# physics/science behind pronation

Discussion in 'Tennis Tips/Instruction' started by Chenx15, Mar 2, 2011.

1. ### gzhpcuProfessional

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My take on this: flexing the wrist locks it and aids acceleration of the racket coming forward at impact. At this point the wrist is very loose and passive. Since the racket whips forward at great speed, it causes the forearm to slow down before impact. This is because the maximum speed must be in the hand and not the forearm. It is a result of the backward force of the handle on the hand. The head of the racket rotating forward cause the handle to rotate backwards, slowing down the forearm.

2. ### tolyHall of Fame

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‘Whip Effect’ etc.

Suppose I have two straight pieces of wood, and I connected them end by end by using rope. There is no gravity. The angle between them is 90 degrees. I move them along the straight line with speed V. There are two kinetic energies:
M1*V*V/2 and M2*V*V/2.
I stop the first one. How many percents of the first stick energy will be transferred to the second one? Because the rope cannot transfer energy at all, the answer will be 0%. I think we can treat our muscles as the rope too.

Let me elaborate on this issue.

First I should think about the expression “I stop the first stick". What way I can do it? For example, it hits tennis net post. This action produces some damage to the post and we lose some stick’s kinetic energy. The stick also gets some damage, again some lost of energy. Then post makes stick to bounce back. I want to prevent this from happening. By using my hand I press the stick back to the post. The stick hits my hand and also creates some damage to it. I push stick back to the post and so on. All this process is harmful for all participants and energy consuming. I believe the stick can lose all its kinetic energy during this struggle.

The rope between sticks allows the second one to rotate in any plane. One end of the second stick abruptly stopped. There also would be damaging and energy consuming fight between rope and second stick, but not so dramatic. Because, the second stick has the option, to convert its linear energy to rotational one. This linear energy (ELin) would be equal ELin = M2*V*V/2 minus some negligible lost of energy, because of the struggle with rope and sticks. The energy (ELin) was converted to the rotational energy (ERot) and ELin = ERot.

What is Rotational energy? Answer: ERot = I*Ѡ* Ѡ/2,

where I is moment of inertia, I = M2*L*L/3,

L is the length of the second stick.

Hence, ERot = (M2*L*L/3)*Ѡ*Ѡ/2.

But, L* Ѡ = Vend,

where Vend is the linear speed of the second stick end.

Thus, ERot = M2* Vend * Vend/6 and we have equation:

V*V = Vend*Vend/3.

Follows from the last equation that the unknown velocity is

Vend = 1.73*V

Hence, the tip of the second stick has the linear speed of 73% faster than its translational speed before impact with tennis post.

This is the secret of the so-called “whip-effect” in relevance to tennis strokes. It is just conversion of the translational kinetic energy into rotational energy. And these energies are equal.

We maybe can freeze our arm during pronation phase of the serve, but we cannot increase kinetic energy of the racquet, which is mostly responsible for the ball speed. Tiw pros never slow down the arm. We also cannot stop torso rotation, because of very big inertia.

I believe all these ideas about: energy transfer, freezing body parts, and ‘whip effect’ are just misunderstanding of the real not so much complicated process.

Last edited: Oct 26, 2011
3. ### spacediverHall of Fame

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I believe you need a minimum of 50 posts before u can do edits.

4. ### Chas TennisLegend

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I wondered where the button was. Thanks.

5. ### bhupaesProfessional

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Hi toly, I agree with your analysis insofar as the stick analogy is concerned. But this is not how the forces work in a forehand, and I will explain. Also, remember, I stated that the whip was just an analogy and does not apply to tennis strokes in practice - but it is effective in illustrating the two different ways of hitting.

Coming to the forehand, let us jump directly to the point where the player is accelerating the racquet towards the ball. In the first way, the player can keep pulling the racquet in a straight line until it collides with the ball. I would consider this the case where the muscles are actively powering the stroke. In the second way, the player pulls the racquet, just before impact, very sharply sideways (inwards) and upwards (instead of pulling back as in the case of the whip), causing the elbow to bend sharply. I believe this impulse will further accelerate the racquet head by causing a rotation centered around the elbow (actually this is a more complex motion, I think, because there is the biceps as well as the internal rotators of the shoulder involved). Plus of course, there is the ulnar-to-radial forearm pronation movement, but this, as I said earlier, I believe is more of an aligning movement than a powering movement - but it doesn't matter for this argument. This second way is as close as we get to the whip analogy, and this type of follow through is the release mechanism for the forehand, IMO.

So, you are right - there is no magical energy appearing out of the ether. The additional energy is supplied by the shoulder muscles and biceps for the most part, at the last stage. Some of this energy will go into translational velocity, and some into spin, as we all know by now.

It would be great if others could add their insights and knowledge about the exact release mechanism of the forehand stroke, other than the follow through - thanks in advance!

Last edited: Mar 10, 2011
6. ### bhupaesProfessional

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Incidentally, I believe this is why it is important to visualize the follow through - it is part of the release mechanism, and has to be executed properly so that energy transfer is optimal. The coaches who have been advocating this for a long time must have known why this was important, just by experience and feel, and didn't need to use Physics for advocating what they did. Or maybe some of them did know Physics, but held back so they wouldn't be pooh-poohed...

7. ### chico9166Guest

Agree wholeheartedly with the whole releasing concept. And it jives with racquet head speed profiles which indicate that the bulk acceleration occurs very close to contact and corresponds with upper arm, forearm rotation. (or release) Indeed, if you look at video of high caliber players, it's pretty clear. Often times the butt cap of the racquet is still facing the ball only a foot or so from it.

You see the same principle in the golf swing. A common characteristic of all good golf swings, is that the acute angle formed between the forearms and shaft is preserved through much of forward swing until close to contact. Hackers tend to "cast the club" whereby this forearm to shaft angle increases. There's no real "release" of the club when this occurs, and club head speed flat lines.

Last edited by a moderator: Mar 11, 2011
8. ### bhupaesProfessional

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Thank you, chico9166. That's a good analogy from golf.

9. ### SystemicAnomalyG.O.A.T.

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Request: Could you pls put more white space (separated paragraphs) in your next long post to make it easier to read. My poor aging eyes can't focus on this (even w/reading glasses), particularly with technical information.

10. ### Chas TennisLegend

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The angle BETA?

For beta. See image 1 & 2 of the Sampras serve or Federer in sennoc post#56.

Beta - is this angle defined somewhere?

Is it

1) The changing angle between the arm (very slightly bent) and the racket axis, a function of time?

2) The angle between the arm (very slightly bent) and the racket axis when the ball is struck?

Both uses? Any idea on how consistently used? Does it have a name?

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12. ### tolyHall of Fame

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I was just trying to hide formulas, because most people hate them.:???:

Last edited: Mar 12, 2011
13. ### gzhpcuProfessional

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Hi toly,
What is meant is to breakup your paragraphs into smaller ones.

14. ### Chas TennisLegend

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Thanks, I see the Safin serve in post #62 above with the angles indicated.

I would say, for example, estimating for the Sampras serve frames, that β(t) varies between 90-10° It starts after the nearly straight arm is up at β0 = 90°. It reaches βi = 10 ° at impact. I guess it is or could be defined by the angle of the racket axis to the forearm axis at anytime so it might be useful earlier in the motion too.

15. ### sennocGuest

@ toly's efforts:
There is no deeper sense at using Newtonian mechanics for tennis strokes analysis. Motions are too complex. This is a trap where you can find virtually every tennis coach...

You should use Lagrangian mechanics instead.

BTW don't you think that's quite funny that the most famous coaches like Bollettieri make money by explaining things they do not understand?

Last edited by a moderator: Mar 13, 2011
16. ### nabrugRookie

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No.

Because understanding of this is only a minor part of developing tennis players.

17. ### sennocGuest

Technique is absolutely fundamental in tennis. The faster you know how to hit and move, the faster you can concentrate on tactics and psychology.

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and.......?

19. ### tolyHall of Fame

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2.2.2 The Slice Serve with Intensive Wrist Flexion, Wrist Ulnar Deviation, and Almost Destroyed Pronation
In order to deceive opponents, pros usually use the same toss to hide a type of the serve they are going to make. To produce pure sidespin they have to bring the long axis of the racquet in vertical position. It means they are forced to keep beta angle around zero. There is for instance Chanelle Scheepers’s slice serve.

During the serve, Chanelle Scheepers bent her wrist backward. That opened the racquet face around 60°, nearly to the water’s tray position. Then she rotates the arm (by using the shoulder joint) in the vertical plane with the angular speed about ΩV =400°/sec and pronates in the horizontal plane. She flexes the wrist in the vertical plane with the angular speed roughly ΩW=2000°/sec. Besides, Scheepers creates extreme ulnar deviation to provide sidespin. The last action nearly destroyed the pronation’s component of the racquet velocity |VLH|, because the arm and the racquet’s long axis represent almost the straight line, hence the angle β is very small. To compensate the lost of the pronation power, she has to extremely flex her wrist.
Chanelle Scheepers the wrist flexion in the vertical plane (the main factor), the slow arm vertical rotation and weak pronation create the flat component of the racquet speed. The wrist ulnar deviation generates significant sidespin of the ball.

Last edited: Jan 11, 2012
20. ### bhupaesProfessional

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I have noticed that there are two types of slice serves. One is the type that toly has described, where the server uses ulnar deviation to carve the outside of the ball, and cause it to curve heavily. Lefties seem to be particularly adept at this type of serve. This serve is not that fast, but it curves a lot in the air and off the bounce and pulls the returner out wide, if not ace him outright.

The second type is pretty much indisinguishable from a regular serve and uses the usual mechanisms, but the ball is contacted a little bit on the outside. I think this is what Sampras does. The ball has a lot more pace, and curves heavily after the bounce (when it slows down, I suppose, and lets the side spin take effect), and the combination of pace and spin can be deadly.

21. ### Chas TennisLegend

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Toly,that is another of your very informative serve sequences with a detailed analysis showing how βi is varied by the pros for special serves.

Elbow Reference Point for Upper Arm Rotation
I have just started to look for upper arm rotation in frames from high speed videos. It is not clear how it would appear in high speed videos especially since we want the bone position but the skin and muscles are recorded. The muscles bounce around. The skin is not connected to the bone. Also, a reference point on the skin is hard to track in a video frame unless a marker, like tape, is used. A good reference for upper arm rotation that should show in many videos is probably the point of the elbow. Most lighting creates shadows that show the elbow point clearly. Close up pictures would be best with the shoulder and impact filling the frame.

Range of Motion. If I hold my arm straight out from the shoulder and stretch the upper arm rotators (pec & lat) I can get about a 270° range of motion. (stretch – thumb pointing back, fully contracted – thumb pointing down). There might be a little more or less possible during the serve including and follow-through. If we could always see the shoulder and the elbow in the videos we might be able to roughly measure the upper arm rotation.

Amount of upper arm rotation leading to impact?
Suppose that leading up to impact the racket went from edge on to facing the ball trajectory and there wasn’t any pronation (lower arm). Then the upper arm would produce all/most of the rotation and it might rotate about 90°, a lot less than its range of motion. For upper rotation does anybody have an idea from videos as to how much is seen leading up to impact (not looking at the wrist but the elbow).

I tried to see the upper arm in the video but without the elbow point it is a very uncertain estimate. Gut feel is that it does not look like that much upper arm rotation.

Wrist Flex Passive - Non-energy adding?
You mention the wrist flexing. I suppose that most of energy for the wrist flex has been produced already by the tricep when the arm was extended, and is the racket kinetic energy. Much less is supplied by the muscles that flex the wrist? Any thoughts on whether the wrist is flexing passively without the wrist flexors adding power? Probably both are contributing to some degree. ?

22. ### Chas TennisLegend

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Arm Rotation & Racket Angle - Jim McLennan

These introductory videos are from TennisOne.com Editor Jim McLennan, who has several on point Youtube videos on serving.

I think that they give a quick orientation for arm rotation (upper arm axial rotation plus lower arm pronation). They discuss the effect of the angle of the racket to arm angle at impact, βi, on the racket head speed.

Just for orientation to the arm rotation issue what do you think of them? I only found one thing, that of the shoulders being too level on the fence demo.

Clear demo but when he demos against the fence his shoulders are probably too level for an actual serve.

The impingement issue and shoulder up by trunk tilt as discussed earlier this thread.

23. ### KevoHall of Fame

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Is that serve on video anywhere. She appears to be hitting the ball more toward the inside than the outside. I'd sure like to see what actually happened to that ball in the air. It's kind of hard for me to tell from the pics what the result of that serve would be. It might not be such a great example. Then again, it might be a clean ace, in which case I might want to look at this more closely.

24. ### Chas TennisLegend

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That serve is a very bad example of most serves. I was not using that serve as an example for serving but I think we were using it to discuss β. I believe that Toly used it an an example of a special serve with a very small β.

As I understand it for the previous few posts

I was not certain on how β was being used and wanted to clarify it for myself and maybe others. Not sure if β is widely used or its mostly Toly's designation. Toly referred to a previous thread example that included a Sampras serve. I mentioned that the Sampras serve only had about βi = 10° Toly used the last example of a special serve as another one with a very small β. β is usually larger for better servers. Data on β being used by the better servers for their bread-and-butter serves would be very useful in understanding the effect of β.

(I'm trying to get βi to be used for impact since I had to spend some time distinguishing between decreasing β leading up to impact and β at impact. ?)

25. ### tolyHall of Fame

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I think, it is obvious, if we keep arm vertically and use wrist ulnar deviation to produce: 1) Pure topspin serve - beta angle should be around 90 degrees. 2) Pure kick serve - beta should be around 45 degrees. 3) Pure slice serve - beta should be around 0 degrees.

The wrist actions must be active. Otherwise, how are we going to control them? For example, during kick serve there should be no wrist flexion at all. During flat serve pros use wrist flexion intensively.

Last edited: Mar 15, 2011
26. ### tolyHall of Fame

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You are right. If wrist ulnar deviation breaks before impact, it will be the pure flat serve. I also (like Chas Tennis) think that would be very bad flat serve routine. Tiw I believe she produced slice serve. If not, then perhaps this explains why she is not number 1, but only #81 in the world.

Last edited: Mar 15, 2011
27. ### Chas TennisLegend

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Then I don't understand at all how the pro's use beta. Your top spin example of impact with the racket at beta = 90 degrees seems at first to be too extreme to be effective and would not have built up racket speed. I believe it would be very stressful for the wrist to add racket head speed with beta = 90 degrees.

I'm watching recorded Roddick & Isner today. Using the DVR I'll see if I can catch Roddick ever impacting with beta near 90 degrees. Unfortunately I won't ever be able to tell what kind of serve is being hit. I could see Isner's arm last week and it looked about 20-25 degrees. Isner probably serves nearly all flat serves, I guess. ? Also, one camera cannot give a completely accurate beta unless the angle is displayed squarely across the frame.

Since maybe I'm the only one who is unclear about this you might refer me back to an earlier thread that has more definition,examples, pro data or references. I read the earlier one that you referred too in this thread.

28. ### SystemicAnomalyG.O.A.T.

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Thnx for the re-formatting that post, toly. Much easier on the eyes now.

BTW, you could always hide it this way:
ERot = (M2*L*L/3)*Ѡ*Ѡ/2

29. ### tolyHall of Fame

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I gave the definition of the pure (ideal) topspin serve where beta should be around 90. But in reality, wrist ulnar deviation needs some time to accelerate the racquet to create brushing motion. During acceleration, before impact, the wrist decreases beta angle notably to 80-50 degrees. Tiw practically there is no pure topspin serve.

Last edited: Apr 13, 2011
30. ### Chas TennisLegend

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Measuring Beta on TV using a DVR

I managed to measure a few serves for Isner at Indian Wells using my DVR. β0 = 30,40,44,50 degrees. These are the angles across the TV display so they might be a little smaller than the actual but they look pretty good. Sizeable angles.

For the the camera looking down the centerline & viewing the players back -

Andy Roddick's serves were all too blurry as were most of Isner's. The shutter speed is slow but a few of Isners serves were aligned to give images without much image blur.

To measure using the DVR - only the deuce court serve viewed from the back would image occasionally without excessive motion blur. That probably indicated that the racket was traveling away from the camera. Isners serve seemed to do that but not Roddick's serve. Both servers produced blurry serves from the ad side. Stop image and step through service motion. Stop on ball contact. Put a clear plastic sheet on the TV and trace the Beta with a Sharpie.

If there is any reference with that data please provide a link.

31. ### Chas TennisLegend

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Angular Velocities for Joint Movements

I found this information on peak angular velocities on joint movements in D. Knudsen, Fundamentals of Biomechanics, an excellent reference -

I also have D. Knudsen's tennis book which has a very broad discussion with insights on many subjects.

The shoulder joint internally rotates with the greatest angular velocity listed. Since the rotation is also driven by large, powerful muscles, the pec & lat, these can also accelerate the racket. So it is easy to see why internal rotation has been recognized as so important to the serve.

32. ### Chas TennisLegend

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High Speed Video to See Upper Upper Arm Internal Rotation

Much of this thread discussed shoulder internal rotation of the upper arm (humerus bone). To observe the upper arm rotation in high speed video the following set up was used:

1) Two pieces of black plastic tape were placed on the protrusions of the humerus bone extending toward the shoulder from the elbow.

2) Two pieces of tape were placed on the bone protrusions at the wrist inside and outside.

3) A piece of tape was placed on the back of the hand.

The tapes were place to avoid muscles that may bounce. The tape does not exactly follow the bone. ? Do you have any other suggestions for additional markers?

I have not practiced increasing the upper arm rotation in my serve very much so this video is mostly to display the video set up and arm rotations.

Camera settings: Casio Ex FH100, Manual, 240fps, shutter: 1/10,000s, ISO 3200

http://vimeo.com/21512296

Serves: Both serves show a β0 of 21°.

Flaws: Racket starts up not completely edge-on but at 30-45°. Arm/racket rotation is timed late perhaps because the racket did not start up edge-on initially. The upper arm rotation is a faster motion but is timed late more at impact than before.

Shouldn’t it be semi-straight arm and then forceful internal rotation? Maybe the arm extension should be more complete and then the complete arm rotation should fire. This seems to be what happened in most thread videos such as the Sampras serve. His arm is semi-straight with the racket at about β = 90° before starting up to impact.

The discussion in Knudsen’s book, Biomechanical Principles of Tennis Technique, emphasizes the stretch-shortening cycle of muscles. I read it to mean that stretch is faster than shortening.

Supination? If the upper arm forcefully internally rotates with the less powerful forearm & wrist lose and the racket is at a larger angle β then probably the wrist would initially supinate in a stretch-shortening cycle before pronating. Has anyone seen information of the forearm supinating a little to stretch?

Anyway, this video set up looks like a workable approach to practicing upper arm internal shoulder rotation and seeing what is going on.

33. ### KevoHall of Fame

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I like the the tape marker idea. I also agree with your observations regarding the serve flaws. I also think your speculation about what you would want to see instead is also correct.

One thing you didn't mention was the shoulders. I can't see what your lower body is doing, so I don't know if that's a contributor or not. However, your shoulders are not nearly steep enough before the swing, so I don't think you will be able to achieve proper internal rotation since the alignment of your shoulder at impact probably will not allow it.

If I had a high speed camera, I would go try your tape marker system though. It looks like it could be quite effective.

34. ### tolyHall of Fame

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Thanks for this great post Chas Tennis. This is really big contribution to the forum!

35. ### Chas TennisLegend

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Thanks for pointing out that a shoulder issue starts before the swing. I agree and will need to work on that. For me that issue probably involves very ingrained muscle memory.

36. ### cliffRookie

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Roddick Pronation Video

Here is a link to a video clip that shows Andy Roddicks forearm internally rotating, whist practising at the London Masters in December.
For me, pronantion helps increase racket head speed which if done at the right time will increase the service speed. The timing of the pronation also allows the server to control the direction of the serve.

37. ### SystemicAnomalyG.O.A.T.

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^ It is the shoulder that internally rotates. The forearm pronates.

38. ### sureshsBionic Poster

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Also the last pic nicely shows how the ball is hit down when it leaves the strings

39. ### SystemicAnomalyG.O.A.T.

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^ Not really, The ball has already left the strings in that last pic. The racket orientation is open in pic #8 and #9 and is fairly neutral in pic #10 & #11. This view of the action is not ideal -- it is not quite directly to the side so it is difficult to make an absolute conclusion as to the orientation of the racket while the ball is on the strings. Note that the ball contact with the stringbed is only 3-4 ms.

With certainty, we can say that the racket face is open just prior to contact and closed just after the ball has left the strings. During contact the orientation seems to be very close to neutral. Whether it is ideally very slightly open, very slightly closed or neutral probably depends on the type of serve and the height of the server.

40. ### gptProfessional

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In reference to another post. I suspect most of you guys are mathematicians and not artists...

41. ### Chas TennisLegend

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Nice video.

As mentioned there is a distinction between pronation and total arm rotation which includes both pronation and shoulder internal rotation. See Reply #65 for clarification. The term pronation has been loosely used. See Reply #160. I vote only for precise nail-it-down definitions for everything after misunderstanding this serving issue for decades!

In the Roddick video the close-ups show that the bones at the elbow (medial eponcondyle and lateral eponcondyle protrusions) and also the bicep are rotating. That is the indication of upper arm (humerus) internal shoulder rotation. An important point throughout this thread is that internal shoulder rotation is the most important driver for the racket. When the racket is at an angle to the semi-straight arm this powerful rotation produces racket head speed. (Toly replies.) This angle is not apparent in the Roddick video, it shows up better when viewed from behind. See above reply on "Measuring Beta on TV using a DVR", try it.

My simplified picture of the internal shoulder rotation sequence -

1) Internal rotators (pec & lat) - initial windup stretch for the stretch-shortening cycle - http://www.fastecimaging.com/video/h...eBackViewE.wmv
2) Internal rotators - more stretch from racket & forearm inertia when the shoulder is accelerated upward both by leg extension and trunk-shoulder reorientation (understand the safety issue).
3) Internal rotators - fully stretched and ready to rotate, arm semi-straight.
4) Internal rotators – fires to accelerate & rotate racket for impact.
5) Internal rotators – rotation decelerated by shoulder external rotators (smaller muscles).

Safety issue –the external shoulder rotators need to be strengthened & conditioned to decelerate the rapidly rotating arm.
Originally Posted by charliefedererer
Sennoc,
…………………………….
http://www.tennisresources.com/index.cfm?Rotator cuff injury ATT=&area=video_detail&basicsearch=1&media_name=&r v=1&vidid=3712
The tennissources link - in-depth discussion of the shoulder injury issue by Tod Ellenberger. Type "Rotator" in the Search block.

I have recently been looking into biomechanical research on this issue with internet searches. There have been many publications extending back more than twenty years. Search “internal rotation tennis serve” or similar (add “shoulder rotation, upper arm,” etc.) and look for B. Elliott and co-authors to start. Many full articles are not available free but the abstracts give you many conclusions. Examples:

http://journals.humankinetics.com/j...mbsegmentrotationsduringthepowerserveintennis

http://w4.ub.uni-konstanz.de/cpa/article/viewFile/2695/2533

Look at the internal rotation rates for lower & advanced level players:

http://onlinelibrary.wiley.com/doi/10.1002/jst.112/pdf

Future 2012 conference:

…..better late than never……..

42. ### Chas TennisLegend

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Last edited: Sep 12, 2011
43. ### dlamSemi-Pro

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There's more pronation in the forearm in a windshield wiper forehand than a serve.
Im glad to see how some of the posters are describing the importance of the body shoulder rotation and wrist hinging is just as important pronation of the forearm.
The other aspect of ther serve that cant be described in mechanical terms is the rhythm.
Good servers look similiar at impact but look very different from in how the get to the trophy position.

44. ### dlamSemi-Pro

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I like the diagram showing the difference between internal rotation of the arm vs true forearm pronation. When the racquet turns in a counterclockwise direction(as observed from the butt of the racket) as it brushes against the tennis ball in a righty serve, there's internal rotation and forearm pronation. Each contributing to the spin, pronation more than internal rotation IMO.
Interesting the single backhand motion, the arm motion is exact opposite. supination with external rotataion of the arm