Discussion in 'Tennis Tips/Instruction' started by Chenx15, Mar 2, 2011.
Sennoc, I just saw your last post... pretty cool stuff! I have to think about this for a while!!
I'll be back in 3 hours, good luck
OK, what's new here? Seems just like the kinetic chain to me, starting from ground up.
For a real good understanding of the biomechanics of the serve, see Brian Gordon's articles on the serve under "biomechanics" on Yandell's tennisplayer.net. The site is not free, but worth every penny...
If I add up all the peaks, and the value of H-R at MER, it seems to add up to something close to the H-R peak value of around 140,000. Does this imply that we can use these values to determine the relative contributions of each component? I'm afraid my insights end here... but I would like to know the conclusion!
Gzhpcu, I think the importance of this is that both qualitatively and quantitatively, it offers an alternate paradigm to that of Brian Gordon. In BG's setup, I believe movements and torques were measured, whereas what Sennoc has presented uses energies. These graphs seem to reflect what we know of the mechanics very accurately - I would never have thought that was possible, but there it is... something quite new, at least, for me.
Video Technique and Estimate of String Velocity Component from Racket Spin
I am average tennis player, not a very good server and have trouble getting a clear idea of what to do. Proper pronation on the various serves is at the top of my list for difficulty. First, how important is racket spin? Second, I need a workable video technique for feedback while practicing.
Continuing from my earlier post, #59, on racket axial rotation, racket spin, and ball hit location this post is for discussion of:
1- A video technique to quickly estimate axial rotation at impact.
2- The relative magnitude of the racket’s axial rotational velocity component to the ball strike.
3- The effect of the location of the ball impact point across the strings of the racket face.
The body’s powering muscles that produce the rotation do not matter for this discussion.
However, due to the informative posts here I’m going to now try and get my upper arm to transfer rotation from my shoulder and be newly aware of the angle, beta, of the racket to the arm’s axis, for sure! Thanks!
If you know of alternate video set ups for a single camera and analysis techniques for observing pronation please post.
Quick Video Analysis for Rotational & Translational Components.
These posted frames were identified as from a kick serve (disregard the added lines, etc.). Toly, post No. 5, posted the frames in another thread which has a thorough & extremely informative analysis at http://tt.tennis-warehouse.com/showthread.php?t=361610.
The following video analysis can give a quick & rough estimate of the relative axial rotational velocity. It locates racket edges and the string center as reference points and could be applied to other serves. I will be able to use this method with my Casio FH100 to quickly look at my serve for feedback on pronation plus shoulder rotation.
The frames show the relative contribution of racket rotation (around the axis parallel to its length) and racket translation to this serve. Some important geometric features of the racket’s 3D path are not available from a single camera and are ignored. This method with a camera viewing parallel to the baseline shows the rotational & translational components in approximately the direction that the ball travels. The term rotation here always refers to the racket’s spin around the long axis.
First Measurement. Let’s look at frame #5, for example. The racket frame’s right side measures (on my 19” monitor) about 8mm from the ball (the ball location at impact would have been more accurate). The frame’s left side measures about 16mm from the ball. The string center measures about 12mm. Continuing the rotation to when the ball is struck, both sides of the racket & center have reached the same plane facing approximately parallel to the ball’s trajectory. In this short time the frame’s right side moved about 8mm, the center moved 12mm and the frame’s fast left side moved 16mm. Assuming that the center of the strings gives the average translational velocity for this serve the frame’s slow right side was moving at about 70% of the average translational velocity and the frame’s fast left side was moving at 130%! In serving the ball does not hit on the frame so these are maximum differences. On the other hand, the rotation is accelerating at #5 so the actual magnitude would be even greater at impact.
Some Conclusions. The axial rotational rate is significant. Does it matter where the ball hits across the strings? For serving with racket axial rotation the string velocity at the impact point varies significantly depending on impact point. For a right hand server it increases from the right to the left across the racket face. The increase is significant. The impact velocity cannot be characterized by a single ‘racket head speed’. This axial rotation results from starting toward the ball ‘edge on’ and hitting the ball after about a 90 degree rotation. Last, I hope that I did not make too many dumb mistakes.
If I understand the posts correctly both the upper arm powered by shoulder rotation and forearm pronation produce the racket spin. These two sources also produce significant racket translation over that same time.
Does anyone have any data to show serve velocity resulting from various racket translational velocities, axial rotation rates and off-center impact points, plots of serve velocities vs these quantities as parameters, etc.? A high speed video study especially of the big servers would be interesting.
Not sure what to do with this information. Hitting more toward the left side of the racket face does not seem like a good idea and might be an injury risk…for starters
http://www.youtube.com/watch?v=lTRvxaBMh8s…take care …..
Of course it's all about kinetic chain. But are you sure, gzhpcu, that you understand everything?
Ke LL + Ke Tr + Ke Uarm + Ke Larm = Ke H+R
This should be true if there is only "loading". So let's check.
20 + 38 + 22 + 30 = 110 J
Great. This is what we did: we produced energy at every part of kinetic chain and moved it into the hand.
But maximum value of Ke H+R is ca. 146 J.
146 J - 110 J = 36 J
36 J is 1/3 of 110 J
So, by "loading" we produced 2/3 of real energy of our hand and racquet.
In our Universe, energy cannot appear from nothing. My question is: where does 1/3 of the stroke energy come from?
I see your line of reasoning now. The "missing" energy has to come from the muscles that are powering the final moving components, the lower arm and the hand mainly, and the rotating upper arm - and this would be mainly due to internal rotation of the upper arm, and some from hand flexion. There is no other source of power I can think of at that point...
You are wrong, Bhupaes. Easy, you are not the only one. The most famous tennis coaches do not understand physics of strokes and it is clearly visible in their "explanations". Soon you will laugh watching Bollettieri movies
Ok, let's look at Ke Tr curve as example. First, it increases and the maximum is at frame no. 28. This energy comes from chemical energy released in our muscles. As a result, at frame 28 the kinetic energy of Ke Tr part of kinetic chain is maximal, what means that trunk moves at the highest velocity.
...energy decreases. Nothing unusual.
In our Universe, energy cannot disapper.
found all my answers in this website
specially this one
Sennoc, the energy doesn't disappear, of course - isn't it transferred to the next link in the chain, as stretched or wound up muscles (potential energy), which is released again to the next link the chain and so on. The released energy doesn't always need to move to the next link, of course - some of it may skip one or more links. The final piece is really the hand, and it seems like it must supply the energy directly to the racquet, since there is no place to store it. Is this the source of missing energy?
I will check after an hour or two - got to run an errand!
I am sure that I do not "know everything". However your equation is wrong:
Ke H +R belongs on the left side of the equation. You completely neglect the kinetic energy of the rotating racket head.
Set of good rules. But the physical explanation is wrong - at least in 1/3
Ke H+R means: kinetic energy of hand AND racquet. Rotational energy generated in H+R is not so important because radius of rotation is small. So, equation is good (or sufficiently good ).
Yes, you are right. But that's what I did: I added maximal energies of each part of kinetic chain. Quite natural, because maximum kinetic energy means maximum velocity. The result was 110 J. So why the maximum of Ke H+R is 146 J, 36 J more? On the diagram 30% of additional energy seems to come from "nothing"!
That's physically not possible. We are talking about HUGE source of energy. HUGE. You can read all tennis books, you can watch all those famous movies made by the best coaches - you will not see any explanation. That's quite funny, don't you think? They are talking about all those "angles", "muscles", "5 stage rocket" - but they say nothing about that additional source of energy and its importance!
OK, last hint before final explanation.
You have to really understand, what physical processes are represent by curves on the diagram. Let's look at the curve Ke LL. In frame 0 the kinetic energy is very small. Then, chemical energy stored in muscles is transformed into kinetic energy - curve rises and has its maximum at frame 24. This means that in frame 24 lower limbs move as fast as possible. But then there is something very interesting: kinetic energy is lower and lower. What does it mean, physically?
I want to clarify this issue with one of the fundamental laws of physics – Conservation of Momentum
The momentum (p) of a particle is defined as the product of its mass (m) times its velocity (V). Btw, velocity is vector. Hence, p is also vector ( p=mV).
An isolated system implies a collection of matter which does not interact with the rest of the universe at all - and as far as we know there are really no such systems. There is no shield against gravity, and the electromagnetic force is infinite in range. But in order to focus on basic principles, it is useful to postulate such a system to clarify the nature of physical laws. In particular, the conservation laws can be presumed to be exact when referring to an isolated system. We can assume that after jump (during serve) our body would be isolated system.
The momentum of an isolated system is a constant. The vector sum of the momenta mV of all the objects of a system cannot be changed by interactions within the system. This puts a strong constraint on the types of motions which can occur in an isolated system. If one part of the system is given a momentum in a given direction, then some other part or parts of the system must simultaneously be given exactly the same momentum in the opposite direction. In case of the serve if we begin moving our arm in the direction of the torso rotation, it would inevitably slow down rotation of the torso. Tiw we cannot sum up the energies and especially their peaks. But we can sum up velocities/momentums as vectors.
I think all this magic stuff is just misunderstanding.
Here's a better copy:
(full text free as journal is open access)
Toly, please don't copy and paste physics definitions from other sources. If you are going to contribute, use your own words. And if not, please cite your sources.
Sennoc, I've read everything you've written here (and I've even just finished reading the paper from which the graph is from), and I don't understand what your apparent paradox is all about.
You seem to be marveling at two facts:
1) the maximum kinetic energy of the hand and racquet exceeds maximum KE of the rest of the limbs.
2) the KE of a particular limb decreases over time.
First, there is something wrong with your analysis - in order to discuss conservation of KE you can't just talk about peaks or maxima. It's area under the curve that counts. You could have a distribution over time that is very narrow and has a high peak, which then evolves into a distribution that is very wide and has a low peak. So long as the area under the curves is the same, KE is conserved.
As for points 1 and 2, Bhupaes has already answered them:
1) KE of H+R can exceed KE of rest of system (even if we are talking about total area under curve, rather than Max KE), since new KE can be generated from chemical sources within the muscles controlling the wrist.
2) KE of a particular limb can decrease since the energy in that limb can be either converted into non-kinetic energy stores, or transferred into other limbs.
Also, it would have been a good idea for the authors to conduct a momentum analysis to validate their methodology. If the momentum of the entire system was stable over time, then that would indicate a sound methodology. As it is, there are internal assumptions built into the modeling regarding the weight distribution of the athletes (they based this on data from other researchers:
These studies investigated the weight distribution of the human body, but there is room for individual variation. In the current study, the authors only knew the mass of the athletes, and the weight distribution of the racquets. They did not have access to information about the weight distribution of these two athletes. A momentum analysis may have allowed them to see how accurate their assumptions were. Either way, this last point is moot with respect to your question: even if the data and methodology are perfect, there seems to be nothing "magical" going on.
Good advice! Thank you very much.
Sennoc, I am not able to see beyond what I have already said... I now await your explanation, very eagerly, of course!
Thanks for pointing out that adding the peak kinetic energy components has little meaning.
It would be integrating the total energy under the curve that would give the actual energy contribution of each component.
And let's face it.
Most of the energy we generate from our legs, core and arms is wasted.
(Just like most of the energy from gasoline, diesel, coal and electicity is wasted.)
One can analyze how much energy is produced by our legs, core and arms.
But we really can't in an experiment precisely measure how much of that energy is going into the serve. Certainly the authors of that paper made no such claim.
It is interesting to graphically see that the legs, core, upper and lower arms fire sequentially.
But no quantitative estimates of the contribution of any one body component are possible from this study.
No proof of a build up energy, and then transfer through a kinetic chain is possible.
So what can we do?
We can maximize each of the components and hope to produce a good serve.
The value of pronation is every bit as important as the energy transfer that occurs through the differential and axle of a car to its wheels. (Even though the energy is being created in the engine.)
The value of pronation is every bit as important as the energy transfer that occurs in the derailleur and chain to the bicycle wheel. (Even though the energy is being created in the cyclist.)
(Well, actually "pronation" as it has popularly become known in tennis circles [as the combinatin of shoulder internal rotation and forearm pronation] is somewhat more important than a differential/axle or derailleur/chain because some energy is being created by the "puny" shoulder rotators and forearm pronators, although of course the real arm "energy" contribution is from shoulder and elbow extension.)
What we are left as tennis players is to try and maximize each component as much as Grand Prix teams try not to just improve their differential, but to eke out maximal horsepower, and minimize the coefficients of drag and lift in their racecars. Or Tour de Fance teams push the bicycle manufactureres maximize every component - even that "puny" little derailleur.
And yet the winning racing teams still need an exceptional individual to put that machinery to work.
I think we all need to go out and practice serving.
I think that neatly summarizes everything.
Theory is one thing, actually doing it is something else.
We are all biomechanically different. Try what works for you on the court.
Spacedriver, you are too fast
Kinetic energy (and momentum) are functions of mass and velocity. Mass is constant, so what really counts is velocity. Unfortunately, there are no laws of conservation for velocity. But they are for energy and momentum. So, if we are interested in velocity, we should talk about kinetic energy and momentum - and vice versa.
Of course, I should add areas. But I added maximal energies - because that's what all coaches are talking about - "loading". Maximum of kinetic energy means maximal momentum - and maximal velocity. So, by adding maximal values of kinetic energies, in reality I "added" (in some way) velocities of each joint. And there was something really strange there.
OK, it's time to explain. We have a kinetic chain. Let say it consists of 5 elements (bones): A, B, C, D and racquet. We are interested in A, B, C and D, because the last part is just a result of what we can do.
We have muscles at every bone, so we can transform their chemical energy into kinetic energy KEa, KEb, KEc, KEd.
Kinetic energies reach their maxima: KEamax, KEbmax, KEcmax, KEdmax. The velocities at KEmax are maximal: Vamax, Vbmax, Vcmax, Vdmax. You can add them ("loading"), and you will never see the value of maximal racquet's velocity.
Kinetic energy cannot appear nor disapper. At bone A, it can increase, because we transform chemical energy into kinetic energy. This process is (basically) finished when kinetic energy reach its maximum.
When kinetic energy at bone A decreases, what does it mean? It means that joint velocity is decreaseing. Velocity can decrease, but energy cannot disapper. It's obvious that it moves to the next bone.
And this is visible as velocities on the diagram too, you do not need to add areas.
Let's look at Ke Tr on the diagram. At frame 26 it has maximal kinetic energy and maximal velocity Vmax Tr. At the moment of contact kinetic energy is much lower, so velocity Vcon Tr is lower too. This difference is a trace of lacking part. If you add these differences (at every part of kinetic chain) to the sum of maximal velocities, you will see that the values will be sufficiently large.
Of course, as Spacedriver said, if we talk about kinetic energy it is more physically precise to add areas, but it's hard to do visually. But we can see this process in some way if we add maximal kinetic energies/velocities and their losses.
What all these things mean for a tennis player? They mean that "loading" is important, but there is also another thing to do, which is a huge source of racquet energy: losing velocities of early joints before impact! We have to stop kinetic chain just before contact!
Wait, "stop"? How? Motion at serves is fluid! No one stops there?
You can really stop your kinetic chain during serve because changes depend on time scales. Does Earth surface change? No, you say. Yes, will say a geologist. Something similar is happening here. And you can see that it works, here:
You see? In sufficiently short time scale, every part of kinetic chain (except racquet) does not move! This is the moment of energy transfer along kinetic chain. This is this magical "whip physics" some coaches are talking about. But they say "be like a whip", that's all. They do not give explanations, no conclusions about tennis methodology.
Now you understand the process. Loading is necessary, without loading there is no "whiping". But pure loading gives quite weak results, ca. 2/3 of real power. You need to stop your kinetic chain just before contact to reach maximum vaules!
How to do that? That's quite easy. Forget about circular motions before contact. The motion should be linear in the direction of the contact point which quarantees the maximal extension of kinetic chain.
"Lead the racquet with the butt" - do you remember this mindless rule for forehands and backhands? It has our process hidden inside. But if you remember this rule, you are unable to produce strong serves without another mindless rule. Now you know that you have to serve not along circular path, but in one direction - between you and the ball at the contact. This is what happens here:
Sampras serves into one direction. Curves are curves, not straight lines due to biomechanics of our body, but he definitely "hits into one direction".
You know the process, you can check how it works at other strokes. Look at Federer's "magically" powerful forehand. How does his hand move before contact? Why the contact is just a few cm in front of hand? This is our process: we have to maximally extend kinetic chain just before contact and the racquet will gain kinetic energy and momentum of the whole kinetic chain.
That's whole "magic" - without magical "be like a whip". Simple, don't you think? Powerful. Now you know, why. What's more, now you understand physical principles. You can use them at serves. At forehands. Topspin backhands. Slices. And, what's really important, you understand what you should do on the court.
Now you can go here as example - and laugh. Yes, you should be like a "kobra". You know why.
"We are all biomechanically different" - this is myth, gzhpcu. Every human has the same number of joints, the same muscles at bones, every tennis player is an object of the same physics. Principles are the same for everyone. It's you what you are doing with them. You introduce differences.
Unlike momentum, KE isn't always conserved. It can be converted into heat, sound, etc. Momentum is what's conserved.
Anyway, moving onto what I think is the gist of your post - you seem to be saying that using the body as a whip is the big magical secret to the serve.
This is hardly news...
See this thread for example:
The whole point of lower (and heavier) links slowing down, or stopping, is so that the subsequent links inherit their momentum. Since the subsequent links are lighter, and momentum is conserved, they move faster.
You are right. And in quantum world, momentum also can be violated Come on, does your arm break sound velocity during serve? Does it glow? We are talking about simple mechanics. We do not need precise numbers nor think about efficiency of energy transfer between bones in kinetic chains. These phenomena are not so important for us as players, they just happens.
I hope you didn't expect the evidence of chi energy (btw phenomenon described here IS what martial artists know as "chi energy").
It was obvious that the difference is in the way of thinking. "Be like a whip" is nothing new. What's new is that you see that this process generates a lot of energy and you know what to do.
Let's read threads here, on the other forums, let's watch famous coach movies. Everybody talks about "loading". About larger angles, larger velocities. Virtually noone talks about velocity decreasing!
"Be like a whip" means nothing for tennis players. If the tip of whip breaks sound barrier, the rest of whip doesn't move. How to apply this process in tennis? At first look, tennis strokes seem to be perfectly different! You won't find any answer between experts. All they do is talking "be like a whip". Now you see how to use whip physics in tennis if you think in different time scale.
Also, you know the fundamental process which works at serves, backhands and forehands. There are no differences between these strokes if you look at the phenomena we are talking about. That fact has huge meaning for a tennis player. If you find right way to use the extension of kinetic chain at, let's say, forehands, it's immediately much easier to use the same method at backhands and serves. Tennis will be the same, as complicated as it was, but internally, for you, it will be much much simpler and you will learn faster.
Energy transfer along kinetic chains is an universal phenomena. That's not important for us that the lower bones in kinetic chain are heavier. It just helps, but has nothing to do with the nature of the process.
If the lower portions of the kinetic chain were lighter than the subsequent portions, there would be no increase in velocity as the movement evolved. It would decrease.
Spacediver, data are different, phenomenon is the same.
In martial arts (they are all about strokes...), transfer of energy along kinetic chain is so important that there is whole philosophy created around it, known as "chi flow" (nothing unusual if you remember that this philosophy was created without knowledge about conservation laws, momentum etc).
In tennis, everybody talks about "loading". Sometimes, somewhere, somebody talks about "whip" as a special kind of tennis stroke.
This is totally wrong.
Both phenomena are important. Loading a bit more, but kinetic chain "freezing" is a very important part of every energetic stroke.
Also, if you concentrate on loading, you loose many possibilities. As example, look at this movie:
Good luck at working on 127 degrees angle. As an amateur player, faster you will hurt yourself, because your body will work "at the edge". So, loading gives you no possibilities. But there are possibilities to produce more power at serves if you concentrate on proper extension of kinetic chain in a very short period of time just before contact.
The same is with forehands. Look at Soderling or Del Potro. Both players use loading as the most important source of energy. Their motion is more circular, the paths longer, there is more time to produce high velocity. Their strokes are powerful. But this technique is very hard to learn. Due to huge moments of forces, it produces many injuries. Also, results are strongly dependant on the condition of your mind, you need more time to hit, your timing must be very precise. On "their days", Soderling and Del Potro win with everybody. But how many good days do they have? And what about their injuries?
Now look at Federer's forehand. Powerful compact strokes, more stable results - because the role of "freezing" is much more important here.
Look at serves in WTA. They are usually very weak. But look at the path of racquet's head motion during and after the stroke. It's almost always circular. Poor women are wasting huge source of energy because they were teached to concentrate on loading only.
More. If you understand that freezing is an integral part of every stroke, the end phase of stroke becomes more and more important. Look at my forehand:
You can compare it with Federer's strokes. My stroke produces more topspin, less power, but this is something you and me can change depending on the situation. The most important difference is at the end phase. Federer's racquet head is far behind his back, almost on his right side. You can't finish this way if the path of the stroke is not "linear". In my case, motion is still too circular. I'm not sure I want to make it more linear - forces are larger and I'm not the youngest one.
Kobra serves, whip or loose grip (like here) - empty words, more magic than knowledge, huge discussions everywhere. If you understand the nature of phenomena you use, if you know how to use them in practice, there are no strange rules. You go on the court and do your job. You know what to do.
Go on the court. Try some serves. Try to hit into one direction. Try to finish with the racquet at your body as fast as possible. And compare results.
Sennoc, i'm not sure if the loading you are referring to is plyometrics. if it is then it is extremely important for power or even your word Freezing. Plyometrics is a slight stretch of the muscle, then hold, with sudden reversal of the stretch into action.
basically, stretch(load), Freeze(hold), Reversal(explosion/muscle action).
Sennoc, this is a very illuminating discussion, even though I am shaking my head at how you got this insight from the energy graph - I would not have got it in a thousand years!
Various people have had insights that made sense - like realizing that some motions needed to be stopped in order to transfer momentum to the racquet. However, that would have been the wrong instruction, because you don't consciously want to stop anything. On the other hand, extension is a way of stopping - at least, slowing down abruptly - like a figure skater slows down and speeds up by extending her/his limbs, or retracting them to speed up. I think extension (at the right time, in the sense you mean) is an excellent cue that will have the desired effect of slowing down without damaging anything.
For some time, I have had some glimmerings of insight where I started believing that energy has to be built up in a way that is most convenient to the body, using big muscles, and then redirected appropriately. For the serve, one swings so there is ulnar support for the wrist (the weak component) and in the last moment, one redirects with pronation. For the forehand, use the big muscles to build up speed, and redirect in the last moment with the wrist. While in the serve I can see how extension can also happen in the last stage, I have to think about how it happens in the forehand and other strokes...
One point regarding the graphs - since they already depict energy level versus time, what do the areas under the curve mean? Energy is already an integrated sum - what am I missing? Just curious, it's been a while since I opened my last Physics text book.
Well, you are right this is an interesting discussion.
It remains interesting, and sennoc is overall right in his pushing of the idea of the total kinetic energy for the serve is produced from various components, and that pronation is more of a transfer of energy than much energy is generated from the small muscles that produce the pronation movement.
But unfortunately he is wrong in his analysis of the graph that it shows that the kinetic chain is responsible for adding a third of the kinetic energy into the the serve. (But in being wrong, he is in good company - Einstein was wrong when he pronounced "God does not throw dice.", failing to see the full implications of quantum mechanics.)
The total amount of energy produced by any one of the components, whether they be leg, trunk, upper arm or hand and racquet, is the area under the kinetic energy curve for each component. It is not the peak energy of of any of the components at one point in time.
There is just no way from this experiment, or from the graph, that one can determine the portion that any one part of the chain contributes to the serve.
The summation of the energy of the leg + trunk + upper arm + lower arm will exceed the energy of the hand + racquet. (We just aren't that efficient.) The summation of the area under all the curves will be the total kinetic energy produced by the tennis player, and will of course greatly exceed the kinetic energy in the ball measured an instant after impact when the ball is at its maximal velocity and spin.
On the graph we can graphically see that different components of the serve fire in a sequence.
That's all we can say.
As to quantify how much the kinetic chain adds to the serve, we are left as uncertain as Heisenberg.
We are left with our thought experiments about rockets and chi, just as Einstein's thought experiments about relativety were open to speculation until the day came when several came up with ways of designing experiments to prove the theory.
No, we have different degrees of flexibility, asymmetries between our left leg and our right leg. One hip might be higher than the other. Just to name a few. This should be taken into consideration in selecting the best technique to be used for a tennis player to avoid injuries.
By the way, Federer's forehand derives power from the stretch-shorten cycle (wrist laid back, hips turn first, racket lags). Where is this in your model?
Charliefedererer, you are of course right about those important details. But I wasn't so wrong as you think If there would be only loading mechanism, the energies should be simple sum. In this sum you can use maximal values for theoretical study - and you will never gain the energy of the stroke. Remember, that in reality you have kinetic energy at present moment only.
Bhupaes, area under curve is build from kinetic energy values at different points in time, so it represents the whole kinetic energy of particular part of kinetic chain during stroke.
How does this way of thinking work in practice? Let's look at some videos.
02:20 - look at her serves. What's the path of the racquet? Circular! She does not transfer kinetic energy along kinetic chain. She uses pure "loading". It costs her a lot. She could reach the same goal almost effortless. But she doesn't know about that possibility.
Now you should see the source of additional power at Federer's forehand
And what's really beautiful, we were talking about serves, not about forehands
This is my way of thinking about tennis: to find the smallest possible set of fundamental laws. Laws, not mindless rules, so you can use them at many strokes. You learn how to serve and then - your forehands are becoming better and better. Your forehands are better and suddenly you discover a small mistake in your backhands, so you can make corrections. Would it be possible without fundamental knowledge? Definitely not. For coaches, forehand, backhand and serve are different strokes. Yes, they are different in many aspects. But they are also the same at many important points.
Gzhpcu, look at the video above. Look for straight lines and kinetic chain extension...
what does extension mean? I keep hearing this term thrown about, but I always assumed it meant elbow extension in serve (straight arm), or wrist extension (so wrist is laid back during forehand).
sennoc, I'm still unclear on what you're getting at.
You seem to be discussing four issues, but I don't yet understand how you're tying them together:
1) circular vs. linear paths of motion
2) loading (as in stretch shorten cycle I presume)
3) whip physics (due to conservation of angular momentum)
4) kinetic chain freezing
What is your main idea? Can you explain it coherently?
btw as has bhupaes has explained, the kinetic chain never completely stops. If you freeze completely, the flow is destroyed.
I wonder if you ever got a chance to looke at those videos from Pat Dougherty, "the serve doctor". (Better yet, download his video at http://www.servedoctor.com/purchase.html - at $19 it's a bargain.)
The idea behind them was that the kinetic chain in a serve is not a series of movements from the legs to the trunk to the arms, but a series of complex movements. It is better to break down the chain into these complex movements (both for understanding the serve and for teaching purposes) than to break them down along anatomical lines.
So the basic components of the serve are the leg drive (up the mountain), uncoiling (the spring), the cartwheel (see-saw), reversing the bow shape (the pole vault), and pronation (hammering the serve).
If you are trying to improve your serve, or get a student to, it is much better to work on these elements, rather than isolate a body part. The body parts have to be used to create motion, and so you work on motions.
The kinetic chain can then be looked as the sequence involving leg pushoff, uncoiling, reversing the bow, cartwheeling and pronating.
Because all this analysis of the flow in the kinetic chain is an interesting topic, but it can't be used to improve your serve if you think of serving as a sequence of body part movements.
This discussion has been going on for a number of pages. Sennoc, could you please just state what you consider to be the bottom line?
So how about using body movements to forming your set of laws, which can be used across multiple strokes.
For instance in that video, the girl has no coil/uncoil (doesn't "wind the spring").
The serve and groundstrokes all share leg drive, coiling/uncoiling and "pronation" (which in common usage now includes the shoulder and foream rotation coming out of a bent wrist and lag in pronation until the last possible moment.)
One can also appreciate why groundstrokes are easier to learn than the serve: Because the stroke occurs in a different plane, it is not possible or required to add in the complex body movements involved in a "cartwheel" or "reversing the bow."
As I said, the most important idea here is that it is possible to reduce complex physics of tennis strokes into small set of fundamental and universal laws. This is the most important part.
Second, there are two methods of gaining energy at tennis strokes: by adding energies at every part of the kinetic chain and by reducing degress of freedom in kinetic chain, what enables energy transfer. Both are important parts, at every stroke, so if you work on loading, you should work on transfering too.
Third, athletic condition of amateur tennis players is significantly worse than pro players. Loading needs good athletic preparations. Freezing kinetic chains needs knowledge. If you cannot fully explore first possibility, you can explore second one.
Fourth, there are laws which are so fundamental that they work at almost every stroke. If you learn a stroke by repeating mindless rules, you need many rules for every stroke. Something is wrong, you do not know a rule, you don't know what to do. If you know laws, tennis is much easier to learn.
Fifth, the myths are gone. Gzhpcu, you said:
"Federer's forehand derives power from the stretch-shorten cycle (wrist laid back, hips turn first, racket lags".
Wrist laid back just prepare forearm for pronation. This is a part of loading.
Hips turn first - this is a method of extending kinetic chain in the direction behind the player. This is exactly what I'm talking about here. I never said that extension of kinetic chain is useful at contact points only. You can use it for better preparation of your muscles, if you extend your kinetic chain behind you. So, it is safely to say that kinetic chain extension is more fundamental law in tennis physics than loading!
Sixth, current methodology of teaching in tennis is wrong as long as it is concentrated around loading only.
Seventh, the extension of kinetic chains changes the role of many elements of the stroke. As you can see on the movie above (Federer's forehand), elbow bend after the stroke is very important, because without it the curvature of racquet's head path won't be "sharp" at contact point.
Eighth, if you change time scale, you can think about kinetic chain as something what can be frozen in time, except the last part (racquet). So, you better know what is an important part of the stroke. As example, let's look at Federer's forehand in the video above. At 05:00 you can see that the kinetic chain is fully extended and "frozen", the racquet is the only part moving really fast. This give you information about the best position of contact point.
...and so on. You know fundamental law. What you can do with it, how can you explore it - depends on you. I wrote just a few examples.
This is the straightening of the body/arm near the point of contact. As per this method, at this point, the angle between the foream and upper arm would be as small as possible; same applies to angle between racquet and forearm. This would "break" the motion as much as possible, and give a boost to the speed of the racquet. The angle probably varies for different people based on their physical attributes, and will probably never be zero.
Incidentally, there was a piece of instruction I've come across where the triceps played an important role in the serve, in extending the arm. I won't cite the source since that gets a lot of people into a tizzy , but you can guess who it is. I could never understand why, but now it makes sense, from the physics point of view - finally!
I believe the key is to accept that there are many ways to hit the ball, and that this is one way, which I personally believe is closer to the optimal than other ways.
Then the graph should be showing joules/second (power) as a function of time, no? Then an integral over time makes sense. I thought the graph was showing the accumulated energy in joules as a function of time, so the area would have units of joules.sec which doesn't make sense.
Anyway, this is tangential to the whole argument, and does not change the fundamental point you have made. Bottom line - I believe your theory is correct, sennoc. I just hit against the wall for two hours keeping these principles in mind, and everything makes more sense!
Edit: It goes without saying I have a long way to go, and hundreds of more hours of hitting to do!
No, the graph is not showing the accumulated energy. It is showing the kinetic energy at that one moment in time on the y axis, and time in the x axis.
(If it was showing accumulated energy, the plot of energy would always be going up, until it plateaued when no more energy was being generated. You actually could do this plot, and then use the slope (derivative) of the curve to judge how rapidly the energy was being generated.)
Way too much blinding us with science, guys. I prefer serving with my head still attached.
Hi Charliefederer, here's my reasoning. Since kinetice energy is measured in Joules, and it's a plot of Joules vs seconds, I assume it reflects the current energy level of the link in question. Somewhere inside the link, muscles are generating force and moving resistant mass whose position is a function of time. This speeds up the mass and at any point in time, it has a kinetic energy that is, abstractly:
which is again:
where v is the velocity of the mass as a function of time, f is the force applied to it as a function of time, and t is time. The y-axis in the curve is a plot of this integral within the limits (0, t), and has the units of Joules, and the x axis is time (t).
Now you can see why I'm asking the question... purely out of curiosity, not related to the main point of this thread. Okay, I got that off my chest, no more about this topic from me!
High Speed Video Measurement of Racket Spin from Pronation, Non-Serving, 90 d. Bent E
High Speed Video Measurement of Racket Spin from Pronation, Non-Serving, 90 d. Bent Elbow.
There has been discussion of rotation produced by pronation and the upper arm rotation powered by the shoulder. This was a revelation to me as I had never thought of shoulder rotation as also contributing to this familiar rotation, obvious now.
Nicely clarifying the distinction between pronation and shoulder rotation:
I wanted to get an idea of the magnitude of the pronation without the shoulder rotation. Not serving, but just standing in the kitchen, I bent my arm at 90 degrees to keep the shoulder out of it. I viewed the top of my racket frame end on with a Casio Ex FH100 camera. ( f/3.2, ISO 3200, 240 fps, 1/800 sec exposure time, a 100 watt incandesant bulb near the racket).
1 – Pronation from Stationary Start. I rotated the racket by pronation as fast as I could from a stationary start. Sometimes I just rotated it and other times I did a bit of a follow-through downward to make it flow a little better. It felt awkward, not as in serving.
The highest velocity on the side of the racket frame from spin was about 10 ft/sec.
2- Pronation with Stretch-Contraction. During serving maybe the pronation muscles receive a stretch and operate in a stretch-contraction mode. ? To look for a stretch-contraction effect I oscillated the racket as fast as I could in a clockwise - counter-clockwise rotation, plyometrics style. It seemed to rotated back and forth only about 90 degrees or less. I also tried to quickly rotate clockwise (load) and then quickly pronate (contract). These motions were felt very awkward. The spin did not increased and was somewhat lower. To test these stretch-contraction motions would need some training, no conclusion.
Conclusions. The magnitude of my pronation, non-serving and with a bent elbow to exclude any shoulder contributions, was measured at about 10 ft/sec at the racket’s edge. If a pronation of this magnitude occurred just before the ball is struck the string velocity would vary by 20 ft/sec across the racket face. For a RH server the string velocity would be lowest on the right side of the racket when the ball is struck. I don't know what happens on a serve but pronation looks like a considerable factor.
Please consider any safety issues. I hope some others who are more athletic and faster than I & have a high speed camera might use this easy test and post some pronation rotation rates. For better video, wear a shirt that contrasts with your tennis racket, white shirt for a dark racket, etc. It is convenient to use the racket frame thickness as a scale. Find the highest rotation rate. Measure how far the racket moves in one frame time. Multiply by the camera’s frame rate to get the velocity. My racket frame measured 1” and it moved one racket frame thickness between frames. 1” X 240 fps = 240”/sec or 10 ft/sec.
Pronation and shoulder rotation rates could also be extracted from high speed videos of pro players by looking at the difference between the rotation of the upper arm and the wrist or racket.
If we could only have seen the ball and racket on this one!
Corrections! 240"/sec is 20 ft/sec!
Can you please post your own video about pronation measurement? If it’s good I buy this camera (Casio FH100).
I got my camera about 6 months ago and like it very much.
Recently I went to try my first post on Youtube. I went to register and ran into some mess where I had to get a Google account to use YT. My wife has a Google account and I think that it would recognize my email address ..and expect her password...to FAQ.... Google now owns YT........ I was very annoyed as were many others that I found on an internet search. No YT account.
To see the YTs of Casio Ex FH100 go to YT and search Tennis Casio Ex FH100 or Tennis Casio FH100 and you will find many examples. One problem with YT videos as I understand it is that you cannot do frame-by-frame because of the video compression used at YT. You can stop start quickly but skips frames. Most YT Tennis are not well taken YT - use <1/2 the frame to view etc. There are many golf users.
As for my video this afternoon. I rushed through it and the racket is hard to see against my dark shirt so I would have to take it again and find a way to post, not YT. You see the end of my racket rotating, I stop it and step frame-by-frame on my computer using Quicktime.
Casio Ex FH100 is about $250 and a 16Gb faster HCSD card probably $40-50. Tripod, extra battery (internet), I have small SD cards too as loaners.........Quicktime on the computer. Look at dpreview for info, Specs, etc.
Serious tennis, stroke analysis, high speed video...................
Thank you very much for information about camera. Looks like I'll buy it.
The whole issue of the kinetic chain and energy transfer in the serve from one link to the other by acceleration - then abrupt deceleration, causing another link to get stretched more in its movement, thereby enhancing its own acceleration is pretty complicated. The model involves rotational energy around a number of different axes (joints), as well as linear energy, terminating at the critical point: the racket face at impact.
All components need to be timed correctly for maximum transfer of energy, otherwise the movement is wasted and contributes nothing to overall speed.
No wonder the whole issue is still being debated and not yet clearly understood.
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