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Putting the mechanics back into ‘biomechanics'

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Craig Payne, Feb 7, 2015.

  1. Craig Payne

    Craig Payne Moderator


    Members do not see these Ads. Sign Up.
    Another gem by Griff:
    Putting the mechanics back into ‘biomechanics'
  2. I agree, this is an excellent article by Ian Griffiths ("Griff"). Everyone who deals with biomechanical pathologies of the foot and lower extremity should read and digest this very well-written article.
  3. Jeff Root

    Jeff Root Well-Known Member

    This is a good article Ian. I do have a few concerns. The brothers are described as being identical except for their stj axis location. However, in the drawing demonstrating the differences in their stj axis locations, the brother with a medially deviated stj axis also demonstrates an abducted forefoot as compared to the brother with the "normal" stj axis location. As a result, I do think there would be observable difference between these to otherwise "identical" brothers including arch height and forefoot angle and as a result, then would not be identical. Similar, yes, but identical, no.

    The other concern I have is the effort (not just in this article) to dismiss or minimize the fact that orthoses do, in many cases, create kinematic changes, and they often occur in all three cardinal body planes. Kinematic changes resulting from foot orthoses in some cases may be so slight, we can't measure them or even observe them with the naked eye. This is especially true at the midtarsal joint where there can be important kinematic (positional) changes in the sagittal and transverse planes that would have contributed to the abducted forefoot position in one brother described in this article.

  4. Jeff Root

    Jeff Root Well-Known Member

    One other observation/comment. The foot with a medially deviated stj axis appears to me to be the same foot type that Merton Root described as having increased or excessive verticality of the oblique axis of the mtj. These feet demonstrate a greater tendency for transverse motion (in this case abduction) of the forefoot to the rearfoot. When this foot bears weight, the reafoot adducts relative to the forefoot or the forefoot abducts relative to the rearfoot, depending on your point of reference. In evaluating these feet, I believe it is equally important to look at the orientation of the mtj axis in addition to the stj axis. If for example, the forefoot does not abduct as much, then the stj axis can't adduct as much. I think some practitioners focus too much attention on the stj axis at the expense of attention paid to the mtj axis. They are equally important and the orientation or position of one axes has an influence on the orientation or position of the other.

  5. Griff

    Griff Moderator

    Hi Jeff

    Thanks for you comments.

    Firstly, don't be concerned... it's just a blog ;-)

    Suffice to say I'm not going to argue with you about the similarities or differences between the two brothers; in much the same way I'm not going to argue with my wife about the anthropometric characteristics of the sheep she counted to get to sleep last night.... neither of them are real. Hopefully other readers of the piece will take the relevant message that I was trying to convey to them out of that particular analogy.

    Furthermore, I did not mention 'Root' once during the article and so I am not sure why it comes up in conversation here? I do however agree with you that some practitioners place too much attention and focus on the subtalar joint - and this was actually the exact reason I picked that joint for all of my examples (if it is a known point of focus then it makes it a very good facilitator of new concepts). Perhaps you didn't make it to the end comments where I stated that this was very simplistic modelling of the foot and that this thought process should be applied to multiple joints in multiple planes?
  6. Jeff Root

    Jeff Root Well-Known Member


    I did read it all and I know it was written for a blog. Actually I thought it was very technical for a blog and I thought it was very well written. The reason that I brought up Root's work is because I don't see it as being mutually exclusive in many regards and this was a good opportunity to show an example of how the same foot type can fit in both paradigms.

  7. Dr. Steven King

    Dr. Steven King Well-Known Member

    "we are unlikely to move our understanding forward if we hold on to old theories just because they are simple."

    Amen Ian Amen!

    •Physics is way cool.

    "Simple" is super cool when we start to consider the use of "simple mechanical machines" in our gait systems, like through the use of levers and incline and declining planes.

    Ian, from the writing of this blog I see no reason why you cannot understand the mechanics of the advanced composite orthotics we tested for the US Department of Defense and Army Medical Research Command for SBIR A11-109 "Advanced Composite Insoles for the Reduction of Stress Fractures."

    I will trust that you will look into simple spring lever machines orthoses with your expansive mechanical acumen and no foam predjudices.


    Here is the final report we produced for the DoD with the assistance of Dr. Benno Nigg and other great mechanically minded educators.

    SBIR A11-109 Final Report for the US DoD. Read it if you like mechanics.


    DARPA's new Google-Boston Dynamic Atlas robot still has flat feet, why?
  8. Jeff and Ian:

    There are a few important points that need to be made here.

    When we say that the subtalar joint (STJ) is medially deviated, it means that the STJ may be medially deviated relative to either the rearfoot and forefoot, but generally to both the rearfoot and forefoot. Therefore, in your example above, Jeff, when an individual has a forefoot that is abducted on the rearfoot, by definition, the STJ axis will become more medially deviated relative to the forefoot since the forefoot has become, now, more abducted relative to the STJ axis.

    It is important to remember that the actual orientation of the STJ axis is determined by the articular facets of the talo-calcaneal joints, all located in the rearfoot, so that when the forefoot abducts on the rearfoot, the STJ axis will adduct relative to the forefoot. I have covered these concepts in great detail in my previous papers and book chapters on this subject (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, 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; Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA, 90:30-34, 2000; Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001).

    These are good points, Jeff, and I totally agree with you here. In fact, there seems to be a notion among some podiatrists that foot orthoses don't produce any kinematic changes whereas the scientific literature shows, in some cases, that they do, in fact, have the potential to produce kinematic change. Here is a list of scientific research papers which show that foot orthoses can produce a kinematic change within the foot and lower extremity:

    Of course, there are other papers show that foot orthoses don't produce any measurable kinematic effects. I suspect that the lack of kinematic change seen in some research studies may be due to one or more of these factors:

    1) A lack of measurement precision within the study where the researchers were unable to detect small changes in joint position.

    2) The foot orthoses used within the study not being sufficiently congruent and/or stiff to produce sufficient magnitudes of kinetic change to produce kinematic changes.

    3) The central nervous systems of the subjects within the study overriding the kinetic effects from the foot orthoses.

    Otherwise, after considering the multiple scientific research studies listed above that do show that foot orthoses can produce a kinematic change in gait, I would find it hard for anyone to deny the potential effects that properly designed and constructed foot orthoses have in changing the kinematics of the foot and lower extremity during weightbearing activities.
  9. For those that don't think foot orthoses change kinematics, I suggest you all go back to first principles and figure out how reaction forces are generated at the foot-orthosis interface.

    The main issue here is that for some, kinematics = rearfoot position / motion.
  10. Dr. Steven King

    Dr. Steven King Well-Known Member


    The concept of using carbon fiber springs as an addition to our conventional orthotics remains to be reviewed, recognized and adapted.

    Are you up to the mechanical task Ian?

    Or is it just more talk than mechanics here on Podiatry Arena as well?

    Where are all the students and female bloggers, why don't they post their thoughts? Or are they too intimidated by the Foam Fan Boys.

    Craig-- another one of my posts went missing on this thread of your site, Why?

    I did not swear or anything... I just dared my colleagues to move.

    A Hui Hou,


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