Posted by: tom.guerry (tom.guerry@kp.org) on Mon Jan 21 15:40:41 2008

Again, I think that analytically speaking, boundaries need to be clarified between different
domains, mechanical vs biomechanical, and terms need to be defined.

One of the simplest ways "linear momentum" is measured in the swing is by the 2D
progress of the body's center of mass or center of gravity (in the case of being on the
earth).

Motionanalysis of baseball swings using the bellybutton as a proxy for center of mass
supports the principle that ideally the center moves forward does not fall back before
contact. See motionanalysis at peavy's site, for example.

From golf it is well accepted that a "good swing" requires that the center go forward as the
swing starts down or even before and not fall back before contact. if the center does fall
back before contact (reverse pivot), then the swing acceleration is less and the swing plane
slices outside in more.

So weight shift could refer to what the center of mass does OR it could refer to what
proportion of the weight is being carried on one leg vs the other.

This is where Yeager's data is helpful using definitions of center or pressure of the feet
and how force shifts during the swing, for instance having force on the front foot max out
during the swing at about 125% of body weight.

From the ground force point of view, Yeager's swing model phases are:

back leg load, back leg push, front leg block, front leg push.

Whatever your swing model or theories about momentum, these data need to be
accounted for.

As mentioned in other threads, it is also important to understand how weight shift
interacts with hip motion and in turn how this effects spine angle and spine tilt in concert
with what is happening at the upper end of the spine and the coiling/uncoiling of the
torso.

This gets into the topic of "posture" and the setup and early portions of the swing as well.
While the overall picture gets very complex, the approach in golf after Hardy based on
understanding distinct swing patterns provides a way to understand things best.