confusion: backward trunk lean and shoulder rotation during backswing (brian gordon)

[spacediver]

Hopefully I will not confuse you with this question!

In the second backswing article, you write:

The twisting rotation of the upper trunk is not, however, as simple as it appears. It's effectiveness in driving the motion of the hitting arm, especially late in the back swing, is related to the angle of the trunk at the end of the backswing.

As we have seen, the server's trunk inclines or leans backward (as seen in a back view) during the windup as a function of the knee bend.

This lean increases during the back swing, causing a shift in the angle of the torso. The increase in this angle that can be difficult to detect with the naked eye, or even with the use of video, but can be measured in 3D dimensional analysis.

Although it is subtle, this increased lean, or tilt, is critical in driving the motion of the shoulder. It is what allows the upper trunk twist to drive the shoulder forward and upward. Without the tilt there would be no upward motion, only the forward component. The upward component, however, is a critical part in creating racket head speed.

Now this is where I am confused. Based on your other articles in the series, I take backward lean to be lateral flexion of the spine:



However, it seems that spine extension is what you should be referring to:




It is this articulation of the body that will produce the angle necessary to drive the shoulder upwards (although I sense that once the spine is already in a position of lateral flexion, you need a more complex combination of spine articulations to bring the torso to face upwards more).

In the context of the quoted text above, I have a feeling you meant to say lateral lean, since in this article you reference a backward lean as seen from the back view; yet in the wind up article, you reference the backward lean as seen from the side view. Either that, or you have changed the meaning of "backward lean" depending on the context of the view.

May I humbly recommend avoiding this terminology? The words "backwards" and "sideways" are confusing for the following reasons:

There are three potential meanings of the word backward or sideways that the reader can conflate:

1) backwards relative to the "cardinal axes" of the body (backward lean to me conjures up an image of spine extension).
2) backwards relative to the net (also confusing since the orientation of the trunk relative to the net changes over the course of the serve)
3) backwards referencing the view from which the serve is observed; i.e. contrasted with side view or overhead view.

Much better to take some time to define the spine articulations in advance and then reference them when needed.


That said, I can understand the rationale behind definine angles relative to a particular view. For example, in your data module in the segment joint angles section, the lateral lean is defined from the backview, and this angle is more of an "emergent angle" based on all the individual articulation angles. Yet this emergent angle is of great diagnostic and computational importance when analyzing three dimensional data.

Perhaps my recommendation is not so sound afterall! (though I think I still caught an error where you should have used lateral lean rather than backward lean in that quoted text).


Really enjoying your series. They're keeping me up very very late...

[BrianGordon]

"In this process I found myself walking a fine line between strict mechanical precision and making the work comprehensible to players and coaches with a wide range of backgrounds. To some extent this may have been a zero-sum game. Some readers probably found it lacking adequate technical specificity, while many others found it still quite complicated."


I'm glad you are getting into the series - I've quoted myself from the end of the last article in the series where I describe the problem I had in expressing the concepts in a way that could get past the editor - JY - I suspect you may be one of the former readers.

Yes, backward in this context is what in other places was refered to as lateral - but it was by design in that case.

In that series I attempted to describe things in a way that would be most understandable from a tennis perspective using concepts and terminology from my several decades of playing and coaching experience and largely using JY as a barometer.

Often, I chose specific terminology, that while perhaps not strictly consistent, I (and John) felt would be most understandable - the trunk being an interesting case because lateral and/or forward/backward, when due to bending of the spine, can be from a variety of anatomical movements in different phases of the swing (i.e. lateral is not a spinal function at the end of the wind up, transitions to slight hyperextension then to slight lateral deviation during the back swing due to twisting of the trunk, and is due primarily to lateral deviation of the lumbar spine near contact).

When I deviated from convention for the intent of enhanced tennis comprehension I tried to add terms like "from a back view" or "as a function of knee bend".

So... I suspect you could find a variety of "inconsistencies" in my effort to take the material out of the classroom and put it on the court - for those I apologize and now I suppose John will reprimand me - thanks a lot!

[johnyandell]

I'm the one in need of reprimand...in my own efforts to try to understand I probably created many inconsistencies. We'll try to get better in the fh articles, right Brian??

[spacediver]

thanks for the clarification brian (btw I'm not one of the former readers - I just joined this site recently).

I empathize with the challenge of maintaining clarity while giving details about angles and body articulations while making it comprehensible to a wide audience, in what may be one of the most complex biomechanical applications of the human body.

I'd also like to express that your series has been a great educational inspiration for me. It's motivated me to learn and look up things like "spinal flexion" and "shoulder abduction", etc.; as well opened my mind up to some very cool biomechanical principles that I previously only intuited kinesthetically(such as the idea of contracting a muscle while it is being stretched by a counteracting force).