Kevin,
I love the 2x4 example. I'll remember the analogy as
a teaching aid. I was reading what Bart had said
about adequate communication of stiffness requiring
"...representation with curves and diagrams...". It
seems that he is correct given the fact that stiffness
is a sum of many factors (joint architecture,
ligamentous orientation/elasticity, muscle activity,
etc.). Each of these factors throws in sub-equations
that may be quadratic to logarithmic. Sometimes a
picture is worth a thousand words (or a thousand
equations). Who can guess how many equations are
responsible for the simple text in this message? So
why not use curves and or diagrams to communicate
clinical stiffness? Don't we already do this with
evaluation of biphasic pulses and peripheral pulse
curves? How about EKG?
If one were to measure just the COM pressure under the
1st MTPJ, I am sure that one could derive a curve.
Example: X(COM pressure) over Y(0-100% of contact
phase). Certainly patterns would evolve that would be
just as telling as an increased Q-T interval or "soft"
P wave.
I think you have me sold on the "stiffness" term. Now
I think we should evaluate the unit or method by which
we can communicate this variable.
Respectfully,
Jay
PS. If you asked the same child in your anaology to
walk back and forth across your steel beam, might that
represent supination and pronation?
--- Kevin Kirby <[log in to unmask]> wrote:
> Bart and Colleagues:
>
> Bart wrote:
>
> <<As for you reply on this mailbase, I agree with
> you when one is working
> in a research oriented environment. But my point is
> that in a clinical
> situation and in discussions with fellow
> podiatrists, it is necessary to
> have easily understandable and conveyable concepts
> and terms. The
> non-linear nature of stiffness does not fit this
> picture because it cannot
> be represented by a single figure but requires a
> more elaborate
> representation with curves and diagrams ( as
> engineers and physicists do)
> and because it needs sophisticated measurement
> tools.>>
>
> Better yet, why not strive to bring podiatrists up
> to the level of the biomechanists and engineers?
>
> The solution that I have chosen is to attempt to
> synthesize analogous mechanical situations from
> real-life to explain more complex mechanical ideas
> of engineering regarding foot function to the
> typical clinician.
>
> Here is an example that I thought of during my
> morning run today:
>
> Take five 2 inch x 4 inch boards of equal length
> and lay them side by side to each other (wide
> surface down) on a wooden deck. The boards would
> overhang the deck by approximately three feet (i.e.
> approximately one meter). The two outside boards
> are made of pine (relatively compliant material),
> the three inside boards are made of oak (relatively
> stiff material). The boards are all nailed down to
> the deck so that when a child stands on the free end
> of each board where it overhangs the deck, it will
> support the weight of the child (35 kg, or 77 lbs).
>
> If the child were to stand on either one of the two
> outside boards, these boards would flex a much
> larger amount than when the child stands on either
> one of the three inside boards since the two outside
> boards are more compliant or less stiff than the
> three inside boards. However, if we were to now
> place a rigid steel beam over the top of the ends of
> all the boards and then have the child stand on the
> steel beam so that all the ends of the boards are
> loaded by the weight of the child, we would find
> that the boards would all flex the same amount since
> the steel beam acts as a flat surface that
> distributes the weight of the child to the steel
> beam requiring all the boards to flex the same
> amount (think now of the metatarsal rays all
> displacing dorsally together to become resting on
> the flat surface of the ground).
>
> In this case, before the child had stood on all the
> beams with the steel beam, would we have said that
> the two outside (more compliant) wood beams were
> "hypermobile" since they deformed more than the
> central three (more stiff) wood beams? If we did, we
> would have been wrong because, when the child stands
> on all the boards with the steel beam under his
> feet, all the beams move the same amount under his
> weight. What we would find is that each of the
> three central beams bear a larger share of the load
> than the two outside beams since the three central
> beams are stiffer that the two outside beams. In
> other words, a stiffer board will take more force to
> deform a given amount than a more compliant board
> will take so that if all the boards are deformed an
> equal amount, the stiffer board will bear a greater
> share of the load than the more compliant board.
>
> Therefore, if one were to observe solely the
> movement patterns of all of the boards acting under
> the influence of the body weight of the child
> standing on them with the steel beam under his feet
> distributing his load to all the boards
> simultaneously, then one would not assume that the
> two outside boards were any different mechanically
> from the three central boards. However, if one were
> to measure the force between each of the boards and
> the steel beam, one would find that the central
> three boards, since they were stiffer, would be
> bearing a much greater loading force than would be
> borne by the two outside, more flexible, boards. In
> other words, the magnitude of force acting between
> each board and the steel beam is proportional to the
> magnitude of its stiffness.
>
> I hope that this example is clear enough to present
> my point that metatarsal ray "stiffness" is a much
> better way to discuss metatarsal ray mechanics than
> by using the term metatarsal ray "hypermobility".
>
>
> Cheers,
>
> Kevin
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