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Old 03-03-2011, 09:40 AM   #61
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Join Date: Oct 2005
Posts: 34,511

Originally Posted by sennoc View Post

The image above is totally wrong. As far as I remember, it was published in a popular tennis magazine, am I right? Do not believe in words and numbers you see in popular magazines and newspapers. Believe me - in my country I work as a science journalist, I spent my last 20+ years as an author and an editor and I know the quality of journalists' work. Perfectly know. And it's low.

This image is EXCELLENT. It perfectly shows one of the most important sources of energy in modern tennis strokes: energy transfer along the kinetic chain.

What do we see?

We see that in a sufficiently short time, kinetic chain does not move! Something wrong is here, don't you think? We have generated so much energy at many parts of kinetic chain, but at the contact everything seems steady. Where is kinetic energy of our legs? Shoulders? Elbow? It cannot disappear. It moves into the only moving part of kinetic chain: into the racquet. This energy transfer produces ca. 30% of stroke energy. Amazing, don't you think? Your image shows a fundamental physical phenomenon which was never discussed here. Until now you were talking about the rest (which is 70% only).

So, if you want to add "power" to your stroke, you need to hold your kinetic chain. "Hold" relatively, of course. How can we do that? By extending kinetic chain into as straight line as possible. Try to serve by moving your palm along a huge circular path, you will see it produces significantly less power. You can use your legs, shoulders, arm, hand, everything you want - and this kind of stroke will never produce powerful hits. Why? You can't transfer energy this way, so you do not have access to 30% of kinetic energy.

Another important part of the energy transfer along kinetic chain is time. Power is not only a function of energy, but also a function of time. If you can transfer energy of your kinetic chain in a very short time, you produce powerful strokes. The longer interaction, the less powerful stroke.

So, there are two important physical phenomena here: effective energy transfer along kinetic chain and short time of interaction - as short as possible. In practice, if you know how to use the first phenomenon, you do the second one.

In your picture, we can also see the work of ulnar deviation and the pronation of forearm. Ulnar deviation moves racquet head significantly, so it is obvious that it is an important source of energy. Pronation starts at frame 5, when the arm, forearm and racquet are almost straight. At this position it cannot produce significant amount of energy. Sorry, that's physically impossible.

Next excellent picture - but you do not describe the most important things here.

Ask yourself: why the main, usually vertical axis of human body is tilted here? Federer is tilted ca 45 degrees. Why? Maybe he doesn't know how to serve?

The answer is simple: internal rotation of upper arm. If you want to use it as a source of energy, you have to find a solution: should I hit with straight hand, as high as possible, or should I hit lower, but tilt my body to extend time when upper arm can increase energy of forearm?

Let's look at Sampras.

Do you see the energy transfer here? The path of racquet's head is not circular here, it's narrow! This is the result of extension of kinetic chain at contact into one direction. Of course, the line is not straight, but it is as straight as it is biomechanically possible.
Interesting. The Roddick and Federer photos also show that the ball is hit down on the first serve.
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