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Average COP trajectory

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Petcu Daniel, Feb 6, 2012.

  1. Petcu Daniel

    Petcu Daniel Well-Known Member


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    Hello,

    Trying to represent 3D position of average GRF vector resulted after a number n of trials [n=10 in my case] using a force platform, I've met the problem of "average COP" trajectory .

    As:
    - the position of each COP trajectory is variable related to the force platform area and
    - the first and the last five percent of stance phase are ignored in order to avoid errors during low vertical forces [Nachiappan Chockalingam et all, Scoliosis 2008, 3:10],

    put me in the front of the problem to find a method of determining an average trajectory of COP.

    In "Effects of Simulated Genu Valgum and Genu Varum on Ground Reaction Forces and Subtalar Joint Function During Gait",Bart Van Gheluwe,Kevin A. Kirby, Friso Hagman it is defined "the inertial axis of the foot" which allow a "pressure-oriented definition of center-of-pressure displacement".

    Will be very helpful any thoughts regarding:
    1. I don't understand very well how is defined "the inertial axis of the foot"- what exactly is "center of weightbearing pressure of the heel". Could somebody help me with an explanation ?
    2.Could be used the inertial axis of the foot as a reference system for defining COP trajectory ?
    3. There is other possibility to calculate an average COP trajectory from the data obtained with a force platform ?

    Thanks in advance,

    Daniel
     
  2. David Smith

    David Smith Well-Known Member

    Daniel

    I take it you didn't have the luxury of 3D video tracking while you took the force plate data. So you don't know the position of or where the foot is on the plate relative to the CoP.
    You could walk the same person over a pressure mat to determine the relative position of the CoP progression with regard to the foot and assume that they were not significantly different on the force plate data.

    You could use the first useable data point as zero and the last useable data point a n and then use the projection between the two as a baseline and find the +/-excursion from the base line for each data point from 0->n and then compute a mean for all the sets.

    Dave
     
  3. efuller

    efuller MVP

    With a force platform, all you get is the location of the center of pressure. You don't know where the foot is in relation to the center of pressure or the force platform. So, you have to define the foot and then know where that definition is in relation to the foot platform. As was mentioned, video can be used for this. Markers can be placed on the foot and this can be calibrated relative to known markers relative to the platform. I recall a paper that described using a special paper placed over the platform. This paper changed color where the foot touched it. Then the cooridnates could be manually retrieved and be calculated relative to the 0,0 location of the force platform. With this special paper you still need to define the foot and asses the possible error in this location. You could define the foot as a line connecting the point at 50% of heel width at 10% of foot length and the center of the distal phalanx of the second toe. I think a saw a another paper that drew a line along the lateral and medial boarders of the foot and then bisected it. Just some suggestions.

    This location issue is still a problem with pressure mats. As the foot hits the pressure mat various sensors will register force. When the foot is half on and half off a sensor that sensor will register. When I was using the EMED platform the sensors were 0.5 cmxcm. Depending where the foot hits you could have a different number of sensors for heel width. The same is true for forefoot width. So, if you were to define the longitudinal foot axis relative to the width of the forefoot and rearfoot then there can be some (significant?) variation in the axis relative to the real foot placement.

    There is also potential for error in using the center of pressure itself for defining the long axis of the foot. The center of pressure is variable. After looking at lots of them for the same foot, there were steps that began and ended in different locations. The problem with using part of the center of pressure as the location of the foot definition is that you are trying to find a relationship between the center of pressure and the location of the foot. Variation in the location of the cop will cause variation in definition of the foot.

    An interesting question is whether variation of the location of the center of pressure is smaller than the measurement error created by the foot definition.

    Hope this helps.
    Eric
     
  4. Daniel:

    In the study on genu valgum that I did with Bart and Friso, a pressure mat was placed on top of the force plate to better define foot position relative to the three-dimensional location of the ground reaction force vector from the force plate. In this way, we were able to estimate how a simulated genu valgum gait pattern affected the moments across the subtalar joint.

    Here is another study done by Bart and Friso using a similar protocol as the one we used in the genu valgum study.

    http://simkinpres.vub.ac.be/verslag december 06/TETRA verslag SimKin december 2006 appendix 3.pdf

    Bart has since retired in France but Friso is still doing research in Belgium. They are the ones that designed the experimental protocol and did the data collection for the study. Contact me privately and I can give you their e-mail addresses and you can ask them directly about the specific experimental setup that they used for our study.

    kevinakirby@comcast.net
     
  5. Petcu Daniel

    Petcu Daniel Well-Known Member

    In fact, I have only one AMTI force platform! Unfortunately we are from this point of view many years behind developed countries so, sometimes we have to reinvent the wheel ! But this could be advantage when you try to find a solution at a problem which doesn't exist with sophisticated motion analysis systems! If you could find the solution !! If not, maybe some people from Podiatry Arena could give a hand...
    Because with a simple 2D video analysis, out of phase with force platform, is almost impossible to correlate the image with force platform data I've tried to see if there could be identified some special points on COP trajectory who could be used as reference points in comparing different COP trajectory. The mentioned article has given to me some hopes !
    Thank you for all your answer !
     
  6. Petcu Daniel

    Petcu Daniel Well-Known Member

    Dear Members,

    I've found 2 articles regarding this subject which might be of interest :

    - Cavanagh PR "A technique for averaging center of pressure paths from a force platform." - http://www.ncbi.nlm.nih.gov/pubmed/730763
    - Motriuk HU, Nigg BM "A technique for normalizing center of pressure paths." - http://www.ncbi.nlm.nih.gov/pubmed/2211737

    One question could be: which one is more relevant from a clinical point of view : COP average or PWA average [ point of wrench application: http://books.google.ch/books?id=wp3...siorsky PWA&hl=de&pg=PA45#v=onepage&q&f=false ]?

    Daniel
     
  7. efuller

    efuller MVP

    It really depends on what clincical question you are asking. Both variables are highly influenced by motor output (relative muscular contraction) so they might be a better measure of behaior than of a purely mechanical phenomenon. You still have the problem of "does the pain cause the gait, or does the gait cause the pain?" "Normal" walking could cause the foot to hurt and then the patient changes the gait to avoid the pain. You may not know if you are seeing "normal", or pain avoidance gait.

    Eric

    Eric
     
  8. Petcu Daniel

    Petcu Daniel Well-Known Member

    My clinical question is related to the possibility to have a confirmation of Functional hallux limitus from COP or PWA trajectory as it is registered with a force platform. I've attached an image with 4 COP trajectories of right foot of a subject with functional hallux limitus and pain between third and fourth intermetatarsal spaces [Morton's neuroma]

    Next step will be to have an average of the trajectories of COP [or PWA]

    Thaks,
    Daniel
     

    Attached Files:

  9. efuller

    efuller MVP

    Daniel,

    Howard Daneneberg has said he has seen a delay in calcaneal unweigthing with functional hallux limitus. So, what you would see with that on the CoP is more dots between the heel and forefoot, assuming that the sampling rate remained the same. Mathematically, there would be a greater dx/dt with a normal foot as compared a functional hallux limitus foot. (assuming dx is the distance in the anterior posterior direction.) You already know dt from your equipment and it should be constant. So a program could be written simply to subtract the x value of consecutive dots. I'd recommend doing this from heel contact to heel off. There are other things you could do as well.


    Eric
     
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