LT/LH and Club Foot - same theories?

While I am not an ME of either Tom or John's stature, my take on it is that the red line is static in its location(ie: Duckett's dot does not change location) so as the angle of the bone either increases or decreases, so does the orientation of DD as represented by the red line. Put another way(in case my explanation sounds like so much gobblydgook ) Remove the red line and replace it with a point on the extensor process and a corresponding point on the ventral aspect of p3. Those loci are, and remain, fixed. If you accept that concept/construct, then it becomes easy to see why the line would move as the angle is increased or decreased. And that's my understanding of it but ymmv.

Tom explains it so much better than I do. That's why he gets the big bucks.

Wait, when did the DDF start inserting on the Extensor Process

Interesting. It's my understanding the GRF will move in accordance to the length of the extensor arm. Increase force to the flexor moment must increase length of the extensor arm which will move GRF forward.

We will have to assume on a club foot their is already more force in the flexor moment and an increase in the extensor arm length? So in your club foot diagram the center of weight bearing would move forward.

When we mechanically wedge a foot, we decrease force to the flexor and decrease the length of the extensor arm which will bring the center of weight bearing back. Which is probably why we don't see the desired effect when adding wedges to crushed heels. Fig 2 shows this: http://www.equipodiatry.com/AspectsofFunctionalAnatomyoftheDistalLimb.html

Now you are speaking of moving the end points or perimeter of GRF which obviously would affect the center of GRF distribution just like moving the end points on a lever.

Well which is it? First you say center of weight bearing moves back now it's obvious increased flexor force and lengthening of EA moves it forward.

Bingo. Duckett's Dot is an anatomical location and irrelevant when discussing the pressure point (net GRF vector). The pressure point is constantly moving around in the foot...when standing during weight shifting and especially during ambulation. The moments about the COA have nothing to do with Duckett's Dot.

The location of the pressure point can be manipulated by trimming, shoe fit, shoe type, etc. It is almost like trying to identify the location of an electron...it just isn't feasible. With both (pressure point and electrons) have the ability to say where it was at a given point in time, but there is no way to track or predict either of them precisely. That being said, we can predict generalities. Ex.: Move a shoe forward, the pressure point moves forward. Move it laterally, the pressure point moves laterally, etc.

The magnitude of the effective pressure point is unchanged in the static (unless you add weight to the horse), the lever arm of the flexor moment is also a fixed length, each foot varying slightly. This only leaves two variables: the tension of the DFT and the corresponding location of the pressure point.

While setting a shoe back, extending heels, adding heel wedge etc., moves the PP back, thus decreases the extensor arm length, which subsequently requires less tension in the DFT. Simultaneously, it increases the pressure in the back of the foot. It has been said many times: Everything we do is a trade off. Decrease force in one area, adds force to another.

Bear in mind, however, that any model or analogy that we use for comprehension is incomplete. Although the models and analogies can help us better frame our understanding, what goes on in the capsule doesn't care about our models and analogies.

I never said that.

The only thing obvious is that you aren't seeing the center of weight bearing as being a point fixed in the bone.

Y'all get it all figured out for me and let me know when you do. I have to go cut firewood.

Jeez.,David. Lighten up, I was just 'funnin', hence the at the end of my comment.......

Yes. This is very easy to measure with a force plate.

Um, yes they do. The COA is at the distal end of P2. Ground forces do not directly interface with this center. P3 exchanges weight bearing and ground forces with P2 through the coffin joint. The moment between the center of P3 and the center of rotation at the end of P2 is fixed by the geometry of the joint.

Therefore the GRF moments acting on the foot act on P3, THEN on P2.

Weight moments acting on P2 are transmitted to P3, then weight moments interact with GRF moments.

All you have to do is drop a plumb line down from the center of p3 to see the dot actually moves forward, on the foot, as the angle of p3 is increased.

Um, Irrelevant. I'm sure we could find a way to incorporate C3 vertebra in the equation but it would be equally irrelevant. The pressure point is dynamic the anthropogenically ordained Duckett's Dot is anatomically fixed.

Extensor moment = flexor moment.

(Magnitude of pressure point) X (horizontal distance from pressure point to COA) = (magnitude of tension in DFT) X (distance from center of DFT over navicular to COA)

...Unless you are just trying to find a way to expand the equation to somehow make the dot have a place somewhere therein. We might can do the same with the mass of fecal material carried and its distance away from the dot. . I do say that in jest Tom, but seriously, the moments are balanced by the above equation and the dot is just not a factor. What applicable relevance any of this has with our daily routine...my jury is still deliberating.

Well that's where you're messing up. The external anatomical reference point for the dot (~3/8" behind the tip of the frog) is only on a plumb line when the line passing through the connections of the extensor and flexor is plumb. Otherwise, it ain't.