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how long does a foot elongate during midstance?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, May 18, 2016.

  1. markjohconley

    markjohconley Well-Known Member

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    Has there been any studies done on just how far the MLA extends during midstance?

    My query relates to the 'how proximal is the anterior edge of the orthoses to the MPJ's' and the 'fitting footwear statically using Brannock device' discussions, mark
  2. Modelling the foot as a simple truss, elongation should be proportional to navicular drop- lots of data out there on that.

    See also Ker: The spring the arch of the human foot.
  3. markjohconley

    markjohconley Well-Known Member

    Thanks Simon, got a copy through 'Mart' on an old thread, 'Plantar fascia has no elastic fibers?', will peruse that tonight.
    Had a rethink, so have deleted the remainder of the post, duh
  4. David Smith

    David Smith Well-Known Member

    Mark you asked how much does the MLA extend thru stance phase and Simon answer that its proportional to the navicular drop (by some trigonometrical equation) The question then is how much does the navicular drop. The answer to both of course is variable dependent on the mechanical properties of the individual foot of interest. Navicular drop would need to be carefully considered since rolling the apex of a triangle over to lay flat on the ground does not change the length of the base but does change the height from the ground reference i.e. in may appear lower relative to the ground but is not lower relative to the original base to apex height.
  5. Agreed
  6. markjohconley

    markjohconley Well-Known Member

    Goodaye David, are you're referring to the dorsiflexion of the metatarsals on the cuneiforms and cuneiform on the navicular? being responsible for the 'rolling over'; if so would it depend on the 'original' relationship between the metatarsals, cuneiforms and navicular; if say the metatarsals are 'originally' relatively plantarflexed to the cuneiforms then the span (base) would extend without a decrease in navicular height (or if the base was fixed there would be an increase in navicular height) till they were in-line? (neutral?) with the cuneiforms then as the metatarsals further dorsiflexed then the 'rolling in' would occur (or the base would decrease in length, not likely i guess)? mark
  7. David Smith

    David Smith Well-Known Member

    What I mean is Mark, as the foot pronates then some degree of the apparent lowering of the navicular height is due to the rolling over of the triangle formed by the Navicular apex, and bases of 1st ray and calcaneus and not due to the bases becoming further apart.
    Anyway surely it would be more relevant to your patient of interest to measure the actual difference in MLA length between non weight bearing, semi weight bearing and fully weight bearing and apply this with respect to how you took the impression of the foot for making the orthoses, i.e. was this impression taken non w/b, semi w/b or full w/b.

  8. markjohconley

    markjohconley Well-Known Member

    Thanks David, I'm with you now, you mean as the 'triangle' everts with STJ pronation, right.
    I imagine the changes in length of the navicular to forefoot interval would be insignificant also.
    With the change in length of the MLA, i was thinking more of the change in length of the MLA span in the dynamic sense during stance, being shod (with orthoses) not so with the casting; have yet to be convinced of the greater merits of casting actually. I remember going into the cast room and noting all the students different cast results; i could tell who did what cast from the shape of the negative casts (well from the major 'biomechanical' students anyway. If we are to attempt to maintain / return all tissues to their ZOOS then surely, as i do, just use the sock lining / insole as a template from which an orthoses can be made, you just get the lab to include the 'desired' prescription variables (a type of over-the-counter but with a prescription)? So far the only problem I have encountered is the lack of choice of MLA height with these manufactured orthoses, mark

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