 30 to (nearly) 90 degree. But, as he goes thru the flexion action, his elbow drop drastically -- below his shoulder line.

IMO, it is better for rec players with WaitersTray to get to that 90 degrees elbow flexion quickly and maintain it to trophy and drop. Avoid the 30 degree elbow flexion as in 2010 motion. Looks like the 2018 motion has elbow flexion at 90 degrees practically from the very start of the motion.

 

[SystemicAnomaly]

What I mean is the arm is at the green circle and not the red one, even if you toss deep into the court.



So Roddicks arm is completely in line with lets call it his back, and in fact at contact was probably slightly behind. But even that does not quite make sense since the red circle represents nearly

Ok, but this still does not explain why you brought up shoulder extension. Extension is not relevant. Did you mean shoulder flexion? But this would still not explain your statement that there should be little or no shoulder "extension". The Red oval represents nearly 150 degrees of flexion whereas the Green is at 180 degrees flexion.

I see what you are saying about the arm in line with the his back (rather than his shoulder). I've never considered that as any checkpoint. Will have to look at it further. In the meantime, Andy Murray at contact:

http://ep.imgci.com/PICTURES/CMS/28500/28570.2.jpg
.​

 

[a12345]

Ok, but this still does not explain why you brought up shoulder extension. Extension is not relevant. Did you mean shoulder flexion? But this would still not explain your statement that there should be little or no shoulder "extension". The Red oval represents nearly 150 degrees of flexion whereas the Green is at 180 degrees flexion.

I see what you are saying about the arm in line with the his back (rather than his shoulder). I've never considered that as any checkpoint. Will have to look at it further. In the meantime, Andy Murray at contact:

http://ep.imgci.com/PICTURES/CMS/28500/28570.2.jpg
.​

I was viewing it from 180 degrees being the starting point and moving to 150 degrees as the extension.

 

[Chas Tennis]

If terms that have been already defined for joint motions are used for other personal meanings, it screws up communication.

There is a very strong tendency to use undefined conversational terms to call things by the most obvious body part seen moving: 'shoulder rotation', 'hip rotation', 'pronation' for the serve. These undefined tennis terms very often conflict with the defined joint terms.

To make matters a little more confusing, the same joint terms are used for both motion and position.
A joint may be doing the motion of flexion but be in a position of extension.

Anatomical terms of motion - Wikipedia

en.wikipedia.org

For terms of joint position view Youtube videos on how to measure each joint angle. Shoulder Extension, Arm Pronation, Hip Rotation

We are in the dark, dark ages of the wonderful new information age - Google it.

 

[SystemicAnomaly]

@Chas Tennis

I was viewing it from 180 degrees being the starting point and moving to 150 degrees as the extension.

Ok, well that's an oddball reference that will only lead to confusion -- unless clearly & explicitly stated. Sure, moving from 180 to 150 degrees can conceivably regarded as a shoulder extension movement. But, positionally, no one would refer to 150 degrees flexion as a position of extension.

 

[a12345]

Yeh basically im referring to movement.

I put the picture up to show the direction of movement between extension and flexion.

 

[Chas Tennis]

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So Roddicks arm is completely in line with lets call it his back, and in fact at contact was probably slightly behind.



This is for flat fast first serves.

For second serves/spin serves to me it looks like the arm actually does come in front of the line of the body and you can probably watch the ball all the way through to contact.

I find myself trying to figure out what you mean.

Search: Roddick serve impact pictures
At impact for the slice and flat serves, the racket shaft appears vertical from the side camera view. But it only appears vertical from the side camera view, it tilts to the player's left so it is not vertical. But for the kick serve from the behind camera view the racket appears about vertical and from the side camera view the racket shaft and head will appear to tilt closed by about 13-15 degrees. See clear high speed videos.

Take in information by looking at high speed videos. Then select pictures that illustrate your points.

We could all benefit from seeing the 9-16 frame array displays made from high speed videos. This array can communicate a lot of information on the serve. I especially like how it shows when Thoracic Extension (11-12) & Thoracic Flexion (13) occur. What is Sampras's elbow angle in frame #12? (It probably needs a better camera angle.)

If you present this kind of information you will find that your word description will often change as you write the post. You will tend to check some of your word descriptions that don't quite match what is in the sequential frames.

Anyone know how to create these arrays from a video?

 

[a12345]

It's more a case of what you shouldn't do rather than what you should.

It's like the difference between a cartwheel hitting motion and a smash.

 

[TennisDawg]

I find myself trying to figure out what you mean.

Search: Roddick serve impact pictures
At impact for the slice and flat serves, the racket shaft appears vertical from the side camera view. But it only appears vertical from the side camera view, it tilts to the player's left so it is not vertical. But for the kick serve from the behind camera view the racket appears about vertical and from the side camera view the racket shaft and head will appear to tilt closed by about 13-15 degrees. See clear high speed videos.

Take in information by looking at high speed videos. Then select pictures that illustrate your points.

We could all benefit from seeing the 9-16 frame array displays made from high speed videos. This array can communicate a lot of information on the serve. I especially like how it shows when Thoracic Extension (11-12) & Thoracic Flexion (13) occur. What is Sampras's elbow angle in frame #12? (It probably needs a better camera angle.)

If you present this kind of information you will find that your word description will often change as you write the post. You will tend to check some of your word descriptions that don't quite match what is in the sequential frames.

Anyone know how to create these arrays from a video?

What I notice is the way both shoulders are in line in frame 12 and how he first drops his racquet before allowing his elbow to lead.

 

[a12345]

Perhaps this is an example. When fed is warming up his arm for the serve there's no cartwheel involved.

The upper body is upright. He's kind of smashing the ball.

You can hit a serve like this and it will go in just fine. But you're not going to get that max power the cartwheel gives you where you're using the body to drag the arm into the ball.

 

[BlueB]

In order to get maximum racket head speed, as the racket drops, the elbow should remain at close to 90 degrees with the upper arm.

You must not bend the elbow in a scratch the back motion. The racket must not drop as a result of the elbow bending.

Instead you'll see that by keeping the elbow at 90 degrees, the racket goes around the shoulder as you turn the body.

Keeping the elbow at 90 degrees allows you to throw the racket with more force.

Fed 90 degrees



Roddick 90 degrees



Sampras 90 degrees



Warwrinka Bent elbow = loss of power

The Sampras vs Wawa pics are misleading. They are taken from different angles and Stan is further down in the racquet drop phase, thus appears as more accurate than 90°, while probably is not.

 

[TennisDawg]

Stan in slow motion. Looks like around 90 degrees.

 

[Chas Tennis]

Perhaps this is an example. When fed is warming up his arm for the serve there's no cartwheel involved.

The upper body is upright. He's kind of smashing the ball.

You can hit a serve like this and it will go in just fine. But you're not going to get that max power the cartwheel gives you where you're using the body to drag the arm into the ball.

I am overwhelmed already with observing the stroke itself and probably won't ever get to warm up serves or serve variations in the follow through. If some says anything about those parts of the serve I look for their evidence. Always ignore displayed confidence as evidence.

 

[Raul_SJ]

The 90 degree elbow has variations

This coaching video says that the path of the elbow flexion move should not be perpendicular to the net. It should be roughly towards the left net post or roughly parallel to baseline. Somewhere between there.

Otherwise, you will be changing directions and it will compromise the drop. And you will end up going palm up (Waiter's Tray) and opening the face too soon on the swing upwards.

 

[Chas Tennis]

This coaching video says that the path of the elbow flexion move should not be perpendicular to the net. It should be roughly towards the left net post or roughly parallel to baseline. Somewhere between there.

Otherwise, you will be changing directions and it will compromise the drop. And you will end up going palm up (Waiter's Tray) and opening the face too soon on the swing upwards.

First, if possible determine by listening to the video, if the sub-motions demonstrated are intended as progressions or are intended to be similar to the actual sub-motions of the serve.

You can also post that video above a serve compilation and check if it looks similar to actual motions of the serve. Compare from the same camera angles.

 

[Raul_SJ]

First, if possible determine by listening to the video, if the sub-motions demonstrated are intended as progressions or are intended to be similar to the actual sub-motions of the serve.

It is not a progression. Coach Clay is demonstrating an incorrect elbow flexion swing path just prior to the drop. The student's elbow flexion swing path is too perpendicular to baseline.

WRONG DROP!! Swing path prior to drop behind back is too perpendicular to baseline.
This incorrect swing path will cause racquet face to open up too soon on upward swing.

There are the motions of elbow flexion and external shoulder rotation (ESR).

Think we have consensus that the racquet drop behind the back (ESR) should occur when the elbow flexion is roughly 90 degrees. Will have to review the discussion but do not recall any post making any convincing argument against the 90 degree elbow flexion position going into the drop behind the back, correct?
Furthermore, the racquet drop behind the back (ESR), (timed with knee extension), should occur when the shoulder is already in a roughly 90 degree externally rotated position, correct?

Note that Djoker changed his motion in 2018 to a more abbreviated motion. Previously, his racquet head tip pointed towards the back fence while in a roughly 0 degrees elbow flexed position, as below:



In 2018 Djoker changed to an abbreviated motion. Note that his racquet head tip never points back fence. He is essentially in the 90 degree elbow flexed position from the very start of the motion. His swing path into the drop will be roughly parallel to baseline, as recommended in the previous coaching video.

 

[Chas Tennis]

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Think we have consensus that the racquet drop behind the back (ESR) should occur when the elbow flexion is roughly 90 degrees. Will have to review the discussion but do not recall any post making any convincing argument against the 90 degree elbow flexion position going into the drop behind the back, correct?
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What consensus? Quote those in the consensus.

I would have to look at a number of high speed servers from the camera angle that shows whether the elbow is about 90 degrees. I haven't done that.

I believe the best angle would be from a high camera viewing angle viewing the forearm and upper arm square on to accurately measure the elbow angle. Use this coordinate axis drawing as a substitute for a proper drawing. Say camera were above at Z , elbow at origin 0,0, forearm ending at Y and upper arm ending at shoulder X.

This overhead view camera angle is not easy to find in internet videos. It is the above camera view.

Frank, are your there? Frank Salazar Fuzzy Yellow Balls video from the above camera view.

Go to maximum racket drop and examine the elbow angle. To single frame on Youtube use the period & comma keys. The tilt of the upper arm is at some angle that is not clear in this camera view, needs a second camera for a more accurate comparison.

Do you see "the racquet drop behind the back (ESR)". It has often been pointed out recently that the racket is not behind the back, but alongside of the back. Single frame through the video and view the position of the maximum racket drop, forearm and upper arm.

There was a recent post asking for overhead camera views of serves. Several are there.

Demo of tricky angle estimates using a 2D camera to estimate an angle in 3D space.
Take a paper clip and bend it at 90 degrees. Hold it by one end and rotate it and look at it from various angles by also rotating your hand. Now bend the paper clip at 135 degrees and view it from various angles, does it ever look to be 90 degrees?


When I look at the first server in this compilation-

Calling his racket drop 'behind his back' really misses the 3D nature of the serve and what that server is doing.

Also, when his elbow angle first becomes visible coming out of the 'racket drop' his elbow angle is considerably less than 90 degrees.

I am not going to try and describe the service motion by tennis terms based on ground cameras that miss the most important features.

Look at videos first and figure it out second.

 

[Raul_SJ]

Do you see "the racquet drop behind the back (ESR)". It has often been pointed out recently that the racket is not behind the back, but alongside of the back. Single frame through the video and view the position of the maximum racket drop, forearm and upper arm.

This is true...
Let's say we examine the second pic below. Racquet butt handle facing the sky, knees have straightened, and there is thoracic extension. Let's call pic#2 the full drop and pic#1 trophy position.

(1) Will the elbow flexion angle at the trophy pic necessarily equal the elbow flexion angle at full drop pic? Suspect that this angle can change. Or it might remain constant... Will have to check overhead view.










Would you agree that the racquet drop alongside of the back, begins roughly near the end range of ESR (shoulder is externally rotated roughly 90 degrees). And at this point, the racquet head tip begins to point downwards, while the knees begin to straighten.

 

[Chas Tennis]

I'd rather pull out a video each time I'm interested and look at the video, as if the video is part of my memory. The server in the compilation that I mentioned had a lot of side motion, I don't want to bury that by attaching a few words to that part of the service motion. I'd rather leave that as 'not studied enough yet'.

I don't want to figure out what the ATP servers are doing. First, I want to observe the serves and their variety.

3D motion capture systems can do a good job on many of these observations like the 90 degree elbow. Not many are available.

Suppose, that if we measured a large number of ATP servers, that 55% had around 90 +/- 10 d. degrees, 35% had less than 80 d. and 10% had greater than 100 d. Also, we had to define a time or position in order to estimate these angles. Maybe the best servers had less that 80 d.? You seem to want to figure it out so that you can declare it is 90 d. Observations might give us the distribution of angles in use. ?

There is another problem in using pictures and believing in 90 degrees - you might be picking the pictures with camera angles that appear to show 90 degrees? That has been done on these forums very often. If I bend a paper clip with a 90 d angle I can orient it in 3D so that a 2D picture can show almost any angle.

Reference standard - whatever the ATP players are doing. Statistics on observed angles vs serve quality.

Djokovic has an interesting history of serving regarding the angle of his shoulder and, I guess, of his elbow also.

"Would you agree that the racquet drop alongside of the back, begins roughly near the end range of ESR (shoulder is externally rotated roughly 90 degrees). And at this point, the racquet head tip begins to point downwards, while the knees begin to straighten."

No.

I would look at a video to see the timing of various events. I know the end of the range of ESR depends on Thoracic Extension, a complication. I can't remember these details and don't know how they vary among servers.

Step through this video and answer your questions on timing and note all parts of the motion that you are not considering. Note - maximum racket drop and the racket head motion to the side. Wrist angle, supination, +

To single frame on Youtube use the period & arrow keys.

 

[Sir Weed]

To single frame on Youtube use the period & arrow keys.

Might as well put that in your signature.

Is this a TTW meme yet?

 

[Raul_SJ]

"Would you agree that the racquet drop alongside of the back, begins roughly near the end range of ESR (shoulder is externally rotated roughly 90 degrees). And at this point, the racquet head tip begins to point downwards, while the knees begin to straighten."

No.

I would look at a video to see the timing of various events. I kno

At trophy position, it feels like I am at the end range of ESR (or very close to it).

At full drop, the butt handle is pointed straight up towards the sky. Thoracic extension and knee extension has occured. As you mentioned, back muscles have been prestretched to set up the upward swing and ISR move... But I am not convinced there is any difference in ESR between trophy and full drop. If there is, it is very slight. Maybe three degrees. move

Both positions below feel like the shoulder has been externally rotated to end range of motion (roughly 90 degrees).

It would be useful if Elliott graph included the trophy position to compare ISR with full drop. But Elliott starts with the full drop.

 

[yossarian]

At trophy position, it feels like I am at the end range of ESR (or very close to it).

At full drop, the butt handle is pointed straight up towards the sky. Thoracic extension and knee extension has occured. As you mentioned, back muscles have been prestretched to set up the upward swing and ISR move... But I am not convinced there is any difference in ESR between trophy and full drop. If there is, it is very slight. Maybe three degrees. move

Both positions below feel like the shoulder has been externally rotated to end range of motion (roughly 90 degrees).

Overhead throwing athletes typically have ER ROM well beyond 90 degrees

you didn’t know that?

 

[mrmarble]

Doubt Djoker’s is less than 120 degrees
Without any back arching that is

 

[Chas Tennis]

At trophy position, it feels like I am at the end range of ESR (or very close to it).

At full drop, the butt handle is pointed straight up towards the sky. Thoracic extension and knee extension has occured. As you mentioned, back muscles have been prestretched to set up the upward swing and ISR move... But I am not convinced there is any difference in ESR between trophy and full drop. If there is, it is very slight. Maybe three degrees. move

Consider only the high level servers and their technique(s) until that issue is settled. Then you would have an understanding of the high level serve to start. Looking at your own serve or other posters, you are on your own. ?

I guess that Trophy Position is an instant around #9. I would interpret maximum external shoulder rotation (ESR) (called MER in some publications) to be near frame #12. There are not enough frames per second. It is a little complicated because the upper arm angle has also changed to higher and more forward and shoulder-over-shoulder is going on. Sampras appears to do active ESR going up to and through Trophy Position, only at TP for an instant with his technique.

 

[Raul_SJ]

Consider only the high level servers until that issue is straight. Then you have the high level serve.

I guess Trophy Position is #9. I would interpret maximum external shoulder rotation (ESR) (called MER in some publications) to be in #12. It is a little complicated because the upper arm angle has also changed, shoulder-over-shoulder is going on.

Would consider #9 to be the same ESR as #12 (or very close to it). Both positions are near the end range of ESR motion of 90 degrees.
Yes, there is thoracic extension but that is an independent movement which does not directly affect ESR.
Will need to see evidence that MER is occurring at #12. And if so, how many degrees more than #9 or #10? Do not think the difference is significant.

Has Bruce Eliott addressed this? Why does his graph exclude the trophy position where the shoulder has already significantly rotated to near its end range of motion ?

 

[Chas Tennis]

Would consider #9 to be the same ESR as #12 (or very close to it). Both positions are near the end range of ESR motion of 90 degrees.
Yes, there is thoracic extension but that is an independent movement which does not directly affect ESR.
Will need to see evidence that MER is occurring at #12. And if so, how many degrees more than #9 or #10? Do not think the difference is significant.

Has Bruce Eliott addressed this? Why does his graph exclude the trophy position where the shoulder has already significantly rotated to near its end range of motion ?

I explained how Thoracic Extension affects ESR. Please search that thread.

Joint angles are measured by defining "0" degrees and specifying details for how the joint angle is to be measured. Instructive videos show how to measure each joint motion. The purpose for such joint measurements is often for medical reasons, for example, to see how far a recovering joint can move. I believe that usually the joint motion is isolated.

During the serve, things are more complicated than words can convey. Then you look at the videos. The purpose is for biomechanical analysis of very complex motions where multiple joints are participating. The joint motion is not isolated but may be affected by other joint motions.

 

[yossarian]

I explained how Thoracic Extension affects ESR. Please search that thread.

Joint angles are measured by defining "0" degrees and specifying details for how the joint angle is to be measured. Instructive videos show how to measure each joint motion. During the serve, things are more complicated than words can convey. Then you look at the videos.

You can’t accurately measure ER just by eyeballing it in that position but my eyeball goniometer tells me it’s well beyond 90 in frame 12. May even be close to 130, which is not that extreme in overhead athletes

The normative value for ER ROM is 90 degrees. Raul is not aware that throwers undergo adaptive changes which increases their ER ROM. This is well documented

because he has me on ignore, he won’t see this. Perhaps pass it along?

 

[Raul_SJ]

I explained how Thoracic Extension affects ESR. Please search that thread.

Joint angles are measured by defining "0" degrees and specifying details for how the joint angle is to be measured. Instructive videos show how to measure each joint motion. During the serve, things are more complicated than words can convey. Then you look at the videos.

Will review again... IIRC, the stretch of the back muscles sets up a more powerful ISR move on the upwards swing. TE is a good thing. But do not see how thoracic extension changes the joint angle measurement and why MER angle occurs at that drop position over the trophy position.

Also might be good to look at Elliott graph. It plots ISR starting from the drop position. Although it excludes trophy position, it shows the differences as it moves upwards towards Big L... Does not appear to be much difference in ISR from drop pic to next three pics... Not clear what that implies for ESR.

[/

 

[Chas Tennis]

Would consider #9 to be the same ESR as #12 (or very close to it).\
........................................................................

I don't agree on that.

The orientation of the forearm is very different and forearm orientation is an indicator of ESR angle.
__________________________________________________________________________________________

What is the importance of ESR angle? To me, it is an indication of how stretched the ISR muscles are. As in pulling back a sling shot.

But when Thoracic Extension occurs it shortens the distance between the attachments of the lat muscle (ISR muscle) (as explained and illustrated in the thread on Thoracic Extension). That reduces the stretch on the ISR muscles. That complicates the biomechanics for the last 100 milliseconds. Again, when Thoracic Flexion then occurs it once again complicates the biomechanics leading to impact. Positions show up in videos but forces are not directly seen.

Better to have these forum posts while looking at one high speed video.

A few frames have been described with text boxes - 1) Start leg thrust. 2) Maximum Thoracic Extension, 3) Back is Thoracic Flexed back to near straight. All other motions can be see. ESR is clear and indicated by the forearm orientation relative to the shoulder joint (on the scapula).

By the way, how does the elbow angle compare to the OP post of "90 degrees".

If @Raul_SJ knows an angle for ESR to great accuracy, fine, but what about all these other motions that everyone can see are in this high speed video.

Collect video information about strokes first and then start to figure it out.

 

[yossarian]

Will review again... IIRC, the stretch of the back muscles sets up a more powerful ISR move on the upwards swing. TE is a good thing. But do not see how thoracic extension changes the joint angle measurement and why MER angle occurs at that drop position over the trophy position.

Also might be good to look at Elliott graph. It plots ISR starting from the drop position. Although it excludes trophy position, it shows the differences as it moves upwards towards Big L... Does not appear to be much difference in ISR from drop pic to next three pics... Not clear what that implies for ESR.

[/

That graph plots speed, not joint position

 

[Raul_SJ]

By the way, how does the elbow angle compare to the OP post of "90 degrees".

Have not measured it yet... But let's say there is a certain elbow flexion angle at trophy. Do we then expect that elbow flexion angle to be maintained while moving to full drop? If this elbow flexion angle does change from trophy to full drop, is there any significance to this?

 

[Dragy]

Have not measured it yet... But let's say there is a certain elbow flexion angle at trophy. Do we then expect that elbow flexion angle to be maintained while moving to full drop? Is there any significance if this elbow flexion angle changes from trophy to full drop?

Racquet drop is a visual attribute of the load (stretch) happening with muscles responsible for ISR. Racquet motion by itself has little purpose, and those focusing on the racquet head loop, backscratch, etc. have always missed the forest for the trees. Your purpose is to get to around this position with max potential energy stored in stretched shoulder:

Now how you achive max shoulder stretch? 2 factors go into it:
- powerful body move - driving to move hitting shoulder up and around with big acceleration;
- maximizing arm momentum (inertia to stay in place and withstand the force pulling arm to follow the shoulder rise/coming around) to expose shoulder muscles to max stretch and load.

Max momentum is achived by putting arm mass farthest from rotation axis. Rotation axis is upper arm. Way to get as much of forearm+hand+racquet mass farther away is to keep +/- 90 deg elbow bend. Keep in mind, wrist is too weak to bear the load, so we relax it fully and "pull" the racquet, don't try to extend it even further from rotation axis.

90 deg is not a mandatory exact angle, but you better be in the ballpark. Here is one strong server who enters the drop with less than 90 deg flextion:

 

[Raul_SJ]

In order to get maximum racket head speed, as the racket drops, the elbow should remain at close to 90 degrees with the upper arm.

90 degrees with the upper arm is what Macci preaches. Does not explain why but Macci
works closely with Brian Gordon.

https://www.facebook.com/permalink.php?story_fbid=3698680403500413&id=143540379014451

https://m.facebook.com/RickMacciTennisAcademy/videos/left-foot-toward-net-post-right-foot-parallel-90-degree-anglebang/3698680403500413/

 

[Raul_SJ]

Max momentum is achived by putting arm mass farthest from rotation axis. Rotation axis is upper arm. Way to get as much of forearm+hand+racquet mass farther away is to keep +/- 90 deg elbow bend. Keep in mind, wrist is too weak to bear the load, so we relax it fully and "pull" the racquet, don't try to extend it even further from rotation axis.

90 deg is not a mandatory exact angle, but you better be in the ballpark. Here is one strong server who enters the drop with less than 90 deg flextion:

What about the 2010 Djokovic motion where he drops from 30 degree elbow flexion? Is that putting arm mass farthest from rotation axis?

 

[Chas Tennis]

What about the 2010 Djokovic motion where he drops from 30 degree elbow flexion? Is that putting arm mass farthest from rotation axis?

Check the anatomy of the lat muscle, in particular, where it attaches to the upper arm. In 2010, considering that attachment to the upper arm, what effect does tilting it down have on the tension in Djokovic's lat? Consider the distance between the several origins of the lat and its insertion on the upper arm.

The rotation axis for ESR & ISR is through the long axis of the humerus. The long axis of the humerus runs near to its centerline.

 

[Dragy]

What about the 2010 Djokovic motion where he drops from 30 degree elbow flexion? Is that putting arm mass farthest from rotation axis?

Rotation axis for ESR/ISR is shoulder-elbow line.