Hi all, Stanley wrote in response to Kevin's comments.
At 01:03 AM 12/15/2004 -0500, you wrote:
>Also please excuse my lack of physics knowledge, as I worked hard to get my
>C. But I remember RXF as torque and the moment as the rotational weight. I
>am trying hard to put together what you are saying, and it doesn't seem to
>make sense. You say that since there is a very small positional change with
>orthotics on the frontal plane, the motion of the joints doesn't matter as
>much as the moments that are changed with the orthotics. To change the
>moment, you have to change either R, or F. Since the force is constant
>(body weight), the only way to change R is by moving the part away from the
>axis. Since little or no motion is occurring, then how do we change R?
In your equation R is the distance from the line of action of the force to
the axis. The part does not have to change its position for there to be a
change in the location of the force (or line of action of the force).
>Craig found there is "no correlation between changes in the pattern of
>rearfoot motion and symptom reduction". This confirms that there is no
>change in rearfoot motion, so therefore there is no change in R.
>
>Can you explain this contradiction to me?
There can be a change in R, and moment, without a change in position. I
have always liked the nutcracker analogy. Place a nut so that it touches
one arm of a nutcracker. Now slowly move the other arm so that it also
touches the nut. (This is analogous to the maximally pronated
position.) Now squeeze harder. The position of the nut has not changed
relative to the arms of the nutcracker. The force, and stress, on the nut
has changed.
Good eating,
Eric Fuller
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