Wednesday, February 1, 2012

A lesson with Crystal Huang: too many degrees of freedom

This is my second blog entry in this series about learning table tennis technique with Crystal Huang. For the first entry and a description of the motivation and goals of the series, see here.

The human arm is said to have 7 degrees of freedom. At the shoulder, we have 3 of those 7 degrees: up/down (i.e. pitch), left/right (i.e. yaw) and the ability to rotate (aka roll). The wrist is independently capable of pitch, yaw and roll as well. That makes 6. Finally the elbow has a single degree of freedom (pitch). So if you like, there are 7 positional parameters to the system.

When you perform an arm stroke, the number of parameters in the system multiplies. For example, we now have acceleration of the various parts of arm to take into account. Then factor in timing, and the position of the torso, center of gravity and feet placement. Add to that subtleties of ball contact and feel, and adjustment for the speed and spin of the incoming ball and where you intend to place the return, and you can see how sometimes it's difficult it is to reproduce a stroke even when we have it demonstrated for us up close and live. Anyway, that's my excuse for being a poor learner.

In the following video, I'm trying to learn a new forehand push stroke. The idea is to be able to receive the underspin (or sidespin) ball with the forehand side of the racquet, control and send it to different positions on the table reliably. I already have a forehand push I've been using for years, it's just not a good one.

The day before the video was recorded last weekend, I had about 15 minutes of instruction on the new stroke. Unfortunately, I confess I didn't truly "grok" or properly understand how the stroke should be executed. Even having the correct stroke demonstrated to me didn't trigger how it should feel. I was still not executing it well.

As this video clip shows, Crystal (after shaking her head) ended up coming around to my side of the table and moving my arm for me. This reduces the number of degrees of freedom of the arm to zero. And only then did I really understand what the stroke should feel like.

The video also contains captioning that I've added of Crystal's instructions during the session. This might be of interest to other table tennis students.

The video clip ends with me using the new forehand push to return service to different parts of the table. And then playing out the point.

Link: here

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