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Determining STJA using SRF device and plantar force matrix measurement

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Mart, Aug 27, 2007.

  1. Mart

    Mart Well-Known Member

    Members do not see these Ads. Sign Up.
    I am drafting a proposal to my local university engineering department to co-design and test some instrumentation for kinetic foot measurement.

    The idea so far develops the work done by Craig Payne and Co-workers on supination resistance force (SRF) measurement and attempts to reduce operator subjectivity by automating some decision making, incorporate ability to look at supination resistance dynamically and record real time force and calcaneal inclination data digitally for post trial analysis.

    It occurs to me that the apparatus Craig used might also be usefully adapted to plot the regions influencing moments around the STJ axis (STJA). Craig’s work published in collaboration with Kevin Kirby found significant correlation between what he defined as supination resistance force and a theoretical STJA location.

    My question is this.

    Firstly position the weight bearing foot, using Craig’s apparatus, just at the point of destabilizing STJ equilibrium, ie the defined measurement point of SRF.

    Using a plantar force matrix sensor system placed under the foot within the same system, would the force measured at that instant represent the pronatory component of forces acting across the STJA and would they therefore also represent the spatial alignment of forces lateral to STJA.

    If this is true then the medial border of this map would represent the outline of neutral position (ie zero moment influence or STJA location of foot in this position) of any forces applied beneath the foot and conveniently provide information for design of FO to influence STJ moments.

    Comments regarding the above welcomed.


    The St. James Foot Clinic
    1749 Portage Ave.
    R3J 0E6

  2. efuller

    efuller MVP


    As I understand it, you are trying to find the position of the axis in the foot by using the location of force applied to the foot. You will know that the SRF = 0 when ground reaction force is pointed directly at the axis. I think that is what you are saying.

    There are several problems with this. If you are doing this to a foot with a connected living brain there will be moments from muscles. As the foot gets close to the point where the supination resistance is zero the brain will notice the potential instability and increase tension in the muscles to prevent sudden unexpected movements.

    Another problem is that you want to know where the axis is relative to the bones of the foot and not relative to the ground. The experiment above will give you one position of the foot where you know where the axis is. The foot is rarely in this position in average daily activity.

    To do kinetics, you have to know where the axis is at the same time you know where the force is.


    Eric Fuller
  3. Mart

    Mart Well-Known Member

    Hi Eric

    Thanks for your reply.

    The points you make are well taken and I feel that I have already planned to deal with them with in the scope of the project.

    To clarify your issues:

    The SRF will equal whatever value is required to initiate calcaneal motion in the weight bearing foot, as a generalization the more medialy shifted the STJA the higher the value expected , it would be a very laterally unstable foot to equal 0.

    If you look at the range of SRF values measured in the experiments done by Craig Payne et al the inter trial values tend to spread quite widely and I have found the same with my own limited number of trials. This could be accounted for by instrumentation limitations or recruitment of stabilising musculature or shifting weight from one foot to the other as you suggested.

    As an aside I would anticipate that the recruitment would likely be of pronators resisting the supination force applied, would you agree?

    My proposal aims to attempt to reject trials where the force/time curve deviates form an acceptably predicable shape, although what constitues "acceptible" is as yet unknown and it's investigation part of the proposal.

    The force is applied by a linear actuatuator (this is a change in design from Craigs apparatus) which provides a reasonably constant rate of change of force applied. I think it likely that we will be able to create rules which can reject data which shows signs of non predicable behavior.

    I accept your belief that the STJ axis likely changes with foot position.

    The reason for creating this instrumentation is to provide a clinical measurement for placement of GRF by FO with predicable effect on STJ moments. Presently this is done by informed trial and error.

    This is NOT intended to create a 3D model of the STJA which it clearly couldn't.

    Given that the STJA likely does shift with STJ position, from a clinical point of view is not necessarily a limitation to FO design. The most important information from this perspective is only concerned with transverse plane component of axis since the FO GRF is applied and measured simply as a vertical GRF with respect to the ground.

    The project instrumentation aim is to present 3 data sets which are synchronized and diplayed simultaneously in real time. SRF/Time, calcaneal motion/time, and segmented plantar GRF/time (via Tekscan sensor).

    The forces measured on the plantar surface once the threshold of STJ movement is reached will be available for examination post trial. This data could then be presented as 2 summed force maps which range before and after the time of SRF measurement. One would show boundaries which would define where supination forces and one where pronatory forces would need to be positioned within the range of shifted boundaries measured during the trials ie the areas where the boundaries shifted across the pronation/supination line would be excluded from FO design.

    My suggestion is that provided the FO provides increased GRF within the boundary indicated by that data (even though it would be expected to shift according to position at any instant in time) then the FO will accomplish it’s objective.

    Please keep challenging – I need to get my proposal honed over the next week and have no one to bounce ideas with outside of this forum


  4. Unless the patient is standing so that their center of mass (CoM) is exactly balanced over the ankle joint axis, then muscles need to have contractile activity to maintain bipedal balance. One problem with the idea that if the center of pressure (CoP) is lateral to the subtalar joint (STJ) axis that there always exists a STJ pronation moment is that this idea does not account for muscle forces.

    For example, during relaxed bipedal stance the CoM in a subject is given to be balanced over the forefoot so that the center of pressure (CoP) is anterior to the ankle joint axis at about the level of the 2nd metatarsal neck. The STJ axis is known to be 3 cm medial to this point, just medial to the first metatarsal neck. In this foot, would there be a net STJ pronation moment? Not necessarily.

    Since the anteriorly positioned CoP requires Achilles tendon force to maintain ankle joint rotational equilibirum, then the STJ supination moment from the Achilles tendon must also be factored in to see if the position of the CoP relative to the STJ axis results in either a net pronation moment or a net supination moment. In other words, it could be very likely that a CoP lateral to the STJ axis at the anterior forefoot may result in a net supination moment acting on the STJ since the required Achilles tendon force acting medial to the STJ axis generates a greater magnitude of STJ supination moment than does the magnitude of STJ pronation moment generated by ground reaction force (i.e. at the CoP).

    Mart, this is an extremely important biomechanical fact to understand, especially considering your proposed experimental design. Eric and I have discussed this in our upcoming chapter and in our lectures a month ago in the UK.
  5. efuller

    efuller MVP


    I don't think I understood your original plan.

    You wrote
    My question is this.

    Firstly position the weight bearing foot, using Craig’s apparatus, just at the point of destabilizing STJ equilibrium, ie the defined measurement point of SRF.

    Using a plantar force matrix sensor system placed under the foot within the same system, would the force measured at that instant represent the pronatory component of forces acting across the STJA and would they therefore also represent the spatial alignment of forces lateral to STJA.

    If this is true then the medial border of this map would represent the outline of neutral position (ie zero moment influence or STJA location of foot in this position) of any forces applied beneath the foot and conveniently provide information for design of FO to influence STJ moments.

    End of Martin's original comments.

    Is the point of destabilizing STJ equilibrium when you first see motion with force applied to the medial arch?

    Is the force matrix sensor a system that measures forces over multiple descrete areas like an EMED or F-scan? Are you then going to use center of pressure to create a number to compare one foot to another? If not how are you going to compare pressure maps?

    What do you mean by medial boarder of the map. Isn't this the outlline of the foot? I don't understand what you think this represents.

    Yes, most likely. You would have to also look out for weight shifts as you mentioned.

    Is the force/time curve for the force in the linear actuator applying force to the medial arch? And not the Teckscan readout.

    Good luck on finding parameters that identify what you want to identify.

    I don't understand what you are saying you do with this measurement. How does this measurement relate to making the functional orthotic (FO?)?

    What is a semented plantar GRF?

    Again, I'm not quite clear as to what you are saying in this last section. What is a boundry and how are they defined? I don't see what you are trying to do with the teckscan information.


    Eric Fuller
  6. Mart

    Mart Well-Known Member

    Hi Kevin and Eric

    Thanks for your replies. Your help is much appreciated.

    Kevin I understand completely what you are concerned about and am pretty sure learnt this concept from your “Intricast newsletters” books where I seem to recall you elegantly dealt with this issue.

    Having looked at my post again I realize that I did not express the most important part of my argument clearly so I want to break this down. Also I realize that the idea is theoretical and if not logically flawed needs testing.

    Please excuse my pedantic approach.

    Here goes.

    Please interrupt each point for agreement/disagreement.

    When we examine a map of an array of plantar pressures, derived from force matrix array system (eg FMat), of the weight-bearing foot, at any instant in time, with subject standing in relaxed bipedal stance it is impossible to determine which cells are measuring GRF medial or lateral to the STJA without being able to superimpose on that map the STJA position.

    Using an apparatus such as the one designed to measure SRF we will apply an external force at approximately the same place as Tib Post insertion.

    When this external force is applied, which we know is medial to the STJA, it will cause a change in the GRF underneath the foot which can be measured spatially by the combined system above.

    On application of the supination force some of the array of cells will show an increase and some a decrease in force, some perhaps no change.

    For this argument ASSUME that NO active tensile elements change their contribution to the system (just ignore COM issues for time being)

    Cells which show increase in force do so because the effect of applying the isolated external force is being opposed by a reactive pronatory force.

    Likewise cells which show a decrease will do so because the externally applied force reduces their supinatory contribution.

    These positive or negative changes in GRF may occur because of forces acting from the point of application of the externally applied force through the musculoskeletal system (MSKS)or because of changes in alignment of the MSKS with the ground.

    Here is the thought which I feel the crux of my idea which I am unable to flaw, but I fear is missing something obvious in my excitement.

    The applied force (adjacent to navicular) will supinate the STJ until it reaches it’s limit of ROM.

    If the applied force is slowly increased there will be a period of time before the STJ moves unless it is fused or already maximally supinated.


    The force applied will act through STJ but may and likely also acts independent of STJ. E.g. through the talo-navicular joint into metatarsal heads 1,2 and 3. So change in GRF seen at MTH 1 2 and 3 may be due to this rather than force acting through STJ.


    My contention is that any changes in GRF which are not acting through the STJ do so because the stiffness of the structures transmitting that force must be greater than those allowing transmission through the STJ.

    As the force is increased and this relative effect of stiffness changes, eventually the force will primarily act through the STJ.

    At the point where the STJ moves the force must be acting primarily through the STJ otherwise it would not move.


    If we examine the events which take place at the instant of STJ motion and after and ignore what happens until then we can assume that the changes occurring do so because primarily of forces acting through the STJ.

    During STJ motion cells showing +ve change must be lateral to STJA and –ve medial to it because they are a reaction to a change in moments across the STJA to the supination force applied externally.


    A map which divides the plantar surface in to 2 regions, those which show cells which have increased in force and those which have decreased during a segment of STJ motion would then predict the effects similarly to GRF applied to the plantar surface at those sites in those various STJ postions.

    Perhaps you can find logical fault already and if so I should simply futher explore the instrumentation for SRF and thank you.

    If not, please let me ,in a further post address the issue you mentioned Kevin regarding effects of position of COM which I have thought through, however this post is already long enough and time for me to get some sleep.


    Last edited: Aug 28, 2007
  7. Mart

    Mart Well-Known Member

    Eric Wrote

    Is the point of destabilizing STJ equilibrium when you first see motion with force applied to the medial arch?


    The SRF as defined in Craigs experiments is that required to cause the intial operator perceived inversion of a posterior heel bisection.


    Is the force/time curve for the force in the linear actuator applying force to the medial arch? And not the Teckscan readout.


    YES the supination force applied via actuator, the FScan bit relates to looking at possibility of plotting effects on GRF according to STJA location and perhaps will prove to be ill founded.


    Good luck on finding parameters that identify what you want to identify.


    do I detect your apparent skeptism?

    If a constant supination force is applied would you not expect to see a repeatable constant heel motion?

    If there is activity from musculature, shifting of weight from one foot to the other or reaction to change in COM I would anticipate the F/T or heel inclination/T curve to deviate from being a nice constantly increasing curve to a gradient reversal or obvious gradient change between trials. How noticeable or predictable this would be is I need to find out but I am curious if you can anticpate problems with this approach.

    ERIC says

    I don't understand what you are saying you do with this measurement. How does this measurement relate to making the functional orthotic (FO?)?

    What is a semented plantar GRF?

    Again, I'm not quite clear as to what you are saying in this last section. What is a boundry and how are they defined? I don't see what you are trying to do with the teckscan information.


    Sorry Eric I agree I made a messy job of explaining my idea, please look at my other reply to hopefuly make more sense!


  8. efuller

    efuller MVP


    The apparatus does not apply the force in the same direction, but that is nit picking. It's close.

    From the pictures of the device I've seen, there are some extreme feet where the applied force will not be medial to the axis. These are probably the feet that broke the machine.

    The models in Kevin's article on rotatoinal equilibrium paper can be used to make a prediction in this case. One of those models has a medially positioned STJ axis and interosseus compression force in the floor of the sinus tarsi. (We've improved that diagram by using center of pressure rather than just medial and lateral foot force.) At rest, with no muscle activity the center of pressure is lateral to the STJ axis and there is a pronation moment from the ground. To maintain equilibrium the interosseous compressive force in the sinus tarsi creates a supination moment. This moment must be present to maintain equilibrium. In the series of diagrams there is a progressive increase in posterior tibial tendon tension. As the tension increases there is a corresponding degrease in compression in the floor of the sinus tarsi. The net moment on the joint would still be zero. In the final picture there is enough tension in the tendon to cause supination momtion and finally there is a lateral shift in the location of the center of pressure.

    The major difference between the tendon and the experimental rig is that the rig is an external force to the body whereas the tendon is an internal force to the body. The force from the device will tend to lift the foot off of the ground so you should see a coresponding decrease in the magnitude of total force on the foot (on both sides of the axis).

    So, with increasing force in the tendon (in a foot with a medially positioned stj axis) you would expect to see no change in force untill the foot began to supinate. With the rig you would expect to see a decrease in total force equal to the vertical component of force in the rig.

    If we make that assumption, then what you say is true, if you take into account my comments above abuot the rig lifting the foot off of the ground. This is a very big assumption. You can make this assumption with cadavers. With living people it will be tough to do in practice.

    To add to your point. The changes in allignment come from the externally applied forces (or change in internal forces from muscles).

    My opinion is that it probably wll not reach the end of range of motion in the direction of supination.
    1. The direction of pull of the rig would have to change to keep its leverage about the axis.
    2. the subject will not let it happen and use thier muscles to prevent it from happening.

    Agreed, for some feet. Not all feet will have a pronation moment from the ground. See discussion above for when there is an internal supintion moment. If a foot has its center of pressure directly beneath the STJ axis then there will be no supination moment and there should be immediate motion with force from the rig. In some feet there is constant contraction of the peroneal muscles to keep the foot plantigrade. It is very difficult to predict what these people will do in response to increased force from the rig.


    I now understand your logic. It may work for those patients who have internal supination moments if you can control muscle activation, the center of mass/ center of pressure issue described by kevin, and account for the total ground reaction force. (The patient shouldn't shift weigth to opposite foot and the rig does not apply so much force that it decreases the force all over location of foot contact.

    My problem is that I always think too much about my experimental design and then don't do the experiment because I'm afraid that it won't work. I would be interested to see if you do notice a decrease in total force with the rig.


    Eric Fuller
  9. Mart

    Mart Well-Known Member

    Eric thanks for your reply, will get at this next.

    I hope this will simplify my thread and end my torment because I have been obsessed by this for the past 3 days( I must increase my medication).

    To speed my rehabilitation I stood on an FMat today perceiving to be in double limb relaxed stance (diag #1) and slowly supinated my foot to approx TNC (dia #2) without allowing it to slide on the mat. I then selected 2 samples of recorded stance, second more supinated than the first and exported the raw data into excel and calculated the of change in force between each of the 2 samples for each cell (diag #3) with colour code for -ve, +ve and 0 values.

    The data is displayed diag 3 represent each force cell with green being decreased force red being increased force and yellow no changethe .

    You can see from each sample that the COP position for both feet and for the measured right foot are fairly unchanged, that the total force and total contact area measured for each sample is similar. I interpret this to indicate that my body COM has moved little.

    The question I was trying to ask in an ungarbled and hopefully less confused form;

    If I position my foot above a FO and apply an increased force to the green mapped area via the FO by modifying it's contour will the foot be supinated in reponce and similarly in the red area will the STJ be pronated at in responce?

    My bright idea (NOT) was that this would be helpful in deciding where to apply a GRF in designing an FO to influence moments around the STJA.

    To me logically this seemed appealing but intuitively I felt this might not work but could not figure out why.

    I think I have figured it out. Please let me know if you agree!

    The answer to the question is:


    Same of course applying to my earlier post with externally applied force from SRF device.

    If this is the answer then clearly this analysis would be unhelpful (unless also using a muscle stimulation device implanted to contract supinators at same time as prescibed FO is used, not exactly a great advantage).

    Anyhow I should be able to sleep again – unless of course my answer is flawed, any takers?



    Attached Files:

    Last edited: Aug 29, 2007
  10. Mart

    Mart Well-Known Member

    Eric thanks for those extra thoughts, as you may realise I dont have any drinking buddies to mull podiatry ideas over with, this is the closest I can get to that.

    I figure after the conclusions from my last post there is no future in my idea anyway.

    Good though to understand why, even though it seems embarasingly obvious now.

    I have been very fascinated by Craig's original idea of SRF ever since I saw him lecture about it in Toronto a couple of years ago and felt more recently it would be interesting to try and develop his apparatus it into a clinical rather than simply a research tool.

    I am going to drop the recent impulsive idea of incorporating plantar force measurement into the plan for obvious reasons.

    I would value some concideration though of the plan.

    Since I seem to have captured your and Kevins interest I will post another thread to look at this which I think will be clearer.

    I think that it will be rewarding to work with supervised engineering students when trying to solve problems of instrumentation, they should be much better at it than most podiatrists. However I would like to make sure that I have my notions as well scrutinised as possible before starting and I dont have any drinking buddies blah blah . . . . . . . . . . . .


  11. Martin:

    I have read over Eric's replies to you and I pretty much agree with what he has said in response to your questions. I think your idea is unique and certainly deserves some further research. I like your idea of pronating and supinating the foot to map out areas of increasing pressure and decreasing pressure on the plantar foot. This technique and the resulting pressure mat data may be of value experimentally in the future for some aspects of foot function.

    Use of the supination resistance device (SRD) would allow you to approximate the actual supination moment being applied externally to the foot if you could at first determine the spatial location of the subtalar joint (STJ) axis relative to the medial navicular and the pulling strap. Simon Spooner and I approximated the STJ axis in our paper on the STJ Axis Locator (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006.) using the method first described by Morris and Jones (Morris JL, Jones LJ: New techniques to establish the subtalar joint's functional axis. Clinics Pod Med Surg., 11(2):301-309, 1994). This approximation was very accurate using our STJ axis locator and could probably be used to determine the actual STJ supination moments being applied relative to the STJ axis position.

    The SRD force magnitude and calculated STJ supination moment (i.e. SRD force x moment arm to STJ axis) could be correlated to actual change in plantar pressure and/or calcaneal inversion with feet of different STJ axis locations (i.e. compare medially deviated STJ axes to laterally deviated STJ axes). You would expect that greater STJ supination moments would be required to supinate a medially deviated STJ axis foot than a normal or laterally deviated STJ axis foot. In addition, it would be interesting to also estimate the sinus tarsi compression force using a model of the forces needed to supinate the STJ the first degree of motion (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.)

    You may also want to see how the center of pressure (CoP) moves laterally with increasing SRD force and correlate this to calcaneal inversion and STJ moments being generated by the SRD. Some foot types may show a "more rapid" lateral shift of CoP compared to others for a given amount of STJ supination moment generated by the SRD. These would be very interesting and worthwhile research projects and would easily be publishable in JAPMA or even the Journal of Biomechanics if technical enough.

    My other idea for you is to see if there is a correlation to force needed to invert the calcaneus with the SRD and the anterior-posterior position of the CoP on the foot (i.e. have the patient lean anteriorly and posteriorly to see if the force needed by the SRD to supinate the foot is changed by these anterior-posterior CoP and center of mass movements). My guess is that, especially in a foot with a medially deviated STJ axis, that as the CoP is moved anteriorly, the SRD force needed to supinate the foot would signficantly increase and, as the CoP moved posteriorly, the SRD force needed to supinate the foot would significantly decrease. (Martin, do you know why??) This would also be a publishable paper in JAPMA.

    Happy thinking and researching! :)
    Last edited: Aug 29, 2007
  12. efuller

    efuller MVP

    I would agree. There is one warning though. The F-scan that I used exhibited creep. That is if you stood on it for 6 minutes the total force read out would be 50% higher without any movement. This is not going to be relevant for measurements that last a couple of seconds.

    In answering the question: where do you want to increase orthotic reactive force; I just looked at the spot with the most leverage. If you want to cause increased supination moment the point most medial to the axis is the medial heel. If you want to cause increased pronation moment the farthest place from the axis is usually the lateral forefoot. If you want to increase pronation moment after heel strike and before full forefoot loading then you increase orthotic reactive force on the lateral heel.

    You also have to think about which structure(s) you are trying to protect with your orthosis. Attempting to supinate the STJ by applying force to the first met head is not really a good thing if you are trying to protect the 1st MPJ.

    So, this answers the question without knowing exactly where the axis is. On the other hand it would be interesting to compare a known axis location to the change in force you have shown.

    It's good to see someone with passion for figuring out how the foot works. I remember when I was doing the biomechanics fellowship at CCPM. I would wake up in the middle of the night with answers to what was puzzling me. It was scary :)


  13. That is why you have accomplished what you have in podiatric biomechanics, Eric. I have had a similar experience with vexing biomechanical problems often keeping me awake at night. Those that don't have this passion or desire for getting to the answer of a difficult problem never lose any sleep over such matters.
  14. Mart

    Mart Well-Known Member

    I understand all your points here Eric, and I have certainly benifited from your publications over the years.

    I should perhaps explain a little more about my interest in this.

    Craig's lectures about his SRF experiments provided for me for the first time some compelling evidence concerning the disparity between lack of evidence regarding the kinematic effects of foot orthoses on the foot, particularly looking at frontal plane motion and the apparent resolution or improvement with certain MSK overload conditions associated with STJ motion.

    Also, and logically coupled with this, was the issue of why foot orthoses which superficially, appear to have the same related functional qualities can have vastly different clinical outcomes in different individuals.

    So, as you are no doubt aware, his evidence pointed to foot orthoses therapeutic effect likely being attributed to moderation of STJ moments rather than motion, and also the location of where foot orthoses GRF is applied with respect to the STJA in an attempt to acheive this.

    Perhaps I had missed some prior studies, but this for me was new work on the back of, ideas developed in the podiatry world at least, largely by Kevin and yourself.

    Using Kevins techniques for palpating STJA and finger testing SRF are useful, and to my knowledge are the best clinical tools we have presently in addressing this particular aspect of foot orthoses design.

    I guess one of my little holy grails (not really so little :eek: ) is to see if it is possible to create instrumentation to objectify this process.

    This might foster further studies along the lines of which Craig has already done (the evidence only comes form his work, someone else needs to repeat this), but possibly and hopefully create a useful clinical tool with reliable results if further studies suggest this would be worthwhile.

    My first step is to attempt to create a clinical tool based on the SRF apparatus.

    This is where I am at presently, I have built an electro-mechanical prototype to apply force which I am starting to use clinically.

    Now I need to do the hard part and develop the instrumentation to piggy back on this.

    I dont have the electronic chops to do this, but have lots of ideas and specifications. I have had some interesting discussion with medical-engineering at U of M and it looks likely we wil work together to create some undergrad projects to explore this.

    I will post more info when I get time because my ideas need some experienced scutiny from a podiatric perspective and to get a reality check from some people whose thoughts I respect. This thread has already helped me reject and consolidate some new ideas which is what is needed.

    If I can get the instrumentation to work, the notion of measuring the effects of foot orthoses on SRF also seems worthwhile and this is one of the reasons I am trying to design the apparatus so that it can be used to record data during single support of a step rather than simply double support stance.

    Anyhow more of this later.


    Last edited: Aug 30, 2007

  15. Here is an excerpt from my 2001 paper (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.) that addresses this issue about why certain feet respond differently to externally applied STJ supination moments than others:


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