Wednesday, December 17, 2008

Rotation Part 2

Excellent research from last year's American Society of Biomechanics meeting from a few Japanese researchers, "A MECHANICAL CAUSE OF BODY ROTATION ABOUT THE VERTICAL AXIS IN BASEBALL BATTING". The authors concluding, "Such a rotation of the entire body could only be generated by the external forces acting eccentric to the center of mass and by the free moments acting on the body. The results clearly indicate that the rotation of the body during baseball batting is generated primarily by the moment of the ground reaction forces acting on the legs around the center of mass".
This drives home what we high school A.T.'s have always known, that is the transverse plane (rotation) is primarily driven by momentum, whether it's swinging a bat or throwing a baseball.
Yet, at last year's combined sections meeting of the APTA, a statement was made that "what is causing the torso to rotate is the muscles, so we have to work on the obliques, the core itself, and hip stability".
Now go back & read Vern's "Rotation is bad?" post from December 13 and decide for yourself who is right.


Jerimiah said...

Simple drill should solve this discussion. Swing a bat using only trunk musculature, then swing normally.

Here's another perspective from someone smarter than me.

Kev said...

Would if be fair to say that the obliques/core/hips strength should be addressed but for a different reason than what the comment from the APTA implied?

In other words, if I interpreted that comment correctly, the APTA comment suggests strengthening because they believe rotation starts in the hips/torso as oppossed to what Joe, Vern, and the person in the blog said in Jeremiah's link above?

I hope I was clear in presneting my question!

JH said...

Ok, I'm it...I'll play devil's advocate on this one.

As we all know, momentum is a mass at a specific velocity, and changes in velocity either an increase or decrease is acceleration or deceleration respectfully. I agree that the acceleration of a baseball swing begins from the ground and is transferred up through the body and hopefully through the bat. However, a good baseball swing is accelerating through contact with the ball and then once contact is made, deceleration begins in the way of the follow through. In doing so, their is very little time for true momentum to occur as momentum is a constant velocity. At a measureable 1500-1700 feet per second, their ain't much time for a constant speed. I would argue that their is a steady ramp up to a peak speed (ball contact) and then a steady decline in speed.

i would argue that the acceleration is ignited by the ground force reaction from the feet upward, then a snowball effect occurs as acceleration increases as more and more links in the kinetic chain assist in producing ever increasing acceleration until ball contact is achieved.

As Jerimiah has pointed out a great test in his post i would suggest trying the same test only this time don't swing the bat, keep your torso stationary and see how much leg/hip pelvis velocity can actually be achieve. Hardly any. The ability to produce a large amount of ground reaction force without the cooperation of the links above the pelvis I would say, is minimal. So in saying this i don't think it's as easy as seperating the upper body and lower body.

Also don't forget the law of angular momentum when discussing the baseball swing. Just before forward transverse plane acceleration begins their is a backward loading of the trunk, scap and hip in which is used to assist in producing a greater GFR.

Don't shoot me, I'm just thinking out load.

JH said...

how does some bicycle kicks, swimming, some volleyball, and kick volleyball fit into this equation? Their are movements that occur rotationally where their is no body part touchuing the ground to produce a ground force reaction.

Jerimiah said...

I think your analysis is pretty dead on JH. I would say that it still applies with your other activities, you just are not able to generate as much force because once you leave the ground the ankle is no longer able to generate force, it can only stabilize to allow the knee to generate force. Or in the case of swimming because of the fluid nature the ankle is not able to generate as much reaction force.