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Has the creator got it wrong again? (STJ axis)

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Sep 9, 2006.

  1. markjohconley

    markjohconley Well-Known Member


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    if the stj axis becomes more laterally deviated with increased supinated position of stj with subsequent increased supinatory moments (from w/b'ing forefoot) about stj axis (and similarly for medial deviated stj axis / pronated stj / ^ pronatory moments stj) then hasn't the creator slipped up again ............ surely it would be advantageous for the opposite to occur ie. an increase in supinatory moments with increased stj pronated position and vica versa ......................also JUST CLARIFYING ....... rearfoot tools to increase supinatory moments about stj axis (blake inverted, medial heel skive, dc wedge) transfer the CoP to a more medial position on the medial plantar calc. tubercle, no other plantar structure receives direct pressure?
     
  2. Mark:

    I think the creator did it all remarkably well....all thing considered. Without the anterior orientation of the STJ axis being pointed toward the first metatarsal head anteriorly and without the posterior orientation of the STJ axis being pointed lateral to the medial calcaneal tubercle and lateral to the Achilles tendon posteriorly, we could not have a stable foot during the stance phase of gait. In other words, unless the spatial location of the STJ axis exists in a very small range of possible spatial locations relative to the osseous structures of the foot and lower extremity, then the human foot and lower extremity will have a extremely poor likelihood of having normal gait function. All one needs to do is produce a model of the foot with different STJ axis spatial locations to see how efficient bipedal gait relies heavily on relatively exact STJ axis spatial location.

    Personally, Mark, I think that as I better understand the biomechanics of the foot and lower extremity with each passing year, the more that I believe that the creator put more thought and effort into designing the foot than in designing any other component of the human locomotor apparatus.
     
  3. davidh

    davidh Podiatry Arena Veteran

    Kevin,
    You said:
    "Personally, Mark, I think that as I better understand the biomechanics of the foot and lower extremity with each passing year, the more that I believe that the creator put more thought and effort into designing the foot than in designing any other component of the human locomotor apparatus."

    Couldn't agree more!
    Cheers,
    david
     
  4. EdGlaser

    EdGlaser Active Member

    Mark,
    Man created the STJ axis as a vast oversimplifcation to try to explain a very complex array of geometry designed to translate ground reactive forces into functional locomotion. Its weaknesses are ours and I just feel priveledged and humbled by the brilliant design that went into this most amazing device.

    Your observation is very acute. You are saying that supination yeilds further supination and pronation yields further pronation. This is the essence of the supination resistance model and the physics bears it out. I am attachig a simple force diagram that demonstrates what you are saying here, but turned 90 degrees to the sagital plane (although the same is true, to a far lesser degree in the frontal plane).
    As far as God is concerned. I often wonder why so many feet, at the end of development are still inverted. There must be a logical survival reason. I am a strict believer in evolution (great article in National Geographic about this which shows the evidence is overwhelming). I will venture a theory, but take it with a grain of salt:
    Humans are slow edible animals with very poor claws and dull (except for the Canine tooth) teeth. We can eat meat but our teeth are certainly a poor weapon. In the wild we were probably food for faster, better equiped predators. Like most primates we hide in trees and caves. Trees are far more common and accessible. It is better, when climbing a tree or walking on a tree limb to have a slightly inverted foot.

    Just a thought.

    Ed
    www.solesupports.com
     
  5. EdGlaser

    EdGlaser Active Member

    I tried to attach the force diagram but it was too large a file size. I will try to reformat it and attach it later.
    Ed
     
  6. Ed,

    I'm glad that you agree with the the test (supination resistance test) that I invented and first published 14 years ago (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992) that was developed in correlation with the subtalar joint axis location/rotational equilibrium theory that I have been working on over the past 20 years (Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987;Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989; Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001).
     
  7. Mark:

    I read your post again, did some more thinking about it and have another answer for you.

    The human foot has two components that mechanically interact with the ground: the rearfoot and the forefoot. Both of these components rely in part on their own plantar shapes to produce their characteristic mechanical interactions with the ground. In addition, both of these components are also affected by the plantar shape of its complementary foot component, which it is firmly attached to, in order to allow the foot to function properly. In other words, the rearfoot is mechanically affected by the plantar shape of the forefoot and the forefoot is mechanically affected by the plantar shape of the rearfoot during weightbearing activities.

    The rearfoot has a relatively narrow, rounded plantar surface that allows frontal plane rotational motions of the rearfoot to occur without hindrance due to its plantar shape. Therefore the plantar shape of the calcaneus in inherently unstable within the frontal plane due to its rounded plantar contour since it's rotational motions are not hindered in any way by its plantar morphology.

    The forefoot, on the other hand, has a relatively wide, flat plantar surface that offers a more stable platform since it tends to resist frontal plane rotational motions of the forefoot due to its wider, flatter plantar contours. Therefore, the plantar shape of the forefoot is inherently stable within the frontal plane due to its wide, flat plantar contours. In other words, the forefoot is inherently unstable when it is not plantigrade.

    Since there are multiple ligaments, muscles and tendons that can exert significant tensile forces and there are also multiple joint surfaces that can exert significant compression forces between the rearfoot and forefoot, frontal plane motions of the rearfoot will always mechanically affect the forefoot and frontal plane motions of the forefoot will always mechanically affect the rearfoot. The frontal plane mechanical interactions between the rearfoot and forefoot can be approximated (i.e. modelled) by imagining a coiled spring (its axis at the intersection of the sagittal and transverse planes) of variable stiffness between the rearfoot and plane of the metatarsal heads. The "midtarsal-midfoot spring" will tend to resist frontal plane motions of the rearfoot relative to the plane of the forefoot while the forefoot is plantigrade since as the rearfoot attempts to invert while the forefoot is plantigrade, the spring will produce an internal forefoot inversion moment (i.e. a tendency of the forefoot to invert relative to the rearfoot). When the rearfoot attempts to evert while the forefoot is plantigrade, the spring will produce an internal forefoot eversion moment (i.e. a tendency of the forefoot to evert relative to the rearfoot).

    These internal inversion and eversion moments of the forefoot are caused by alterations in magnitude and plantar locations of ground reaction force (GRF) on the plantar forefoot that occur with frontal rotational motions of the rearfoot while the forefoot is plantigrade. As the rearfoot inverts, the GRF is shifted laterally on the forefoot that produces an external forefoot eversion moment and that, in turn, is resisted by an internal forefoot inversion moment from the "midtarsal-midfoot spring". As the rearfoot everts, the GRF is shifted medially on the forefoot that produces an external forefoot inversion moment and that, in turn, is resisted by an internal forefoot eversion moment from the "midtarsal-midfoot spring".

    However, it is not just the midtarsal joint-midfoot complex which is mechanically affected by these rearfoot-forefoot interactions. As you mentioned in your original question, the subtalar joint (STJ) also plays an important part in these mechanical interactions since the STJ is the primary joint that allows frontal plane rotational motions of the rearfoot to the tibia during weightbearing activities.

    The rearfoot relies on the wider, flatter shape of the plantar forefoot so that the rearfoot may possess additional frontal plane stability during weightbearing activities that allows it to better resist excessive pronation or supination motions of the STJ. The forefoot, on the other hand, relies on the narrower, rounder shaped rearfoot, to allow less restricted supination and pronation motions of the STJ to occur that, in turn, allows the tibia and femur to also undergo less restricted transverse plane rotations while the forefoot is plantigrade during weightbearing activities.

    The medially deviated STJ axis, that you mention above, will cause a STJ pronation moment which will, in turn, tend to cause STJ pronation motion. This STJ pronation motion will then cause a shift in GRF medially on the plantar forefoot. This shift in GRF medially on the forefoot causes an increase in external STJ supination moment (or decrease in external STJ pronation moment) which will be mechanically transmitted back to the rearfoot by the "midtarsal-midfoot spring" to help prevent further STJ pronation motion and, overall, give the STJ and foot as a whole, increased frontal plane stability.

    The laterally deviated STJ axis, that you mention above, will cause a STJ supination moment which will, in turn, tend to cause STJ supination motion. This will then cause a shift in GRF laterally on the forefoot which will cause an increase in external STJ pronation moment (or decrease in external STJ supination moment). This increase in STJ pronation moment will be mechanically transmitted back to the rearfoot via the "midtarsal-midfoot spring" to help prevent further STJ supination motion.

    Therefore, Mark, one should not try to understand the mechanical function of the foot by isolating the STJ (or rearfoot) from the forefoot since both of these components of the foot are mechanically linked to each other via a complex mechanism that can be approximated by this "midtarsal-midtarsal spring", with each component of the foot contributing very specific characteristics to the mechanical function of the foot as a whole. In other words, I would not suggest trying to understand the mechanical effects of STJ axis deviation without also understanding the very significant mechanical effects that STJ axis deviation has on the mechanical function of the forefoot and on the loading forces that act on the forefoot from GRF during weightbearing activities. In addition, the variable stiffnesses of this "midtarsal-midfoot spring" that occur within the human population will also greatly affect the mechanics of the rearfoot and forefoot,STJ, and foot as a whole. In conclusion, the mechancal function of the rearfoot and forefoot are so closely and integrally related to each other that to discuss one without the other just does not make good mechanical sense.
     
    Last edited: Sep 10, 2006
  8. markjohconley

    markjohconley Well-Known Member

    thanks gents for your posts ............. kevin thanks again for that last post, i comprehend and now to "stew over it"
     
  9. Mark:

    I have added more information to my last posting to include a frontal plane spring mechanism between the rearfoot and forefoot, or the "midtarsal-midfoot spring" that should help further clarify the mechanical interactions involved.
     
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