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Re: Re: Jack - Leg Power


Posted by: Mike Myers (mike.myers@bell.ca) on Fri Sep 17 11:27:11 2004


>>> Jack,
>
> I have been studying your general assertion that both legs contribute evenly to rotation around a stationary axis.
>
> I disagree. The rear leg always supplies more power.
>
> Reasoning is as follows:
>
> Due to the relative position af arms and bat (to legs) throughout the swing, the body's center of mass, from intitiation to contact is always closer to the rear leg. One can demonstrate by comparing scale weight on both feet at various points in a good swing.
>
> The body's axis of rotation is equidistant from both legs.
>
> Torque = F x d (from axis of rotation).
>
> Since the axis of rotation does not coincide with the (body + bat) center of mass, the rear leg must drive more mass, and maintain the same acceleration as the front leg. Thus, the rear leg must supply a greater force component than the front leg.
>
> The fact that both legs exhibit rotational symmetry around a stationary axis of rotation, does not indicate both legs contribute equal torque, or power. <<<
>
> Hi Mike.
>
> Which leg supplies most of the power for body rotation has been the subject of much discussion on the Board. I do not suggest you are incorrect in claiming the back-leg does most of the work. It may very well be true with some mechanics. However, with my students (and my own swing), I never mention the back-leg. They obtain their optimum rotation around a stationary axis when they have ample flex in both legs and use the extension of the lead-leg to drive rotation. While concentrating on rotating and extending the lead-leg, the back-leg always seems to rotate to the proper position without even mentioning it.
>
> My question regarding the back leg has always been; if the back-leg is supplying most of the force – how is it doing it? It must not come from extension, at least it doesn’t with a rotational hitter like Bonds. The bend in his back-leg remains about the same (or increases) during rotation.
>
> The rotation of the knee toward the pitcher does not necessarily induce hip rotation. In fact, many coaches teach rotating the back-knee toward the pitcher while keeping the hips closed before opening the hips and then initiating the swing, --- So, what are the muscles in the back-leg doing that drives more of a batter’s rotation than the extension of the lead-leg? – How much of the back-hip rotating forward is actually generated from hip and torso muscle contractions?
>
> Mike, as I stated earlier, you may or may not be correct, but these are questions I have long pondered.
>
> Jack Mankin
>

Jack,

Barry Bonds has an incredibly fast swing (1/8s). With such speed, conventional video (30 frames/s) is of rather limited probative value.

The question is:
Is there ever a point in Bond's swing where his back leg extends?

Yes. By chance, you can see it between frames 3 and 4 in the following:

http://www.usatoday.com/sports/gallery/bonds/flash.htm

Another important consideration is very strong upper body strength. The more forcefull the upper torso (shoulders and chest) initiation, the more forcefull the lower body (hips and legs) reactionary counter force. So, even if Bonds' back leg does not extend, it can be reasoned that just preventing/reducing intial backward rotation is a feat in itself.

As proof, sit in a swivel chair, suspend your legs, and quickly rotate your upper body in one direction. You will find the chair beneath you rotates in the opposite direction.

Mike.

>


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