In our Universe, energy cannot appear from nothing. My question is: where does 1/3 of the stroke energy come from?
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.
And then...
...energy decreases. Nothing unusual.
Really?
In our Universe, energy cannot disapper.
I am sure that I do not "know everything". However your equation is wrong: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?
Ke H +R belongs on the left side of the equation. You completely neglect the kinetic energy of the rotating racket head.Ke LL + Ke Tr + Ke Uarm + Ke Larm = Ke H+R
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.
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.
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 as vectors.
I think all this magic stuff is just misunderstanding.
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?
The mechanical model was adapted from Clauser, McConville, & Young (1969) and Zatsiorsky, Seluyanov, & Chugunova (1990) taking a 28 point model definition into consideration (Figure 3). Twenty three points were from the body (foot toe, ankle, heel, knee,
Good advice! Thank you very much.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.
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?
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.
I think we all need to go out and practice serving.
It is interesting to graphically see that the legs, core, upper and lower arms fire sequentially.
QUOTE]
I think you will see the same graphics while walking, biking, swimming etc..
That is to say from athletes with the correct motoric skills. For me the real interesting thing would be to compare these grahics with the graphics of lesser skilled athletes.
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.
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:
http://tt.tennis-warehouse.com/showthread.php?t=345688
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.
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.
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.
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."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.
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.
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?
Really?
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.
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.
http://www.youtube.com/watch?v=8JcldQ78M_k
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.
Next video.
http://www.youtube.com/watch?v=pPLmCqGIotM
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).
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.
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!
Way too much blinding us with science, guys. I prefer serving with my head still attached.
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.)
I think some of the misunderstanding is perhaps due to misidentification of biomechanical definitions:
Pronation alludes solely to rotation of the radioulnar joint at the elbow around a superoinferior axis, and is confused with internal shouder rotation which occurs at the shoulder.
Seems to me that the active internal shoulder rotation results in the passive pronation if the arm is kept loose...
To be clear: we are talking about multilevel movements around two separate joints - shoulder and elbow
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 20 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 20 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 40 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 20 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!
http://www.fastecimaging.com/video/highSpeed/TennisServeBackViewE.wmv
Chas Tennis,Toly,
I got my camera about 6 months ago and like it very much.