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Challenging SALRE

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Robertisaacs, Nov 20, 2008.

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  1. Members do not see these Ads. Sign Up.
    I hope no one will mind if i break this off the superiority thread. There was a lot going on there and this is such a great question i feel it deserves its own home. Makes it easier to find too.

    Ed said
    Some wiser and better heads than mine have given some great answers on the other thread. This is my humble and simple attempt.

    There are several parts of this question which need examination

    1. The Axis does not have a single location. The medially deviated axial foot refers (i beleive) to the range, that is that the whole range is medially deviated.

    So, for eg. Here's the foot swinging through the air, nicely inverted, perhaps even more so because of the orthotic. The heel hits the ground and the force equations start to kick in. At this point the axis is towards the lateral most extramity and at this point the orthotic starts to generate ORF. Supination moments are therefore happening earlier and the part of the orthotic under the navicular MAY STILL BE EXERTING A SUUPINATION MOMENT because at that instant that point is medial to the axis.

    As this takes place earlier the foot will supinate more slowly / less. Therefore the axis remains more lateral for longer and therefore supination moments * time will be higher.

    Notwithstanding the above, even if the axis DOES travel medial to the nav, we must remember (THought experiments 101) that supination moments are coming from more than just GRF and ORF. If the orthotic has increased GRF under the nav by inference it must have DECREASED it under the lateral part of the foot. By moving the COP nearer to the axis it may therefore have decreased the Pronation moments to the point where the other supination moments (muscles for eg) can now overcome the pronation moments

    3. You said
    Orthotics don't exert a vertical force, they exert force at an angle dependant on the angle of the orthotic plate and the friction co - efficient. This must be kept in mind, although we are looking at SALRE mainly in the transverse plane it works in three planes.

    You said

    I have seen, in paeds with GROSS axial deviation because of gross hypermobility the foot stand with the lateral border waving in the breeze and the medial border weight bearing alone. Apply increased vertical upward force to that navicular (for eg by giving the child a brick to hold) it will still be a pronatory force.

    Of course an angled surface like an orthotic may still make it supinate. But an angled surface will be exerting a force on a vector which takes it superior to the axis in a med -> Lat direction and therefore be working across the top part of the rotation. In other words consider a clock in the frontal plane. the ground is at a quarter to three (ish). A vertical force pushed against the minute hand will make it turn. But put the hands now at 5 to 5. A force from right to left will turn the minute hand more effectivly now. I'm sure Dave "the engineer" Smith is weeping into his coffee as he reads this debasement of science but thats how i can understand it.

    Great discussion :drinks

    Last edited by a moderator: Nov 20, 2008
  2. Nice work, Robert. You learning.:drinks
    And as the rest of your post points out, in time too.

    In other words, we could have a CoP which in the transverse plane is located medially to the transverse plane projection of the STJ axis, but because of its 3D nature, the vector actually passes lateral to the axis and has a pronatory effect.
  3. Which got me thinking about sagittal plane theory. Deceleration of the CoP pathway has been proposed as one of the "indicators" of sagittal plane blockage. Yet we could have a CoP whose position relative to the transverse plane is unchanging. But the vector and thus it's mechanical effect is changing. Hmmm, need to think on that one.
  4. For those of you trying to figure out what "SALRE" means, it refers to my paper from seven years ago - Kirby KA: Subtalar joint Axis Location and Rotational Equilibrium theory of foot function. JAPMA, 91:465-488, 2001. I have attached a copy here for those who would like to read it again, or for the first time.

    For those of you who are further interested in what I went through to get a paper such as this published in the Journal of the American Podiatric Medical Association, neither of the two reviewers that looked at my paper thought it should be published as it is now. One thought that I was not respectful enough of the contributions of Mert Root, DPM, in my paper and thought it should be split into two papers and more credit given to Dr. Root and his ideas. The other one thought that I should choose between a clinical paper and a scientific paper, and not mix up clinical findings with scientific ideas in the same paper. I had to fight for nearly 6 months to get this paper published as is and, after all that fighting with the reviewers, the paper won the "Journal of American Podiatric Medical Association Scholl Award for Outstanding Paper Published in 2001".

    Sometimes it is worthwhile fighting for something you so strongly believe in and have worked for so many hours to create. Hopefully this will serve as a helpful lesson to those of you who are also interested in trying to get papers published in peer-reviewed scientific journals.
    Last edited: Nov 20, 2008
  5. Nothing more than osmosis ;)

    Everyone (who has not already done so) should download and save a copy of the SALRE paper against the internet ever blows up. I drove 40 miles to steal it from the university library at eastbourne and i'm glad i did, its worth it. Read it slowly and over a few days then try the thought experiments. It really will change the way you view things.

    THEN read Ed's question and think very hard about that!

    Thankyou, Kevin, for making this revolutionary (whether you like it or loath it) paper public!

    Kind regards
  6. As in. Place your thumb under your nav and push up, you get supination. Same spot, same COP, push medial to lateral the foot pronates. Find the angle at which the foot does neither and you have a force vector which would bisect the joint axis.

    I love this stuff. Its like sudoku but better. :D:drinks

  7. Very good! This technique could be employed to estimate the angle for the orthotic shell at this point to obtain desired mechanical effect, friction complicates it a little though, as does the rest of the orthosis and foot! Still, I'm sure it'll be useful for something, so you better rush out and patent that test Robeer!!!!
  8. Last edited: Nov 20, 2008
  9. David Smith

    David Smith Well-Known Member


    Quote by ED Glazer

    There are two misconceptions here.

    1) A misinterpretation of Kirby's theory IE That misinterpretation being, that puting a medial wedge lateral to a medialy deviated STJ axis will cause pronation.

    One must consider the relative actions and not the absolute action.
    I.E. Consider diagram 1 - Represents a medially deviated axis. There is equlibrium since positive and negative moments are balanced because forces act at both sides of the fulcrum C.

    Diagram 2 The heel tilted to the ground at heel strike so that +moments cause pronation because forces acts at point B only.

    Diagram 3 Insert a wedge from point P to B. Clearly there would be some forces acting all along its full length in contact with the ground. The mean of the summation of the forces and their distances from point P would result in a convenient CoF acting at point B2 corresponding to application of force F2.
    Clearly the relationship of F2 to F1 along PB is less than F1 along PB ie the lever arm is shorter. Therefore while the absolute effect of putting a wedge lateral to point fulcrum C is +M = pronation the relative effect is -M = supination.


    Therefore an while arch support pushing in the MLA and lateral to the STJ axis may in absolute terms cause pronation the relative effect can be supination.

    2) Pushing up anywhere on the MLA will cause the arch to rise and the base length to shorten i.e. increase the archiness as Robert might put it.:D

    There is infact only one point where this will happen, pushing up in any other point will in fact decrease the archiness of the MLA. So unless you can predict the exact CoP of the orthosis reaction force at any spatial tempral point of interest then it is not possible to predict that outcome.

    In the diagrams below (extracted from some of my previous work) there is a analysis of forces in an arch when loaded assymetrically as it is in a foot.

    The second diagrams indicate where the bending moments would be in two arches of different profiles for the same type of loading.

    Clearly then an arch support would have to push in diffrent directions and places to achieve more archiness in the differing examples.

    If you wished to increase the height of a simple arch structure by inducing appropriate moments about the apex node then clearly this could only be achieved by pushing up under the node itself, anywhere else would tend to cause moments in the opposite direction and so flatten the arch. How could an arch support be designed to always cause a CoP at this exact point?


    All the best Dave
  10. David Wedemeyer

    David Wedemeyer Well-Known Member


  11. This is a major problem with estimating internal joint forces by looking only at the results from a pressure insole system or pressure mat, the three dimensional orientation of the ground reaction force vector (acting at the center of pressure) is only obtainable via use of a force plate, not by use of pressure insoles or pressure mats.

    Another problem with using pressure insoles/mats or force plates and relying only on center of pressure (CoP) location or CoP movement, is that unless the STJ axis spatial location can be determined relative to the GRF vector, the actual rotational effects of the CoP on the STJ can not be determined. At late midstance, assuming 500 N of force through the 2nd metatarsal head area at the CoP, the interindividual variation of STJ moments seen within the human population with this specific CoP location in late midstance could easily range from 50 Nm of STJ pronation moment (assuming a 10 cm pronation moment arm) to 10 Nm of STJ supination moment (assuming a 2 cm supination moment arm). This is why only tracking CoP migration will only give us very superficial information as to the function of the foot and lower extremity and as to how foot orthoses should be prescribed for our patients.
  12. Stick a pin in it at this point; that's not a recommendation BTW, just an answer to the question.;)

    With a little less tongue in cheek you could: substantially increase the stiffness of the orthosis shell in this area relative to the rest of the orthosis; make this point the apex of the curve of your orthosis; put a vertical "prop" between the shell and the shoe interface at this point; thicken the material at this point and use a higher density material here. This may help. Time, time, time though.... That 4th dimension is a bitch. You got to use some kind of "pin track" to get it where you want it on time. Don't ask me, I'm just a chiropodist.
    Last edited: Nov 20, 2008
  13. efuller

    efuller MVP

    Robert, I think what you wrote was pretty good, but I'd like to take a stab at it. A good reference in this area was the work of Van Langaallan 1983 where he took cadaver feet and put metal balls in the bones and x-rayed the foot while loaded in a device with progressive 5 degree increments of internal leg rotation. He found a "bundle" of axes of motion for all the bones of the rearfoot. So, there was some small amount of variation of the axis relative to the bones, but it would not be too far off to create an average for that bundle. The average of the bundle of one foot could be compared to the average of the bundle of another foot. The average of the bundle could also be projected to the transverse plane to get a sense of how much foot is on each side of the STJ axis.

    If a person is in static stance, the force, from the ground acting on the feet has to equal body weight. So, the vertical component of orthotic reactive force has to equal body wieght. Also, if there is no horizontal acceleration the net horizontal force has to be zero. So, if you stand on a heel cup that has a medial heel skive there will be some horizontal force because the force has to be perpendicular to the surface. This horizontal force that has to be resisted by frictional force or the foot will slide until the lateral heel cup is touched and there will be a horizontal force from lateral wall of the heel cup. The frictional force and the force from the heel cup have to equal the horizontal component of ground reaction force caused by the inclined plane of the heel cup.

    Agreed, Good discussion. It is always good to examine your assumptions.:drinks

  14. :good: I like that a lot, Eric. Take the excursion of the axis during a given dynamic function and use the midpoint as the datum for that function. :cool: Moreover, why stick at one plane? Man, you know sometimes that the future of podiatric biomechanics is going to be so cool and you just want to be around to shape and be part of that future. Think I might try and stop smoking again. The futures so bright I gotta wear... :cool:

    And during dynamic function.....

    This evaluation of SALRE is long over due and there is already some good (did I say good? brilliant might have been better) stuff coming out of it. Thanks Ed.
    Last edited: Nov 20, 2008
  15. Simon:

    In reference to your statement above, a wise man once told me, "Science is the antidote to the poison of enthusiasm......";)
  16. Dave:

    Good posting. As I said in my earlier posting on the "Claims of Foot Orthosis Superiority" thread, it is a common myth that when an orthosis presses lateral to the STJ axis that the net effect of the orthosis will always be to increase the external STJ pronation moment. As you showed in your free body diagrams, a varus wedge that redirects ground reaction force (GRF) from a more lateral position to a more medial position, but with the whole varus wedge still being lateral to the STJ axis, the varus wedge will still cause a net decrease in external STJ pronation moment which will, in effect, mechanically tend to cause a decrease in symptoms caused by excessive STJ pronation moments versus a barefoot conditon.

    In order to optimize the clarity of these mechanical scenarios we are now discussing, we must make a distinction between the condition of:

    1) an equilibrium condition of the STJ (e.g. holding the supine foot with the examiner's fingers plantarly loading the 4th and 5th metatarsal heads in the plantar parallel position) where the external STJ pronation moment from the examiner's hand pressing lateral to the STJ axis is exactly counterbalanced by the internal STJ supination moment from the tensile force within the Achilles tendon pulling medial to the STJ axis so that the STJ is resting in a static condition (i.e. not rotating),

    from the condition of

    2) a perturbation of an equilibrium condition of the STJ (e.g. the examiner using their opposite thumb to press plantar to the medial navicular in a superior direction) where the additional external STJ supination moment that results from the thumb pushing medial to the STJ axis causes an angular acceleration of the STJ in a supination direction.

    Unless we make a distinction between these equilibrium and non-equilbrium conditions in our postings, and clearly state the conditions and external and internal forces acting on and within the foot that cause these equilibrium and non-equilbrium conditions, then things will get very confusing for the vast majority of those trying to follow along....including myself.:morning:
    Last edited: Nov 21, 2008
  17. :D

    That would never do;)

    Perhaps not the thread for it but i'd love at some point to look more closely at the vector and friction aspect of orthotics. I have always found a huge difference between orthotics cast corrected so the plate meets the medial shoe at a right angle and those like UCBLS where the plate is swept up around the medial part of the arch.

    I've also been trying to get my head around the friction aspect. If you took the same prescription and coated one pair with a mythical zero friction coating and the other with superglue how would they perform differently? Assuming that an orthotic with a high wrap contacted the foot first under the medial aspect in the superglue case the point of contact would in essence create a whole new pivot!

    Obviously not a good one to try out but as a thought experiment, quite interesting!

    These are not just theoretically fascinating but also potentially practically applicable things to seek to understand!

    But that is, perhaps, another thread entirely.

    Thanks to all for this thread!:drinks


    My kind of language!:D But you have GOT to learn to shrink your diagrams M8!!
  18. Peter

    Peter Well-Known Member

    Hi Kevin,

    You wrote

    "Good posting. As I said in my earlier posting on the "Claims of Foot Orthosis Superiority" thread, it is a common myth that when an orthosis presses lateral to the STJ axis that the net effect of the orthosis will always be to increase the external STJ pronation moment. As you showed in your free body diagrams, a varus wedge that redirects ground reaction force (GRF) from a more lateral position to a more medial position, but with the whole varus wedge still being lateral to the STJ axis, the varus wedge will still cause a net decrease in external STJ pronation moment which will, in effect, mechanically tend to cause a decrease in symptoms caused by excessive STJ pronation moments versus a barefoot conditon."

    Is this because you are moving the GRF closer to the STJ Axis ( smaller lever arm, net decrease of pronation moment)?

    Or am I "forgetting sesamoids"?

    Kind Regards

  19. Peter:

    Yes. Even a varus wedge that is placed under a foot so that the whole varus wedge is lateral to the STJ may reduce the net external STJ pronation moment since now, with the wedge in place, the ground reaction force (GRF) will be shifted more medial which will, in turn, decrease the pronation moment arm for GRF.

    This medial shifting in GRF on the plantar foot, such as would occur using such anti-pronation orthosis techniques as a medial heel skive or a higher medial longitudinal arch height, will either increase the external STJ supination moment or decrease the external STJ pronation moment. The net effect of these orthosis modifications will, therefore, be to reduce the STJ pronation moment acting on the foot during weightbearing activities.
  20. David Smith

    David Smith Well-Known Member


    Is this the type of thing you are thinking of in regards to an inclined surface acting on the foot.

    As you can see the spatial position of the physical axis will determine the direction of rotation ie positive or negative moments.


  21. David Smith

    David Smith Well-Known Member

    By the way you can test this if you want.

    Take your calculator in your right hand, hold it between thumb and forefinger near the top. Hold the other hand out, finger pointing away from you and palm inclined 45dgs or so and facing to your right. Push the bottom leading corner of the calculator directly downwards (only vertical, no horizontal motion) so that it strikes the palm of your inclined hand. Notice the Direction of rotation. Now do the same but hold the calculator near the bottom.
    Notice the directrion of rotation is in the opposite way.

  22. And there is the problem in a nutshell, in-shoe sensors provide an additional interface that is both slippery and two dimensional, so difficult to see what is really going on at the orthosis/ sock interface. And no doubt influential in the in-shoe data reported. Sensors are generally too stiff and too two dimensional to see what is happening at the sock/ foot interface. How sweaty are your feet? Too many layers to really get to grips with... Just like onions, which is where the word bunion comes from BTW.

    Have a nice weekend y'all.

    BTW increase friction on an inclined plane= more vertical vector, so in a medial heel skive, the more sticky the better. No more of those vinyl top covers in taupe- right?
    Last edited: Nov 21, 2008
  23. :mad:

    Damn you Simon,:mad:

    I've been up half the night with SARS (at the very least, possibly H5N1) with my head in a bucket of vicks thrashing my febrile and frazzeled brain about this! I just crack it at about 3 am with an analogy about ski slopes and how lower friction = more lateral force and higher = more vertical, i come on here this morning all excited to post it and you've just casually dropped it in there already in a single sentance like its the most obvious thing in the world.:mad::butcher:

    I hate you so much right now.

    Thanks for the diagrams Dave! Really helps to see a rotational diagram in the frontal plane. A whole new area there!

    So if the actual vector of force is a composite of the friction and the angle of the orthotic we can actually have a good crack at controlling, with our prescription, thae force vector in the frontal plane. Grabby material and flattish (but raised) plate gives a more vertical force, slippy material and high medial wrap / angle gives a more horizontal force.

    This is great stuff! I've often thought the covering of an orthotic was a somewhat neglected area, now i have some ideas for how i can use this to my advantage!

    Thanks Dave (and Simon i guess, mutter mutter darkly mutter);)


  24. Sorry Robert. You may want to read the section on calculation of forces here:

    Then follow the external link at the bottom to play with the simulation. This should calm you down.

    Hope you're feeling better and your bird flu is clearing up; I had "the alien" last week, not nice.
  25. David Smith

    David Smith Well-Known Member


    Lets check to see if this strictly true?

    Using the analysis technique cited by Simon earlier, which is probably easier to understand than rotating the applied forces then:-

    Diagram 1 & 2 indicate how as frictional force on the incline plane (IFF) increases then so does force due to mass x Gravity (Fmg) and therefore Normal force (NF), which = the resistive force from the inclined plane. IE if friction induces an increase in IFF then there must be a corresponding relative increase in all other forces.


    Diagram 3 & 4 show how this applies to a 10dg medial post / wedge under a heel.
    It can be seen that the wedge changes the application point of the centre of force (CoF), which reduces the force and lever arm available to anti clockwis moments (AC/W), and also induces an additional IFF that acts against a long lever arm L3, increasing clockwise (C/w) moments.


    The diagrams above show that in terms of direction of moments then there is no change due to magnitude of friction. In terms of magnitude of moments then as friction increases the net moments in the same direction increase. The verical applied force must increase as the friction increases but so must the normal force.

    This would appear to show that it may make no difference what the coefficient of friction (CeF) is between two contacting surfaces of the orthosis foot interface.
    However, I believe that it does especially when timing is considered. The coefficient of friction acts to attenuate the applied forces, less friction = less attenuation or a longer lower attenuation curve in terms of a graph. If the frictional force grips the foot earlier in the contact phase then this will alow a greater magnitude of (hopefully) supination moments (if that is required) to be applied earlier to the STJ before it reaches a position that reduces the possibility of applying those forces to an advantage eg causing supination moments (if that is what is required). Of course if a greater magnitude of moments is required then more friction (increased coefficient of friction) may be the answer. However it must be remembered that the Fmg is somewhat finite and this will be a limiting factor. The additional consideration of horizontal forces as the positional application of body weight / CoM changes thru stance may have some bearing on choice of orthosis covering material in terms of CeF.

    All the beast Dave
    Last edited: Nov 22, 2008
  26. David Smith

    David Smith Well-Known Member


    sorry you might have noticed a small error, I put 1600 N as the normal force in the diagram and calculations when I already calculated it to be 1385.6N ie NF = Fmg cos30dgs => 1600cos30 = 1385.6N. So the revised Ac/W moments in diag2 is 5.57N. Principle still aplies tho.

  27. David Smith

    David Smith Well-Known Member

    Oh! good you didn't notice my deliberate mistake or were kind enough not to highlight it.
    In diagram 1 & 2 I drew 60dg inclines and analysed as 30dgs inclines.:eek:
    Anyway this doesn't change much in terms of principles. In my defence I was doing this in between seeing customers, making orthoses, washing the floor and sterilising and general paper work. That's my excuse and I'm sticking to it.:eek:

    Thinking about Simon's and Roberts queries about coefficient of friction (CeF) of the orthosis foot interface, how does friction relate to orthosis design in terms of surface camber / inclination?

    The ground reaction force and bodyweight are going to be fairly constant and we can see that the frictional force cannot rise on its own there must be a corresponding and proportional.

    The key, it seems to me, is that the incline of the orthosis must be at least matched to or greater than the maximum frictional force available to the foot, orthosis interface.

    So if a certain interface of interest say leather and wool have a coefficient of friction 0.55 and the maximum possible normal force is 800N. Then the maximum frictional force would be 800 x 0.55 =440N. The max resultant force will be square root of 440^2 + 800^2 = 913N at an incline of 440 / 800 = 0.55tan-1 = 28.8dgs.
    Therefore the greatest incline that you could use with a wool - leather interface with a CeF of 0.55 and a max load of 913N would be 28.8dgs. Above those parameters and the grip would be lost and slipping would take place.

    Of course this doen't take account of the fact that say the lateral edge of the foot may also at the same time be be pushing on a flat surface but hey you can't have it all at once.
  28. EdGlaser

    EdGlaser Active Member

    Sorry that this took so long.
    Interesting that this thread is called challenging SALRE but I have yet to see a single challenge......so here goes. I have several more pages to add but when I gave it to Don to add pictures (I had trouble adding them where I wanted them) he gave it back to me in a .pdf format which I cannot edit on my computer. It is attached but still needs more graphics. I added some grammatical edits which should be visible in adobe acrobat 7 if you click on them.

    So, although this "Introduction to SALRE criticisms" is incomplete, it may give some of you something to think about.

    Let me state, that while doing this little paper, it became very clear to me that Kevin has been unable to see the forest because he got lost rotating around a singular tree, the STJ axis that his instructors pointed out to him as being the most important.

    Also, when I thought more about this problem it seems clear that there is some variation in STJ axis orientation occurs both between feet within the same foot by virtue of postural changes. By far the factor that is the greatest contributor to this variation is foot posture and not anatomical variance. Therefore in saying that a medially deviated STJ axis is different than just saying a flat or pronated foot or saying a laterally deviated axis is different than saying high arched or supinated is the same as saying that there is little or no correlation between the more lateral / vertical axis seen in a supiated foot posture and the preponderance of laterally deviated axes in cavus feet or visa versa for flat feet.

    The STJ axis leaves the foot at the point of heel strike. Anatomical variance should predict that some pronated feet will hit on the medial side of the axis and others on the lateral side. The medial tuberosity being a little bigger will also contribute. So, if the SALRE theory were correct, some feet would be applying a GRF medial to the STJ axis at HS and create a supination moment and therefore would necessarily supinate immediately following heel strike.....and these should be the more pronated feet. I only see these feet pronate after heel strike. Why do they not supinate?

    So what has a greater effect:
    a) The anatomical variation in the medial to lateral placement of the STJ axis shadow in the transverse plane.
    b) The translation of the axis which is possible secondary to a change in foot posture.

    My main criticism of SALRE theory is that it reduces the foot to a uni-axial 2D see saw or physics problem.....ignoring the postural component which is the dominant force in foot function.

    SALRE theory recommends taking this bag of bones that has collapsed medially, leave it collapsed and worry about the tilt of the ground or where lumps and bumps are placed to reduce tissue stresses. I would rather push that bag of bones back into alignment by giving it the postural support it needs to function optimally.

    I could go on forever but I have to stop somewhere.


    Attached Files:

  29. Ed:

    I can see that you really have not changed one bit. And to think that you were actually going to play nice this time. I should have known better.

    You are a real work of art, Ed.....that is the nicest thing I can say after reading your ramblings on how you think the foot works.
  30. efuller

    efuller MVP


    It is very difficult to address the issues you raise for two reasons. It's too hard to quote what you say from a pdf document. I feel that I might miss some of your argument because you address the same points in different locations in a seven page document. It would help you convince people of your points if you could put all of your arguments about a particular point in one place and not repeat them.

    The pictures are not necessary. All of the points you are trying to make with the pictures can be made in text. It would help you make your point to repost a more concise critique.

    The reason that there have been few challanges to SALRE is that a lot of people here on podiatry arena accept it. If you want to convince people otherwise concise arguments would be helpful to your cause. Volume (repeating the same point 3 times) and diagrams may impress the uncritical, but you should be able to make your point concisely.

    I believe the STJ is the most important joint for determining foot function. If I had to pick one parameter to predict foot function, I'd pick the STJ axis location. That doesn't mean that I don't look at relative length of the 1st and 2nd metatarsals and other things. Ed, what parameter do you feel is most important for predicting foot function? Should everyone be casted in MASS regardless of foot type?

    Ed, I've read the above a couple of times and I'm not clear on what you are saying.

    What is the distinction between foot posture and joint position and arch height.

    An aside. STJ axis position is independent of arch height. You can see the rare flat flexible foot with a laterally positioned STJ axis. More often you will see a high arched foot with a laterally deviated STJ axis. And more often than that you will see the flat medially deviated axis and the high arched laterally deviated axis foot.

    Muscle activity. The laterally deviated STJ axis feet are much more likely to develop peroneal tendonitis or peroneal muscle fatigue, because if these muscles were not active these feet would supinate at heel contact.

    In comparison of one foot to another, a) the anatomical variation in the location of the STJ axis is more important than the movement of the axis that occurs with a change in joint position. The reason that I believe this is that orthoses don't change he postion of the STJ that much. Ed. we have talked about this before. On average, how many degrees does the calcaneus invert when they stand on a MASS casted orthosis when compared to standing barefoot on the floor?

    SALRE theory is a three dimensional dynamic physics problem that can be simplified to static two dimensions for ease of understanding. I agree with what I think your postural argument is. The STJ axis does move with changing position of the talus and calcaneus. You do want to use the position of the axis in which the foot is functioning, because that is the position of the axis in which you would want to calculate the moment from the center of pressure of ground reaction force.

    Is a 0-2 degree change in position of the STJ significant enough to make a medially deviated axis foot a non medially deviated axis foot? I don't think so.

    Ed, I will address the points in your PDF document. However, I'm not going to organize your thoughts for you. I'll just pick the first instance of a point. If you repost a concise organized version it would save us both some time.


  31. Oh dear. And things were going so well!

    It was your question!!

    You may well think that. But does it really help the discussion? Will this sentance convince anybody that there is a flaw in the model under discussion? Or are you just seeking to convey that you saw something that kevin could not? This smacks of ego!

    I honestly don't know if your posts seem combatative because you want them that way, or if you do not realise how unhelpful it is! I thought we had agreed this was going to be a discussion of the science not the people behind it!
    Did you see Davids diagrams? It can ONLY be a 2d expression on paper because paper is 2D but the principles can be applied in all three dimensions!
    Oh SO easy and tempting to argue with this. However that would take us back around the old stump. Would it be possible, as we agreed, to discuss the flaws in SALRE without looking at MASS as an alternative? Because otherwise people will just start on about flaws in MASS instead and the point of the thread (critiquing SALRE) will be lost!

    Let me see if i have understood you here because you lost me a bit. Are you saying that the position of the STJ and therefor th position of the axis within the bundle is more important than any anatomical variation in the stj position?

    This is a good question and one which perhaps (although hope is fading) we can discuss in a civilised way as befits professionals.

    I would say that one cannot separate these two. Its akin to saying, what has a greater effect on the speed of a car, the amount of throttle to the engine or the size of an engine. Of course the position of the Axis shadow (good term btw) within the axial bundle has more variation than the position of the centre point of the bundle. HOWEVER the starting point of the axis is IMO, what defines the degree of excessive excursions of the axis which are more likely to generate pathological forces!

    So yes you can get a speeding ticket in a ford fiesta if you throttle it hard enough. But you can go faster and get a bigger ticket in a subaru impreza much more easily if you are not careful!

    So, to continue the analogy and again IMO, you can avoid tickets by revving less (attempting to controll the position of the axis within the bundle), but it is informative when trying to do so to know what car is being driven! 4000 revs in a ford fiesta a pootling. In the subaru its flying! Can't treat them the same way!

    PLEASE guys, this can be a really great discussion! Can we leave the personalities and egos at home!?


    For me?

    And the lurkers who would rather read an intelligent debate than a slanging match?

    Kofi Anan
  32. Ed Glaser seems to think that I believe that my Subtalar Joint Axis Location and Rotational Equilibrium (SALRE) theory of foot function explains everything that is important mechanically about the foot. Unfortunately, Ed again completely misses the point from my paper. I very clearly state, at the end of the paper, that in order to thoroughly explain my theory in one paper, other important information needed to be left out. Here is one of the last paragraphs from my paper.

    I thought this was going to be a discussion about SALRE theory but rather it seems that Ed wants to make it all about his self-proclaimed "best casting technique", the "MASS" technique. Since I am working on my third book now and Thanksgiving vacation is nearly here for my family, free time is currently in short supply. I will try to comment every now and then once I can see there is a little more clarity in what we are actually discussing.
  33. Dave,

    Sorry I didn't come back to this earlier, I was just too busy over the weekend.

    "In the case where friction is supposed to be absent, the line of action of the reaction between the surfaces in contact is perpendicular to the plane, but in the actual case, where friction exists, the line of action of the reaction is no longer perpendicular to the plane, but makes an angle with normal equal to the angle of friction." The inclined plane. In: Ingham E.: Applied Mechanics. The Bennett College LTD, Sheffield.

    Isn't the angle of friction a function of the coefficient of friction between two surfaces? This means that it depends on the nature of surfaces and is unique for the two surfaces in contact. It is also my understanding that the angle of friction does not depend on maximum static force or normal force. This being the case, I'm not sure how two dissimilar top-cover materials would result in identical angles of friction and ergo the same line of action as you suggest?
    Last edited: Nov 24, 2008
  34. Actually, I didn't mention the centre of pressure. What I said was:
    Despite your inaccuracy, having had time to reflect on my initial thoughts here, you've actually hit the nail on the head. In a foot with an STJ axis exiting through the tuberosity of navicular, if you were to push too hard with a high arched device with the maximal height of the orthosis plate focused beneath the navicular, this is one potential outcome- injury to the spring ligament or subluxation of the navicular. We can also extrapolate from this a similar situation regarding the medial cuneiform and it's ligaments and a device that pushes too hard there also. Indeed, given the right foot (for example, a foot with PT dysfunction) and a poorly designed orthosis, the force exerted by the orthosis may even rupture the spring ligament if it were to push to hard in the medial longitudinal arch, lateral to the STJ axis. Luckily, I wouldn't make a device like this, but I have seen devices that could potential cause such a problem. I'm sure you are familiar with the kind of device I'm talking about Ed.;)
    Last edited: Nov 24, 2008
  35. Steve The Footman

    Steve The Footman Active Member

    Modeling of foot function is a simplification with the goal of making a complex structure easier to understand. If it is a good model then it can be used both in research and by clinicians. As with any model there will be limitations in order for the model to be clinically useful. The main clinical question is how valid and relevant the model is to pathology.

    Based on more recent research on pronation I would question the level of importance of pronation at the STj to foot pathology. Are we missing more important structural issues by this focus on frontal plane theory? Is there a way to differentiate different feet and their pathologies to identify whether the STj is significant to their pathology/injury? Should SALRE theory be transferred to other joints in the foot like the MTJ complex to make them easier to understand?

    While the axis location is significant to whether the foot pronates or supinates the clinically relevant question is whether an abnormal axis location is more significant than other factors to pathology. I think that a significantly reduced or an excessive STj ROM could be more significant than the axis position to the risk of injury in athletes.

    Is SALRE more relevant to walking and does its significance diminish when dealing with the greater forces of running - particularly in sports that primarily require forefoot rather than rearfoot contact?

    I have used SALRE in examination of many patients but in practice have found other factors of more relevance to interventions. (many from precision intercast books I might add!)
  36. Steve:

    This is an extremely insightful comment. Bravo!! How refreshing, especially after reading the ramblings of an orthosis lab owner last night!

    The actual kinematics of whether subtalar joint (STJ) pronation occurs or not are not that important. What is important is the kinetics of the STJ. In other words, whether STJ pronation moments exist or not is the important consideration since it is the body responding to excessive external STJ pronation moments that can cause injury to the structural components of the foot and lower extremity. For example, if the posterior tibial muscle/tendon is exerting an internal STJ supination moment to decelerate pronation motion to a normal level, it may become injured even though STJ pronation seems kinematically normal. Or, for another example, if the floor of the sinus tarsi of the calcaneus is preventing the lateral process of the talus from pronating further, then "normal" STJ pronation may seem to be occuring kinematically while the abnormal interosseous compression forces within the sinus tarsi that result may be causing sinus tarsi pain. Therefore, pronation motion of the STJ is not that important, but STJ pronation moments are very important.

    I believe that there is, but the studies have not been done yet. Here is what I have seen clinically over the past 24 years of clinically correlating STJ axis spatial location to foot pathology (and yes, I have had a full time clinical practice for the past 23 years). The pathologies that seem to be caused in many instances by medial deviation of the STJ axis and excessive external STJ pronation moments include: 1) posterior tibial tendon dysfunction/tendinitis, 2) sinus tarsi syndrome, 3) patellofemoral syndrome, 4) medial tibial stress syndrome, 5) tarsal tunnel syndrome, 6) pes anserinus bursitis, 7) medial collateral ligament strain of knee, 8) plantar fasciitis, 9) medial plantar ligament stress syndrome, 10) functional hallux limitus, 11) medial dorsal midfoot interossous compression syndrome and possibly others. The pathologies that seem to be caused in many instances by lateral deviation of the STJ axis and excessive external STJ supination moments include: 1) chronic lateral ankle instability, 2) peroneal tendinopathy, 3) lateral dorsal midfoot interosseous compression syndrome, and possibly others.

    I have already lectured on the concepts of midtarsal joint rotational equilibrium at the 2007 PFOLA meeting in San Diego titled "Rotational Equilibrium Across the Midtarsal Joint Axes: A Kinetic Explanation for Longitudinal Arch Stability". I wrote about these subjects in my second book (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 141-152) and will be including further discussion of these concepts in my third Precision Intricast Newsletter book that I am working on currently.

    I agree. STJ axis location is only one variable that affects the kinetics of the foot. However, a factor that superficially seems unrelated to STJ axis location, such as a reduced or excessive STJ range of motion (ROM) may also significantly affect STJ kinetics. For example, an excessive STJ pronation ROM may affect STJ axis spatial location by allowing more medial translation/internal rotation of the STJ axis (due to more medial translation/internal rotation of the talar head/neck relative to the plantar foot) during weightbearing activities and thus increase the external STJ pronation moments when compared to normal. In addition, if there is a severe restriction of STJ eversion ROM, then this may cause the maximally pronated position of the STJ to have a relatively lateral STJ spatial location that may cause the foot to not only be maximally pronated at the STJ but also be laterally unstable. This unusual combination of mechanical circumstances can be seen in congenital varus "clubfoot" conditions where calcaneus is highly inverted in the STJ maximally pronated position .

    Both walking and running, and any other weightbearing activity for that matter, will be affected by the spatial location of the STJ axis. The magnitude, direction, plantar location and temporal patterns of the ground reaction force (GRF) vector are very important, along with the STJ spatial location, in determining STJ kinetics during weightbearing activities. Since a GRF vector acting only on the plantar forefoot directly transmits the mechanical effects of GRF back to the rearfoot via the midfoot and midtarsal joints, then one only needs to know the spatial location of the STJ axis and the spatial location and magnitude of the GRF vector acting on the forefoot to determine the magnitude and direction of STJ moments that are being caused by a weightbearing activity where only the forefoot is contacting the ground.

    You should enjoy the third book, Steve. It is shaping up to be the most sophisticated, but also very practical, of my three books. Your observations are excellent. You show an advanced understanding of the concepts that are important in gaining a more complete understanding of foot and lower extremity biomechanics and foot orthosis therapy. Good questions!!:drinks
    Last edited: Nov 25, 2008
  37. David Smith

    David Smith Well-Known Member

    Simon, not sure if I understood your query??

    I haven't studied frictional forces/rheology/tribology in depth but I have never come across a statement that indicates that the frictional line of action is variable dependent on the coefficient of friction and / or the type of interface materials.
    As far as I know only the magnitude of frictional force is dependent on the coefficient of friction, which is variable with varying interface materials.

    I said
    Because changing the coefficient of friction does not change the direction of moments. This is because the increases in relative forces are proportional therefore only the magnitude of moments change about a point of interest.
    Which is why I later wrote the following;

    In other words the appied force can only be as high as the reaction force and vice versa. Therefore to increase the magniyude of the reaction force we need to increase the coefficient of friction, changing the angle of inclination only changes the action of the forces not the magnitude. At some point the adhesive nature of friction ie the grip will fail and slipping will take place to stop this we must increase the coefficient of friction or reduce the inclination angle, which will reduce the frictional forces but increase the normal force. By increasing the inclination angle we reduce the normal force and increase the frictional force until we reach a point where the two forces equal a ratio equivalent to the coefficient of friction. Therefore if the CeF is 0.5 then as the frictional force approaches 1/2 the normal force then slippage will begin. If the sliding frictional force is only 0.4 then slipping will only stop when the frictional force is 0.4 of the normal force. This requires reducing the incline or reducing the applied force.

    Is this the answer you were looking for Simon

    Oh! hang on do you mean the resultant reaction force will change with the CeF?
    No it doesn't for the above reasons ie all the forces change proportionally as the frictional forces increase.

    (I think I'm right aren't I ? Someone jump in here - Eric?:dizzy:)

  38. I am talking about the resultant, and according to the books I'm reading the angle is influenced by the co-efficient of friction.
  39. EdGlaser

    EdGlaser Active Member

    Actually I was talking about a COP and you transformed it into an arrow vector.:bang:

    Once again you think that the force of an orthosis is "focused beneath the navicular". Do I need to explain COP to you? You can have a smooth curved orthosis that evenly distributes force per unit area over the entire plantar surface of the foot and still it may have a COP under the navicular. Such an orthosis will never dislocate the navicular relative to adjacent bones and only act to decrease stress on the spring ligament. Supination brings the sustentaculum tali closer to the navicular which will necessarily relax the tension on the spring ligament. But, as you said, you would never make an othotic that does that.

  40. David Smith

    David Smith Well-Known Member


    Are you sure they are not saying that the absolute resultant angle of force can change with the increase of CeF as this allows the angle of inclination to increase before slipping occurs?

    Place a block on an inclined surface and the block slides, increasing the coefficient of friction will just slow or stop sliding. It will not cause tumbling or take off which should happen if you change the relative resultant force vector of the frictional force. The downward vertical force of gravity is the opposing equal and opposite force to the upward vertical resultant force from the force vectors of friction and normal force. This must always be so and since Normal force must be a cosine of the gravitational force the relative frictional force angle cannot change.

    Can you post any diagrams that explain what you have read?

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