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Effect of foot orthoses on COP

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Mart, Jan 17, 2013.

  1. Mart

    Mart Well-Known Member


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    Hi All

    Just wanted to seek opinion regarding proper interpretation of in-shoe pressure data

    please refer to

    http://www.tekscan.com/plantar-fasciitis-pressure-mapping-williams

    The accompanying interpretation of data states

    Pressure Profile With Orthotics

    Notice the improvement in the mid-line position for the center of force.

    Question is

    Would we not expect to see a medial shift in COP for ALL feet with addition of any increased contact into the inner longitudinal arch where there was little or no contact with floor without presence of foot orthoses?

    Also would it not be unusual to see a central COP in an unshod or non total contact foot orthoses in-shoe measurement unless foot was pes planus weight-bearing?

    If this is true then can we really interpret this as an improvement since what the medial shift of COP is showing here is simply increased ground reaction force in the inner longitudinal arch - anything else would be surprising ?


    Cheers

    Martin

    Foot and Ankle Clinic
    1365 Grant Ave.
    Winnipeg Manitoba R3M 1Z8
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
    www.winnipegfootclinic.com
     
  2. Martin,
    The medial shift in COP could be due to the foot pronating more in association with the orthoses, it could just be that more of the medial longitudinal arch is in contact with the in-shoe sensor as you suggested.

    Remember force is a vector; COP doesn't tell us anything about it's magnitude or direction, only it's point of application. Nor does it tell us the sense of the moment acting about a joint axis. Centre of Pressure (COP) is a rather blunt instrument when applied in isolation.

    See:
    http://www.ncbi.nlm.nih.gov/pubmed/16988168

    and
    http://www.ncbi.nlm.nih.gov/pubmed/21084541
     
  3. David Smith

    David Smith Well-Known Member

    Mart
    I don't fully agree with he interpretation given by Bruce of the pressure characterisation.

    In your first question, yes the CoP will move medially when there is medial posting/medial arched orthosis (under the conditions you state)

    To your second question the answer is no its not unusual

    your third question, Disregarding the former is not true: The medial shift of the CoP on its own cannot be interpreted as 'an improvement' it is what it is, [Yes anything else would be surprising (except if the STJ axis was lateral and the pronation was caused by early or long peroneal firing)] if the intention was to move the CoP medially and you deduced that this medial shift would result in reduced tissue strain in the tissue of interest then this can be interpreted as an improvement. The real improvement of course is when the customer returns with a smile on their face and the pain is gone.

    Regards Dave
     
  4. Simon and David have some excellent comments here. Analyzing the CoP position and motion patterns are fairly limited in allowing us to determine the actual moments acting across the joint axes of the foot and lower extremity due to their inability to detect the non-vertical shearing ground reaction forces that act on our feet at all times with every step. In addition, without a knowledge of the three-dimensional spatial location of the subtalar joint axis and the ground reaction force vector, one is only guessing as to the magnitude of STJ pronation/supination moments that the CoP is creating at any instant in time.

    However, analyzing the CoP does give us information regarding how the ground reaction forces are basically moving along the plantar foot. In turn, this information may, in some instances, allow us, along with other clinical observations, to make more informed clinical decisions as to how the foot and lower extremity are functioning during gait and how best to treat the patient with foot orthoses or shoe modifications.
     
  5. efuller

    efuller MVP

    I also disagree that you can categorically state that the COP line becoming more central is an improvement. It certainly will be for some people, but not all.



    When you look at center of pressure lines of gait: At the time of heel contact the center of pressure will be under the heel. When there is lateral forefoot contact the average point of force (center of pressure) will be (numerically) pulled laterally and anteriorly. Immediately after lateral forefoot contact there will be rapid pronation of the STj and medial forefoot contact will happen. This will pull the average point of force medially and as more force is placed on the forefoot the center of pressure will move anteriorly. So, with a sampling rate of 50 Hz the first couple of dots (location of center of pressure) will be pretty close to the same location under the heel. Then the next couple of dots will move toward the fifth met head, then when medial forefoot loading occurs the dots will take a medial turn and continue moving toward the forefoot.

    I don't recall looking at this part of the curve and comparing with and without orthotics, but with feet that pronate enough so that there are high loads under the first met head, the curve will be pretty far medial even without an orthotic.


    No as I described above, the Center of pressure is an average point of force and can be located in an area that does not actually touch the ground. High pressure on the medial forefoot and pressure on the heel will average to a location under the medial longitudenal arch.

    Eric
     
  6. efuller

    efuller MVP

    I'd have to disagree and say sometimes it will and sometimes it wont. Take a foot that sits with the center of pressure directly under the foot in static stance. In this foot no moments are required in the foot above to maintain equilibrium. If you placed a varus wedge under the heel of this foot you would create a supination moment and the STJ would supinate. As the STJ supinates, there will be an increase in force further laterally, most likely on the forefoot. So the medial wedge moved the center of pressure medially under the heel and the resulting motion will move the center of pressure laterally under the forefoot, so, when you look at the center of pressure for the whole foot it will not have moved. It's like two people sitting on a see-saw half way from the fulcrum and then both people moving further away from the fulcrum to maintain the same position. However, with the medial wedge, you should be able to detect a change of center of pressure udner the heel. (I've done some prelilinary work with this and did see a medial shift of cop under the heel.

    In gait, you can't predict where the center of pressure will go because where the center of pressure will be located is determined by muscle activation. You can choose to walk more pronated or more supinated depending on where the thumbtack is stuck in your shoe. (As long as you have range of motion)

    You may see some changes in specific locations. I've seen medial shifts in the heel with dual density midsole shoes. In the forefoot the location of the center of pressure will more likely be determined by the relative activation of the peroneal muscles and the posterior tibial muscles.

    Eric
     
  7. Mart

    Mart Well-Known Member

    Thanks to comments from you all; if you will indulge me I would like to push some ideas which follow from your combined and excellent review cited below (Spooner, Smith et al. 2010).

    I will try and take very small “bites” over time so that this doesn’t become onerously time consuming. I think that there is likely some interest across podarena from those using or contemplating using pressure matrix technology beyond a rather disingenuous marketing tool.

    As Simon mentioned – no definitive text seems available to educate users regarding interpretation and without good grounding in “proper biomechanics” there are plenty of horrible pitfalls to fall into.

    If we ignore issue of instrument error right now, as you discussed in your paper there are importantly issues in properly interpreting the data not only because it often represents only the vertical component of ground reaction force but that limitation then becomes complicated when the sensor is not aligned to the ground surface because the actual orientation of the sensor is unknown and likely indeterminate because around heel cup and inner longitudinal arch each individual sencell orientation in space varies incrementally from horizontal to at extreme vertical.

    Considering this, I would initially like to only consider interpretation of forefoot pressures since these are largely excluded from complication of sencell orientation relative tio graound

    Some basic statements for comment even if simply true/false (T/F).

    ONLY in consideration of forefoot (sub metatarsal head distally) pressures in the same individual under same conditions:

    1 We can reasonably assume that we are looking at sencells parallel with walking surface.

    2 If we want to estimate the dominance of the vertical component of the ground reaction force (VCGRF) at any instant in time we may be able to usefully generalize that; at constant walking velocity in a straight line; it will vary closely coupled to the distance the subject’s COM deviates from the COP. When COM is above COP the value of resolved vector will approximate to VCGRF and as the COM moves away from COP then it will increasing lessen and the relationship between difference of resolved vector and VCGRF will be largely dependent on velocity of COM.

    3 The value (implied importance) of the anterior/ posterior (AP) and medial/ lateral (ML) reaction force component at any instant in time can be estimated by considering any changes to line of progression of COM compared to COP; more precisely they will be inversely related to its direction. Therefore walking forward in a straight line changes will be predominanlty in AP axis.


    BTW I do have an inductive position I am heading towards so there is some point to these arguments – just want to check their face value step at a time (pun noticed but initialy unintended)

    Cheers

    Martin
     
  8. Mart

    Mart Well-Known Member

    No as I described above, the Center of pressure is an average point of force and can be located in an area that does not actually touch the ground. High pressure on the medial forefoot and pressure on the heel will average to a location under the medial longitudenal arch.

    Eric[/quote]

    Thanks Eric - just want to clarify this.

    When we examine the graphical representation (as in the example cited at start of thread) of the position of COP it represents the cumulative history of a sequence of events. One problem when considering the data as a complete stance in this single snapshot is that it is unclear what part of the foot is contributing to the sum of forces for each point on the line, we would need to examine the indivual samples to properly understand that.

    I agree that it would be possible to see a COP in an area of non contact but this would only be clear if we look sequentially at each sample to see how this might happen.

    Cheers

    Martin
     
  9. Interestingly, the one bit we were asked to alter in our paper was related to this; we opted to remove it because it was such a minor aspect within the paper. Point one is arguable, but go on, go ahead and spoil us Martin, where are you leading? I suspect where you are going is that there is a trigonometrical function which can be used to calculate the anterior-posterior component of force when using a pressure system that only measures the vertical component of force, given a constant velocity and a pressure sensor that is perpendicular to the supporting surface (can these assumption be given?). Moreover, you'd need to know the COM position too. From Orlin and Mcpoil: "Cornwall and McPoil19 were able to predict fore-aft or anterior-posterior shear force with a pressure sensor platform using a combination of peak force, time to peak pressure, and stance phase duration. To accomplish this, the authors attached a pressure sensor platform to a force platform and collected data with both systems recording simultaneously at a similar sampling frequency. No researchers to date, however, have been able to determine the medial-lateral " http://www.physther.net/content/80/4/399.full Or, do you have something else in mind with this? I hate teasers.
     
  10. Mart

    Mart Well-Known Member

    Thanks Eric, I agree and an important point. A little aside whilst thinking about this - I have until recently largely regarded recruitment of peroneals largely as protective for lateral ankle ligaments when shifting direction - providing internal protective pronatory moment around sub-talar joint axis.

    During single limb stance period of forward walking gait, the tendancy of gravity to pull COM medial to COP may be influenced the peroneals "pulling the lower limb to "balance" COM over the COP.

    Is this likley an important contribution of normal gait or largely subserviant to effect of hip abbductors?

    Cheers

    Martin
     
  11. efuller

    efuller MVP

    The picture that the software usually gives you is the maximum pressure plot with all of the path of the center of pressure (each individual center of pressure point for each sample) laid on top. The maximum pressure plot (MPP) gives you the maximum pressure at each sensor over the entire step. So, it doesn't actually give you reality. Well, it might look like a foot print in sand, but it is not representative any single point in time. After looking at a lot of rollover "movies" you might be able to make some sense of the maximum pressure plot, but starting with the MPP will probably just confuse. Is that what you were asking?

    Eric
     
  12. Mart

    Mart Well-Known Member

    Yes - thanks Eric - perhaps confirming the obvious but those following the post may not be familiar with exactly what the graphic represents.

    Cheers

    Martin
     
  13. efuller

    efuller MVP

    It depends. Think rigid rocer bottom shoe vs. very flexible shoe.




    What do you mean by dominance of the vertical component? I'm sure what you are looking at here.

    David Winter had some excellent papers on the physics of gait in the 90's. He described how center of pressure and center of mass were used in walking. One of the things he showed was that the cener of mass stayed in between the feet during gait. The center of mass and the center of pressure create a force couple that will cause a rotation. In the frontal plane with the left foot on the ground the center of mass (or gravity acting on the center of mass) will be to the right of the left foot. This will cause a rotation toward the right until the right foot accepts weight and the center of pressure is moved toward the right foot. Then a force couple will be formed to accelerate the center of mass back toward the other foot.

    In gait, when both feet are on the ground, the center of pressure acting on the whole body, will be in between both feet. So, you can't just look at the Cop of one foot.

    Eric
     
  14. Mart

    Mart Well-Known Member

    Yes you spotted part of my chain of thought - thanks for the reference I wasn't aware that it had been tried - will check it out.

    Where I am heading tries to push past that - after reading what I intended to post it was too much of a rambler - sorry to sound so coy . .. I'll see if I can make it sensible after a bit more thought and post later.
    Cheers

    Martin
     
  15. Mart

    Mart Well-Known Member

    My thoughts are that clinically, when concidering forefoot pressures and force time integrals we are often weighing up the posibility of elevated localised tissue stress as potential for injury or non healing and if that if percieved goal to use foot-wear/ foot orthoses to mittigate that stress we are usually seeking evidence of usefully reducing those forces. Since major limitation of pressure mats is not measuring shear, also that there is no proper understanding predictively of relative role of compressive vs shear as cause of injury and that may vary individually both as threshold level and relative cause, the true value of measuring vertical GRF is somewhat questionable. If we are going to use this tool without the benefit of a force plate then there may be useful reasoning we can apply regarding probalilty of "predominate" activity of force according to relative position of COM to COP. So if that is true there may be some crude algorythm that can be applied to approximate inferred proportion of each(dominance of forces)which has some relationship to that COM/COP relationship. At basic level this might be: proportion of shear force greatest at HC and TO and least at MS - idea then is replace dominance (poor choise of word on my part) with proportion to make my comment understandable.

    Cheers

    Martin
     
  16. Martin:

    I'm having difficulty following what you are exactly asking for here. However, from what I can decipher, you are looking for a method by which you can reliably estimate the medial-lateral and anterior-posterior shearing forces during walking gait with either a pressure mat or pressure insole system. Am I right?

    Suffice it to say that shearing forces are not totally predictable from looking only at the spatial location of the center of mass (CoM) relative to the center of pressure (CoP), but may probably be estimated somewhat. Bart VanGheluwe, Friso Hagman and I published a study over six years ago where we looked at how simulated genu valgum/varum walking styles affected the angle of the ground reaction force (GRF) vector during walking (Van Gheluwe B, Kirby KA, Hagman F: Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait. JAPMA, 95:531-541, 2005). Bart and Friso used a pressure mat placed on top of a force plate in order to allow both precise localization of the CoP relative to the foot along with the spatial orientation of the GRF vector at any instant in time.

    The shearing forces changed, of course, with different placements of the foot relative to the CoM, which we expected, and shearing forces were also affected by genu varum/valgum angle. I simply don't think you can reliably extract this information from just a pressure mat or pressure insole that does not measure non-vertical shearing force, no matter how accurately you measure the spatial location of the CoM relative to the CoP. Part of the reason that I believe you will never be able to do this is because muscle activity has both interindividual and in- step variability. In other words, muscle activity will instantaneously affect the shearing forces present under the foot before the spatial location of the CoM has had a chance to accelerate to a new CoM spatial location (i.e. the body has inertia and, as a result, there will be a finite amount of time for a shearing force to move it medial, lateral, posterior or anterior).

    I would suggest that, before you get too far along with this idea, you try to find a lab where you can put a pressure mat on top of a force plate to see if you can somehow reliably predict the shearing forces seen with the force plate from using only the data generated by the pressure mat. I think you will find that using CoM estimation to estimate shearing forces will not only be only a crude approximation of shearing forces but will require a 3D video system to do so with any reliability.

    Hope this helps.:drinks
     
  17. Mart

    Mart Well-Known Member

    Yes exactly - the reason I was meandering around ideas was to question the reasoning to bother doing this and to question the face value of the idea so as not to :deadhorse:.

    I am just about to install a a couple of AMTI force plates and 3D system here and am considering at this as a possible project - I havent done a proper literature search yet and appreciate the views of the "4 podarena biomechanics musketeers" which usefully speed up learning curve.

    Cheers

    Martin
     
  18. Mart

    Mart Well-Known Member

    Simon cited

    http://www.physther.net/content/80/4/399.full

    this in turn cited

    1. Cornwall MW, McPoil TG. The relationship between maximum plantar pressures and anterior-posterior shear during walking. Lower Extremity.1995 ;2:141–146.
    Pub Med search cant find this and I have tried variant searches without luck. Anyone able to send me a copy.

    Thanks in anticipation.

    Cheers

    Martin
     
  19. Mart

    Mart Well-Known Member

    Thanks everyone for contributing; below I have pulled out some points from conversation so far which I would like to add to.
    -----------------------------------------------------------------------------------------------------
    “Centre of Pressure (COP) is a rather blunt instrument when applied in isolation”.

    “In gait, when both feet are on the ground, the center of pressure acting on the whole body, will be in between both feet. So, you can't just look at the Cop of one foot”

    No question a rather blunt instrument but could it be sharper if COP is confined to a region of interest (ROI)?

    COP is typically measured over an entire plantar surface or as Eric mentioned the surface boundary of both feet. COP can be defined though however we chose; if we limit the boundary say to a metatarsal head, then we are constraining the information and perhaps, because it is more specific, allowing that information to have clearer interpretation.

    An example might be

    “So the medial wedge moved the center of pressure medially under the heel and the resulting motion will move the center of pressure laterally under the forefoot, so, when you look at the center of pressure for the whole foot it will not have moved”.
    Eric commented on the important limitation of not defining the ROI according to task. If we had defined 2 ROIs however, one at medial heel and one at lateral FF then we could have understood properly what had happened by noticing the timing and direction of the COP changes within in those ROI.
    --------------------------------------------------------------------------------------------------------

    “Shift of the CoP on its own cannot be interpreted as 'an improvement' it is what it is . . . . but if you deduced that this medial shift would result in reduced tissue strain in the tissue of interest then this can be interpreted as an improvement.”

    “Analyzing the CoP does give us information regarding how the ground reaction forces are basically moving along the plantar foot. In turn, this information may, in some instances, allow us, along with other clinical observations, to make more informed clinical decisions as to how the foot and lower extremity are functioning during gait and how best to treat the patient with foot orthoses or shoe modifications.”

    Exactly why there may be gold in them thar hills

    --------------------------------------------------------------------------------------------------------
    “I simply don't think you can reliably extract this information from just a pressure mat or pressure insole that does not measure non-vertical shearing force, no matter how accurately you measure the spatial location of the CoM relative to the CoP.”

    So far that is assumed true, unfortunately I haven’t been able to get hold of Cornwall MW, McPoil,s paper to see how reliable and generalizable the relationship they established proved to be.

    Cornwall MW, McPoil TG. The relationship between maximum plantar pressures and anterior-posterior shear during walking. Lower Extremity.1995 ;2:141–146.
    --------------------------------------------------------------------------------------------------------
    “Part of the reason that I believe you will never be able to do this is because muscle activity has both interindividual and in- step variability.”

    That variability is measurable and I believe if we confine the COP to a small ROI this may not be such a problem . . . more to follow. . .

    --------------------------------------------------------------------------------------------------------
    “In other words, muscle activity will instantaneously affect the shearing forces present under the foot before the spatial location of the CoM has had a chance to accelerate to a new CoM spatial location (i.e. the body has inertia and, as a result, there will be a finite amount of time for a shearing force to move it medial, lateral, posterior or anterior).”

    Kevin - fascinating point which I hadn’t thought about before.

    If I understand the implications of this properly, the mechanical properties (primarily plantar calcaneal area and plantar metatarsal area plantar fibro-fatty pad and to certain extent joint ligaments across hip/knee/ankle), because they stretch will create a lag between the motion of the COM and a change in COP?

    Is that lag then an index of the stiffness of those tissues and perhaps a measure of clinical importance because that lag may be related to potential for injury.

    If the lag is predominately a function of the plantar fibro-fatty pad deformation then that may have useful measurement value (if that is possible); if an intervention reduced shearing at a site subject to intolerance to shearing then the measured lag would be expected to decrease? I guess there remains the issue of signal/noise ratio in measuring that.

    Where I was leading though at the start of the thread was to think about the rate of change of COP in a given direction rather than simply its trajectory.

    For example; isolate the sencells at the 2nd metatarsal head. Consider the centre of pressure ONLY within boundaries of the MTH. Information about the forces acting on the skin in that area will be limited by size and spacing of sencells under MTH and sample rate. Depending on the resolution of those parameters there may be lack of meaningful data if sencells are too large, too widely spaced or noisy data if sampled too fast (this could be filtered).

    What I believe we could infer is that regardless of cause, the magnitude of shear will be related to the rate of change of COP and that its direction will likewise be related to inverse of direction of COP.

    Whilst there will be a lag between measuring change of COP and associated change in COM, COP and COM remain interdependent and if this deformation remains fairly constant possibly coupled by some “summed coefficient of tissue deformation”.

    Information contained within the ROI, because it is isolated, has greater acuity for determining non vertical GRF than if we used the COP for the entire plantar surface PROVIDED the time period of measurement was constrained so that, within the ROI, the number of sencells recording pressure values above a reliable threshold (measured area) remained constant.

    The parameters required to calculate the rate of change is extractable from FScan files. Currently there is no way to make that data obvious within the packaged software other than look at the length of the lines between the dots.

    The natural variability which Kevin talked about is a problem if the goal is to isolate and measure change in functions which predict the COP.

    However if we are more concerned regarding the effect of an intervention, to know the nature of the COP at a ROI may have value, if we can assume it is caused by the intervention.

    To do this the variability would need to be measured both with and without intervention and data constrained within predetermined confidence limits.

    How might we characterize the nature of COP? I have some ideas but before blathering on how does this hold up so far?

    Cheers

    Martin
    Foot and Ankle Clinic
    1365 Grant Ave.
    Winnipeg Manitoba R3M 1Z8
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
    www.winnipegfootclinic.com
     
  20. Martin, I don't have time to answer all of your points, but I will address your first point. Nope, it's still a blunt instrument since: you still do not know the kinematics of the body above the sensor; you still don't know the magnitude nor direction of the ground reaction force vector at any instant in time; you still don't know the spatial orientation of the joint axes relative to the vector; you still don't know the spatial orientation of each of the sensors and the influence this is having on the calculation of COP; you still don't know how the stiffness of the surface that the sensors are placed upon and how the bending of the sensors might be influencing the output from them; and you still don't know how the change in frictional characteristics induced by the imposition of the sensor is influencing the kinetics / kinematics compared to if the sensor were not there. Other than that, I guess you're on the money :rolleyes:
     
  21. Martin:

    Here is the paper mentioned earlier in pdf format. I'll look in my garage to see if I have those old issues of "Lower Extremity" still.
     
  22. Mart

    Mart Well-Known Member

    Oh dear . . . I just spent a couple of hours replying to to this and describing my idea and of course it was . . . a bit sad really. Anyhow thanks for humouring that, I have to say that I have a much more profound understanding of COP as a result of this thread and thinking stuff through with these challenges, hope some others did too - as always thanks for your help - over and out :eek:

    Cheers

    Martin

    Foot and Ankle Clinic
    1365 Grant Ave.
    Winnipeg Manitoba R3M 1Z8
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
    www.winnipegfootclinic.com
     
  23. Mart

    Mart Well-Known Member

    Thanks Kevin - The Cornwall, McPoil paper doesn't seem to be linked properly in the online databases

    Cheers

    Martin
     
  24. Couldn't find my old issues of Lower Extremity...must have thrown them out.
     
  25. David Smith

    David Smith Well-Known Member

    Martin

    No! thats not the truth, I would encourage you to keep on exploring and examining what comes out of that exploration - its good stuff.


    Why is the Centre of Pressure confined to the Republic of Ireland Does this mean the UK is pronating to the west?:D
     
  26. David Smith

    David Smith Well-Known Member

    Hi Mart

    I printed off and read thru the full thread last night to get the right feel of what you are trying to discover. (Luckily Simon and Kevin are more perceptive than me :eek:)
    Anyway this hastily drawn diagram might help.

    You might notice that I am trying to convey that wherever you determine or measure the CoP or CoF to be in the horizontal flat plane of the ground there is no reliable way to interpret that within the shoe in the plane of the reactive surface the foot is acting against. Well I suppose you could track the foot and estimate the relative position of the orthosis and foot interface for each individual case but that might be very time consuming.

    [​IMG]

    Dave
     
  27. efuller

    efuller MVP

    Center of pressure is different from magnitude of force. It would not make much sense to examine the CoP over a small region. The CoP is the location of the average point of force and I see no reason to figure out the location of the average point of force for a small region. Now, the magnitude of force in that region might be very relevant. For example, if you were interested in stress fractures of the 2nd metarsal then the magnitude of force on the second metatarsal head would indeed be interesting. Also locations of ulcerations would be interesting as well.


    My example was of one specific situation. One in which the foot was balanced with theoretically no internal moments. Another example is one where the center of pressure is lateral to the STJ axis and there is a pronation moment from the ground and there must be a corresponding internal, to the foot, supination moment to maintain equilibrium. In this case, the rearfoot varus wedge would cause the the whole center of pressure to shift more medially and the internal supination moment to reduce a corresponding amount. Both of these examples are in static stance. As I said before, in gait the relative activity of the peroneal and posterior tibial muscles will determine the location of the center of pressure on the forefoot.

    The regions of interest would have to be divided along the STJ axis to see the effect of the original example.





    The rate of change of the CoP is just a measure of how fast the average point of force is moving. With high plantar flexion moments at the ankle, the center of pressure will shift from posterior to anterior faster than if there are low plantar flexion moments at the ankle. This might be an important indicator of something. But we have to be careful about saying what that something is. Howard has talked about a delay in calcaneal unweighting when there is a sagittal plane blockade. A delay in calcaneal unseigthing would be seen as a slower progression of the center of pressure plot. My explanation of that delay is a pain avoidance mechanism where the person chooses to use the calf muscles less and create less plantar flexion moment to avoid putting weight on the forefoot.


    To describe a force you need a direction, a magnitude and a point of application. The center of pressure is the point of application and is independent of magnitude. In some of your suggestions, I think, you are mixing magnitude and location.

    Eric
     
  28. David Smith

    David Smith Well-Known Member

    Just to clarify for the reader


    [​IMG]


    Regards Dave
     
  29. Bruce Williams

    Bruce Williams Well-Known Member

    Martin;

    if the medial arch of the orthotic is not comfortable for the patient or restricts 1st ray motion of plantarfleion in midstance then the CoP can and will remain lateral.
    I will not disagree that the pressure of the medial arch will usually increase with a good form fitting cfo. But if it is impeding progression the the CoP will deviate appropriately.

    For you final question I disagree in part, mainly because of what I stated above.

    CoP ultimately is only one of the variable from in-shoe pressure that I use to grade success in treatment with a CFO. It is the amalgam that is ultimately the predominant indicator of success to me.

    Sincerely;
    Bruce Williams
     
  30. Mart

    Mart Well-Known Member

    Thanks for that Dave, the diagram will make useful reference point and the ROI meaning went right over my head :). As far as my initial flawed idea; it was based on premises which I had not though carefully enough and fell apart after considering confounding factors. I am still mulling this over but won't post further unless I can see a solution. I want to look at Eric comments next - see where this leads.

    Cheers

    Martin
     
  31. efuller

    efuller MVP


    This is exactly why I don't like the terms external and internal. I was talking about a plantara flexion moment applied to the foot by the gastroc and soleus muscles. If you define the system as the foot and leg, the momemnt from the muscles is internal. If you define the system as just the foot and the lower leg is external to the foot then the moment from the leg is external. With either of those definitions of system the force from the ground applied to the foot is external. So, when you use internal and external you have to define the boundries of the system that you are talking about.

    Eric
     
  32. efuller

    efuller MVP

    I'm not quite sure what you are saying. Are you saying that force is not a vector? Force is a vector. When you multiply a scaler quantity (e.g. mass) by a vector quantity (e.g. acceleration) then you get a vector. F = ma.

    Center of pressure is used to figure out a point of application of forces applied over a surface. From this point you can calculate moments. This was elegantly demonstrated in the old Hicks paper that looked at the function of muscles. In that paper, Hicks found a balance point when a loaded foot was placed on a flat surface and pin was moved under the flat surface until balance was achieved. Hicks didn't call it that, but that balance point is the center of pressure. Reading that paper should help people understand how muscle activation changes the location of the center of pressure. I believe that paper can be found in the classic biomechanics papers thread.

    Eric
     
  33. Here is a nice video on CoP and Free Moment, specific to force plates.

     
    Last edited by a moderator: Sep 22, 2016
  34. The concept of center of pressure is also used in understanding the physics of flight and also the forces exerted on the sails of sailboats and sailboards (i.e. windsurfing).

     
    Last edited by a moderator: Sep 22, 2016
  35. Here is a nice little science experiment which demonstrates how understanding the concepts of center of mass (called "center of gravity" in this video) and center of pressure can help explain the physics of balance and also how a soda can can be balanced on its edge.

    Physics can be phun!

     
    Last edited by a moderator: Sep 22, 2016
  36. Mart

    Mart Well-Known Member

    Thanks Dave - the ROI was invisible until you pointed it out. Thanks too for your diagrams which usefully illustrated limitation in interpreting curved surfaces.

    As has been pointed out COP is used usefully as a way of determining the point of application of a force when the force is applied over an identifiable area - this allows for calculation of moments.

    I want to focus on one idea which is the crux of the flawed idea I had originally and see if there is useful way to exploit some information.

    This is entirely a thought experiment at this stage simply to test the idea in context.

    Consider the 2nd metatarsal head barefoot on top of a rigid FMat. Subject is standing normally vertically bipedal, aligned in relaxed calcaneal stance position and is steadily balanced with minimal shifting of COM.

    I create a 5X5 sencell matrix whose central sencell corresponds to the COP of that matrix under above conditions.

    If I examine the pressure data in real time under the following conditions this is what I would expect to see.

    Push foot parallel to sensor surface with sufficient force to displace metatarsal head within limits of coefficient of friction for skin so that MTH shifts over the plantar fibro-fatty pad without skin surface sliding on the sensor.

    If there is no muscle activity, the metatarsal doesn’t bend or rotate and COM remains unchanged (this is purely hypothetical) then:

    The COP will move in the direction of the applied force,

    the distance moved by the COP will correlate with the magnitude of applied force and

    the rate of change of the applied force will be correlated with the velocity of the COP.

    The measured ground reaction force will increase in the cell underneath the area of greatest vertical tissue deformation, decreasing to adjacent cells at any given instant in time.

    So what we have is the makings of a crude single axis accelerometer?

    What it detects in those tightly controlled conditions corresponds to the effect of shear stress by considering compression within the plantar fibro-fatty pad?

    If that holds up I will move to next condition ….

    Any comments . . . or so far so good?

    Cheers

    Martin
     
  37. Mart

    Mart Well-Known Member

    As has been pointed out COP is used usefully as a way of determining the point of application of a force when the force is applied over an identifiable area - this allows for calculation of moments.

    I want to focus on one idea which is the crux of the flawed idea I had originally and see if there is useful way to exploit some information.

    This is entirely a thought experiment at this stage simply to test the idea in context.

    Consider the 2nd metatarsal head barefoot on top of a rigid FMat. Subject is standing normally vertically bipedal, aligned in relaxed calcaneal stance position and is steadily balanced with minimal shifting of COM.

    I create a 5X5 sencell matrix whose central sencell corresponds to the COP of that matrix under above conditions.

    If I examine the pressure data in real time under the following conditions this is what I would expect to see.

    Push foot parallel to sensor surface with sufficient force to displace metatarsal head within limits of coefficient of friction for skin so that MTH shifts over the plantar fibro-fatty pad without skin surface sliding on the sensor.

    If there is no muscle activity, the metatarsal doesn’t bend or rotate and COM remains unchanged (this is purely hypothetical) then:

    The COP will move in the direction of the applied force,

    the distance moved by the COP will correlate with the magnitude of applied force and

    the rate of change of the applied force will be correlated with the velocity of the COP.

    The measured ground reaction force will increase in the cell underneath the area of greatest vertical tissue deformation, decreasing to adjacent cells at any given instant in time.

    So what we have is the makings of a crude single axis accelerometer?

    What it detects in those tightly controlled conditions corresponds to the effect of shear stress by considering compression within the plantar fibro-fatty pad?

    If that holds up I will move to next condition ….

    Any comments . . . or so far so good?

    Cheers

    Martin[/QUOTE]
     
  38. David Smith

    David Smith Well-Known Member

    Eric, I was explaining why a force is a vector and a pressure is not an indicator of the force vector because pressure is always considered perpendicular to the surface of interest. Obviously I wasn't very clear DoH!:confused:

    Regarding pressure and moments, what I was saying was that you cannot work out moments from pressure unless you know the distribution of the forces causing the pressure, which as you point out is the CoP. And so referring to the first paragraph above CoP is not CoF because the pressure only has direction relative to the surface of reference. Therefore moments calculated by CoP do not result in the actual moments about an axis of point of interest (as has been pointed out before by Simon et al) I was just trying to reiterate that in terms of this discussion and back up and agree with the points you were making earlier in the thread. :dizzy:

    Dave
     
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