Hi all, Stanley wrote:

At 02:10 AM 12/16/2004 -0500, you wrote:
>Eric,
>
>You wrote:
>
>>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.
>
>Thanks for explaining this to me. But I am now further confused. My limited
>knowledge on the subject is RXF.
>Radius times Force. Since we are not changing Radius, we must be changing
>Force, which is what I seem to understand from your analogy. Since force is
>a function of body weight, and body weight has not changed (unless too many
>nuts have been eaten), where exactly does this increase in force come from
>
>\

My nutcracker analogy was not good in explaining the situation you asked
about.  The analogy was good for understanding why position of parts is not
the best way to look at stress.

To answer your question: the magnitude of force or body weight is not
changed, but the point of application of that force is changed.  When the
point of application of the force is changed the moment arm (R) that force
has about an axis is changed. So R changes even though the position of the
calcaneus does not.  The concept of center of pressure may be helpful in
understanding this.
Fuller, E.A. Center of pressure and its theoretical relationship to foot
pathology.
J Am Podiatr Med Assoc. 1999 Jun;89(6):278-91.

I was channel surfing a while ago and came across a NASCAR interview.  The
driver was saying that he had to touch the breaks to shift the center of
pressure toward the front wheels so that he could get better traction on
the wheels that directed the turn.  The weight of the car is the same, all
four wheels are on the ground, but there is more weight on the front wheels
after touching the brakes.  If you think about bicycyling it is possible to
hit the front brakes so hard that you can actually lift the back wheel off
of the ground.  In this instance, just before the wheel lifts the position
is unchanged, but just after the wheel lift the position is
changed.    After standing on a varus wedge, the position of the force is
different even though the position of the foot did not change.


Cheers,

Eric Fuller

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