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Supination resistance and bodyweight

Discussion in 'Biomechanics, Sports and Foot orthoses' started by lcp, Aug 14, 2008.

  1. lcp

    lcp Active Member


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    Gday all,
    I have a quick question regarding the supination resistance test. I understand the need for the test, and am pretty sure I have the technique down pat, but how do you differentiate a high supination resistance from just a "bulky" person? There is probably a very common sense answer, but I have seen to fellas this week that I reckon my car jack would struggle to supinate, simply because they are big, heavy blokes. Am I doing something wrong here, or just trying to make a simple answer very hard???
    Cheers
    Paul

    (PS Go Eamonn, get that gold son!!!!!)
     
  2. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    Re: supination resistance

    Hopefully in the next week or two I will finish a video explaining the test and it will be on You Tube (and cross posted here).

    Effectively the test is a measure of how much force is needed to supinate the foot. The hypothesis is that this force is proportional to the force needed from an orthotic to supinate the foot (other things being equal and the static/dynamic issues; etc).

    Bodyweight may or may not be an issue:
    1. Bodyweight explains about a third of the force needed to supinate the foot (see this ref)
    2. If you seen me talk about this, you would have seen my diagram in which I hypothetically rank different types of foot orthoses in how much force they apply -- the hypothesis is to match the foot orthotic force to the patients supination resistance ---- in that case the bodyweight is not an issue (eg heavier people will be moved down the orthotic list as they will have higher supination resistance because of their bodyweight)

    For research purposes and depending on the research hypothesis, we may divide the force gauge value for supination resistance by the bodyweight.
     
  3. Admin2

    Admin2 Administrator Staff Member

  4. lcp

    lcp Active Member

    Hi Craig,
    Thanks for your quick reply. Yes, I did see you talk in Melb on Sunday, which I guess is why Im thinking about this more. By the sounds of it, I have looked past the obvious and made an easy answer hard! These fellas would have weighed a good 120kg on a good day, so of course a "soft" orhtotic is not going to apply adequate force. Gotta love when common sense takes a holiday, or maybe the synapses in my brain are more "stiff" during times like these.................Look forward to the video.
    Thanks again
    Paul
     
  5. The supination resistance test that I originally described 16 years ago (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992) is a test done manually where the examiner estimates the force required to supinate the foot by pulling up only on the medial navicular area of the plantar foot.

    However, Craig's supination resistance machine has a strap that is placed across the whole plantar foot, from lateral to medial with the medial side of the strap pulling upward to try and supinate the foot and is measured more precisely with a electronic device (Payne C, Munteaunu S, Miller K: Position of the subtalar joint axis and resistance of the rearfoot to supination. JAPMA, 93(2):131-135, 2003). Because of this mechanical difference between the manual supination resistance test I originally described, and Craig's supination resistance machine, I believe that the force required to supinate the foot is less with the manual test than with Craig's supination resistance machine. Craig's machine must pull up both on the lateral and medial arch of the foot whereas, manually, the medial aspect of the arch only is being lifted up. That all being said, Craig's machine certainly has much more accuracy since the actual magnitude of force can be measured whereas the manual test force is only estimated by the examiner.

    This is the main reason why I think Craig's study showed such a strong effect with body weight. With the manual test, I have not noted such a strong effect from body weight. Further experimentation is obviously required to find a more accurate way of assessing supination resistance that would not be so strongly affected by body weight. Possibly an "L-shaped" rubber device could be placed under the plantar navicular to "grab" the medial navicular and then attach to the strap of Craig's machine to supinate the foot without having to also lift up the lateral arch of the test subject. I think this testing method would show a lesser effect from body weight than with the current design of Craig's machine.

    By the way, I think Craig's machine and his work with it is brilliant. Good job Craig and coworkers!
     
  6. Personally I think that body weight will always account for a high proportion of the variance within supination resistance due to the inherent nature of what it is that you are measuring. What's of more interest is the factors that account for the remaining variance such as STJ axial position. Craig can you remind me, what were the other predictors you looked at and what was the r square of the model?
     
  7. Simon and Colleagues:

    I also think that body weight is an important factor but in Craig's study it was found to be just as important, if not more important, than STJ axis location. I believe that once Craig's supination resistance device is modified to only place an upward plantar loading force at the medial navicular, to more reliably reproduce the mechanics of the manual supination resistance test, then more of the association of the supination force will be determined to come from STJ axis location and less from body weight. My model of this mechanical system is illustrated below. The medial navicular force (MF) needed to supinate the STJ is related to body weight (BW), the CoP constant that determines the percentage of body weight that is borne at the midtarsal joint level of the foot (k), the perpendicular distance from STJ axis to 5th metatarsal head (X) and the perpendicular distance from medial navicular to 5th metatarsal head (Y) as represented by the following equation:

    The CoP constant (k) is determined from relative positions of the center of pressure (CoP) acting on the plantar foot to the medial calcaneal tubercle, ankle joint, Achilles tendon and to the navicular within the sagittal plane. In other words, the more the subject is leaning forward and the more anterior the CoP, the greater the percentage of body weight will be needed to supinate the STJ.

    I expect Dave Smith, Eric Fuller and Simon Spooner to check over my equation for suggestions for better accuracy.
     
    Last edited: Aug 17, 2008
  8. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    I no have the paper handy at moment; but it was not a regression model, just a pearson's correlation between some parameters to describe transverse plane axis location, the bodyweight and supination resistance .... the r was around .5 (+/-); giving a r2 of ~25-30% ... meaning both STJ axis and bodyweight both explained about a third of the variability in the supination resistance,
     
  9. I'm actually surprised that body weight didn't account for more than 30%. I think we need to build a regression model and normalise data for body weight, then we will see how important STJ axial position and any of the other variables really are.
     
    Last edited: Aug 17, 2008
  10. efuller

    efuller MVP

    The direction of pull from the supination resistance machine should only matter if the direction of pull changes from one measurement to the next. The vertical component is equal to the sin of angle times force in the strap. So, if the angle of pull remains the same then the force required to produce supination will be proportional to the pull in the strap.

    I agree that forward body lean is important and it is variable. When I was playing with the EMED a lot I noticed a wide variation in the position of the center of pressure as percentage of foot length. I would also predict that this would vary between trials as well as between individuals. The old Hicks paper showed that the more tension in the Achilles tendon the farther forward the center of pressure was. The amount of body lean is a behavioral. As I was looking at center of pressure I was surprised how rarely the "resting" center of pressure was anterior to the ankle joint axis.

    When the center of pressure is close to the axis there will be a low moment from ground reaction force about the ankle. Therefore there is low moment required by the muscles to maintain equilibrium about the STJ axis. (Equilibrium is when the moment from the ground = moment from the muscle and there will be no motion of the joint as required for maintaining static stance.) The explanation of why the center of pressure would be anterior to the ankle joint is probably related to balance. When trying to maintain upright stance there is postural sway. (Try balancing a pencil with the tip of the pencil on your finger.) There is usually constant shifting of the center of pressure relative to the center of mass to maintain balance.

    When the center of pressure is at the ankle joint, different muscles will have to both work to shift the center of pressure under the foot. (Anterior tibial and Achilles tendon muscles) When the center of pressure is anterior to the ankle axis the center of pressure can be shifted with a small change in the tension of the Achilles tendon. It may require less mental energy, but more physical energy to maintain balance with center of pressure anterior to the ankle axis. Or it could be that there is more margin for error in sway if the center of pressure is closer to the middle of the foot.

    With either explanation of why most people choose to have their center of pressure anterior to the ankle axis you still have to account for changes in anterior-posterior position of center of pressure when looking at supination resistance. There's 2 research projects. Where is the center of pressure relative to the ankle joint axis across people and how much does it vary. And with a center of pressure measuring device under the foot and the foot in the supination resistance machine, have the subject look at the computer screen and keep their center of pressure at different percentages of foot length. It would be a way of combining direct supination moment from the tendon with change in moment at the STJ from shifting location of center of pressure.

    Cheers,

    Eric
     
  11. With the supination resistance machine, the strap pushes up not only on the the medial navicular, as in the manual test, but pushes up on the lateral longitudinal arch, middle longitudinal arch and medial longitudinal arch. Therefore, for a given pulling force, the supination resistance machine will tend to pronate the foot more (or generate less STJ supination moment) than will the manual test.
     
  12. Adrian Misseri

    Adrian Misseri Active Member

    G'day all,

    Craig, quick question, is this the same supination resistance test machine that you were trialing when I was back under your wing at La Trobe? (I believe you broke it once testing my feet.....) If so, from my memory, the angulation of the strap that went from lateral to medial passed (if I remember) from the cuboid area and up under the proximal navicular/talo-navicular joint region. Would this not then mimic the direction of pull of the tibialis posterior and to a lessor extent, tibialis anterior tendons, the major muscles involved in supinating the foot in gait? I do see what you're suggesting Kevin that it will also apply a pronatory force on the lateral column of the foot. as there would be dorsiflexory moment along the area of the peroneals though.

    I actually thought it was quite an informative test, and use the manual version in my everyday orthoses prescribing. As has been suggested, I have found a correlation between force required for supination and the angulation of the longitiudinal STJ axis, with those being medially deviated requiring more force. Funnily enough, I've never found bodyweight an issue, with some heavily obese people with cavoid feet often being quite light, and kids often being quite difficult to supinate manually with their abducted, medial bulging, pes-pancakus feet. In response to Eric, I think that this postural center orf pressure is quite important. Everyone who has performed a weightbearing 'palpate the talar head to find subtalarjoing neutral' test will agree how reliabilty can be altered whether the patinet is looking down or standing up straight with head forward, and if we look at the ankle joint as a fulcrum, any adjustment of the center of body mass forwards will effectively dorsiflex the foot, which is a component of pronation, affecting how the foot deals with the supinatory moment of the test.

    Looking forward to the U-tube video Craig, and nice to see the supination resistance test make a re-emmergence. Kudos!

    Cheers!
     
  13. Craig Payne

    Craig Payne Moderator

    Articles:
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    yes
    The strap is attached to the ground around the cboid area, then come up around the medial talo-navicular area. We have experimented with moving it further froward and further back, but the positions are so highly correlated that it does not matter... so yes it sort of mimics the post tib muscle
     
  14. Adrian:

    Excellent posting.:good:

    In the past 20 years since I invented the supination resistance test (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), I have also seen, like you, Adrian, that body weight has only a minor effect on the supination resistance test. It seems to be much more highly correlated to spatial location of the subtalar joint (STJ) axis.

    For example, I have had children that weigh 50 pounds with significantly medially deviated STJ axes have much higher force needed to supinate their feet than large adults that weigh 250+ pounds that have signficantly laterally deviated STJ axes. This is why I feel that doing the STJ supination test by just adding force on the plantar foot to the medial navicular, and not across the whole plantar foot, is much more highly correlated to STJ spatial location and not so highly correlated to body weight.
     
  15. efuller

    efuller MVP

    Kevin, and all those who have stood on the machine,

    Thinking about mechanical analysis of the strap. The forces on the strap: at top, in the direction of strap at the top. At the bottom, in the direction of pull at the bottom. Force on the strap from the foot has to be in a direction to create equilibrium in the strap. So, the force from the foot acting on the strap has to have some lateral to medial force in addition to dorsal to plantar force. So, the equal and opposite force from the strap on to the foot will be from medial to lateral and dorsal to plantar. It seems that highest forces from the strap being applied to the foot would be at the medial part of the foot.

    Questions, how far under the cuboid is the ground attachment of the strap?
    For those who have stood on the machinge do you feel much pressure under the lateral part of the foot from the strap.

    Cheers,

    Eric Fuller
     
  16. Moment = force x perpendicular distance

    Going back to the see-saw analogy. Weight of fat kids on "pronatory" end of see-saw x distance from axis is the "resistance" that needs to be overcome. Let the distance from the navicular to the STJ axis be the lever we have to lift them with. Lets assume that these are the only two factors of significance. Why should the length of the lever be "much more highly correlated" than the load?
     
    Last edited: Aug 19, 2008

  17. Simon:

    Good question. Why would the 50 pound child have more force required to supinate the foot than the 250 pound adult? If we assume that the 50 pound child with a medially deviated STJ axis had a 2 mm supination moment arm and the 250 pound adult with a laterally deviated STJ axis had a 30 mm supination moment arm to the STJ axis from the medial navicular, then this lateral spatial shift in STJ axis location caused a15 fold increase in STJ supination moment arm for the supination resistance test (i.e. posterior tibial muscle moment arm length) whereas the increase in body weight was only 5 fold from child to adult.

    In other words, if we don't model the foot correctly with the proper mathematical equations, then the variables for the equation will not correctly represent reality when we compare it to the actual mechanics of the supination resistance test. I believe that the relatively large changes in STJ spatial location that can and do occur in human feet cause proportionally much greater changes in the magnitude of supination moment arm than do the changes in body weight that occur with the supination resistance test.
     
  18. Kevin, I understand this. What you are saying here is that the rate of change in supination resistance will be higher per unit change in lever arm than it will be per unit change in body weight. However, correlation measures the association between two or more variables. Assuming Craig performed a linear correlation, the r value tells us how well the data fits the linear model: y= mx +c where: y is the dependent variable, m is the slope or gradient of the line, x is the independent variable and c is the intercept constant. So if we draw two scattergrams for the data, one with supination resistance on the y axis and lever arm on the x axis and one with supination resistance on the y axis and bodyweight on the x axis, what we should see is "steeper" line on the lever arm graph compared to the body weight graph (higher value for m with lever arm). This is what you described above. However, this does not mean that the lever arm graph will be any "straighter", i.e. fit the linear model better than the body weight graph. This is what the correlation co-efficient measures; how closely the data follows a straight line. So I don't think that the rationale you provide above explains why the lever arm should be "much more highly correlated" than the load, rather it explains why the change in lever arm results in a greater change in supination resistance per unit than body weight does, this is not the same as correlation.
     

  19. Simon:

    If you can't say that a variable (i.e. STJ axis position) that is 2 times as likely to affect the experimental result as another variable (i.e. body weight) is "much more highly correlated", then what term would you use to describe this experimental result where one variable has more affect on an experimental result than another?
     
  20. Kevin,
    Apologies for the delay in responding to this. The Family Spooner have been on a surfin' safari in the kombi.

    I'm not sure what term I would use, other than those that I have used in my previous post. But I wouldn't use "correlation" since rate of change and correlation are not the same thing.
     
  21. Simon:

    After going back and reviewing the concept of correlation in statistics, I believe I was right initially that there should be a higher correlation between STJ axis position and supination resistance than between body weight and supination resistance (assuming supination resistance test is done with the force only on the medial navicular and not across the lateral plantar midfoot area also). The reason I believe this is because, in my clinical experience, supination resistance seems to be more highly related to STJ axis position than does body weight. Try performing the manual supination resistance test on a number of patients and see if you don't agree with me. Maybe I'm wrong......but I don't think so.
     
  22. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    It could be because of the parameters we used to measure the STJ axis position ...ie perpendicular distance from axis to 5th met head. If we had a parameter close to the rearfoot (difficult to actually measure), the correlation may well have been different.
     
  23. Kevin, can you explain why a higher correlation should be observed in statistical terms please? Maybe I'm slipping with my statistical knowledge as I'm not using it so much these days.
     
  24. johnatperth

    johnatperth Member

    Hello Kevin, Craige and Simon.

    Having simillar observations to you on supination resistance, I believe your right. I think Craige's supination resistance measuring device is working at a mechanical disadvantage. The straighter the strap is, the more force must be used to maintain the same supination force in the medial navicular. This will amplify the result obtained for narrower feet, higher archs' and heavier people (because of loading on the strap plantar to the cuboid etc). Trialing my own interpretation of Craige's device on myself I found the force required to supinate my foot varied with the angle of pull, (that is the bend in the strap). The more bend the easier it was to supinate my foot.

    Thanks for your discussion
     
  25. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    John, You are right, the angle of pull of the strap is crucial. We make sure the the medial border of the foot is in the same place for each foot, so the angle of pull is the same. That way we a consistent within a study - I would not, however, want to compare our data to orther devices being used for measuring this for this reason and a few others.
     
  26. Dr. Raymond Duck (formerly known as Simon Spooner):

    Even though statistics was probably my worst math subject, and you have a much better understanding of the subject than I do, I will still make an attempt.

    Higher correlations will be observed with STJ axis location since a change in STJ axis location will cause a more predictable change in supination resistance. Lower correlations will be observed with body weight since only small changes in STJ axis location will affect supination resistance greatly, and therefore body weight to supination resistance will be less predicatable and more variable.

    OK?....Brother Duck?
     
  27. Unfortunately until we get research which demonstrates vastly different r square values for lever arm and body weight we cannot say that lever arm is more predictable of supination resistance. In a big enough sample, load should also provide a predictable change in supination resistance. I'm not sure whether we will see a vast difference in the r square values. Which is what Craig's research has already demonstrated. We may have to agree to disagree on this one Kevin until the data comes in. Then we can split this atom properly, in the time honoured manner. :drinks I look forward to being proved wrong, but I look forward more to a multiple regression model which takes into account these variables simultaneously and provides a predictive model that can be used clinically. Thinking about it, this data could be re-arranged to build a model that could be used to predict STJ axial position.

    Quack, Quack
     
    Last edited: Aug 26, 2008
  28. Another complicating factor is that in those patients with significantly laterally deviated subtalar joint (STJ) axes, the effect of ground reaction force and Achilles tendon tension is to cause the STJ to have sufficient supination moment to maximally supinate the STJ. These patients use tonic peroneus longus and/or peroneus brevis muscular activity to add a STJ pronation moment to their foot and, therefore, use the peroneals tonically to keep their feet plantigrade and their STJs from supinating maximally.

    In these patients, "body weight", so to speak, is causing STJ supination, not STJ pronation. This was not accounted for either by Root and colleagues in their "STJ neutral theory" or is accounted for by the supination resistance test, by itself. It is accounted for in the STJ axis location and rotational equilibrium theory. Therefore, until we can account for supination/pronation moments added into the foot by the extrinsic foot muscles that cross the STJ axis during standing and other weightbearing activities, the equations for calculating supination resistance from either STJ axis and/or body weight alone will be problematic.

    Good discussion.:drinks
     
  29. Griff

    Griff Moderator

    Craig,

    Did this video ever make it onto You Tube? Had a cursory glance but could not find it

    Thanks

    Ian
     
  30. I've been thinking about this one again, looking at Craig's experimental set-up here:
    http://www.latrobe.edu.au/podiatry/supination-resistance-device.htm
    it seems that supination of the rearfoot was achieved by increasing the tension in the strap under the foot via the pulley mechanism. How did you control the acceleration induced by the "pulling" on the pulley mechanism to increase the strap tension, Craig? I'm trying to do something similar at the moment and just thought about this potential source of error- any help you can offer should be much appreciated
     
  31. joejared

    joejared Active Member

    Not that it matters too much, but typically y=mx+b is used to define a line. It would probably provide better results to go with a non-linear model, however, once enough data was acquired. Here, nth order non-linear regression in 2 or 3 dimensions are simple library operations and it would take little difficulty to provide a graph of output of either.

    From a prior thread or comment, I was thinking along the lines of a mechanism that would apply force as suggested with a pressure sensor of some sort in a repetative fashion so that the human element could be minimized throwing out the upper and lower quadrant and averaging the remaining results. It would seem that the practitioner would have to lean heavily on the subjective as to whether or not the patient was relaxed and whether or not their measurement was good.
     

  32. As I defined my algebra it REALLY doesn't matter does it Joe. If a linear model fits the data well, why use a non-linear model? Moreover, how do you get 3D or n>2 degree from two variables? A little statistical test for all, as we increase the x terms in the model, what is lost?

    Patient relaxation is difficult to control, but repeatability analyses give an insight into error due to this.
     
    Last edited: Jan 5, 2009
  33. Simon:

    The accelerations of the subtalar joint supination are probably so small in using these types of supination-producing machines that the foot could be modelled effectively using quasi-static methods. In other words, make sure the pull or push from the machine is slow and steady, and then quasi-static approximations should be able to be used with mininal error.
     
  34. Thanks for that. I'll e-mail you with details of my experimental set-up for your once over.

    Happy new year BTW
     
  35. joejared

    joejared Active Member

    Oh, in your case, what you're describing is a 2D dataset. I can't disagree that linear interpolation would work, but typically I find non-linear interpolation to be a bit more accurate for most things. This might be because most of the datasets I work with relate to contours. I'd be inclined to believe range of motion and force required varies more dramatically as the BMI (Body Mass Index) changes, mostly because things start to collapse as the high end of bmi, but that's hypothetical from my side.

    I take that to mean we might be on the same page? If I were to guess, there would be a higher initial resistance to motion and towards the end of a cycle of tests, the patient might even become overcooperative, helping the test along.
     
  36. Griff

    Griff Moderator

    Looking at these pictures reminded me of something I thought a while ago - I have always performed the supination resisitance test with the patient static and on two feet. Do we think it would be different (and/or relevant) if we tested this with the patient in single limb support; which is obviously where we spend most of our dynamic life?
     
  37. CraigT

    CraigT Well-Known Member

    Simon
    The company which makes the force gauge used (I think) also make test racks which are designed to apply a force at a constant speed (Mecmesin). That might solve this potential problem.
    Have also wondered about the accelerations. Comments Craig P?
     
  38. Have been doing this....
     
  39. Craig Payne

    Craig Payne Moderator

    Articles:
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    Sorry about late replies --- barely keeping 'head above water' at the moment ...
    It still on the 'to-do' list with a zillion other things - will get to it soon.
    The acceleration is a problem. We have found that the intratester relibility of this device is extremely good; but the intertester reliability is not so good, purely as two different people will pull at different velocities --- fortunately everyone is consistent with the velocity that they pull with (hence the good intratester reliability). Martin Colledge has been working on a device with that uses a servo motor so that can be controlled for.
    Have tested that - subjects were measured in double limb stance and single limb stance and the pearsons correlation was 0.91 (which is obviously good); problem with single limb stance is that many subjects were unsteady and muscles start fireing; so given the 0.91 correlation, we keep doing it double limb support.
    Its a problem (see reply to simon above).
     
  40. Mart

    Mart Well-Known Member


    Just wanted to elaborate a bit on this. I have copied and I hope refined Craigs original apparatus a little, replacing the manual force applied to the strap with an electrical actuator which is heavily geared and applies a slow motion which is consistent for the individuals weight. I have not attempted to calculate how the applied torque varies with weight. Since there is no electrical compensation for load I suspect the acceleration slows somewhat according to subject resistance but would expect this to be consitent for the subject's limb over multiple tests.

    My suspicion is that intertester reliablility may be influenced by the perceived motion threshold causing different observers to record a different range of values. I have done some crude investigation for this by applying an accurate digital linear distance probe against the sustentaculum tali and incrementally recording force vs horisontal motion. What seemed to happen was that motion was detectable as soon as the supination force was applied in the several subjects I examined but this was damped by the compressible tissue underlying the probe. Increasing the surface area of probe contact as might be expected helped to reduce this effect. I feel that this issue needs sorting out if a weight normalised value is to be attempted.

    My time is currently overloaded with my MSc studies and I have had to put this project on hold for a while. I feel that this idea can yet provide some interesting observations of foot behaviour and if some of the confounding variables can be isolated a normalised value may be obtainable and useful.


    cheers

    Martin


    The St. James Foot Clinic
    1749 Portage Ave.
    Winnipeg
    Manitoba
    R3J 0E6
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
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