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Calculating joint moments using Optotrak

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  • Calculating joint moments using Optotrak

    Hello,
    I'm a PhD student with a Physical Therapy background who will be using an Optotrak 3D Investigator system with a force plate to evaluate the kinematics and kinetics of a jump landing.

    I have done some reading on using kinematics and GRFs to calculate joint moments, but alot of it is beyond my level of expertise. Can anyone recommend some articles or other sources that can explain what data I will need to collect to calculate knee and hip joint moments, and how I go about doing the calculations, in relatively easy to understand language??

    Thanks for any help or advice you can offer.
    Joanne

  • #2
    Re: Calculating joint moments using Optotrak

    Hi Joanne,

    A good question to nail down first is whether you want to compute the joint moments in 3D or in 2D (e.g. just the flexion/extension moment at each joint). The data you collect is essentially the same either way (you'll probably use a different marker set for 3D) but the calculations and modeling in 3D are considerably more involved.

    Ross

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    • #3
      Re: Calculating joint moments using Optotrak

      Thanks Ross,
      I was hoping to do 3D - knee abduction moment. However nothing has been set in stone yet and I'm looking at feasible options.
      Any advice?

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      • #4
        Re: Calculating joint moments using Optotrak

        Joanne,

        For gait, you can probably ignore the inertial terms (due to accelerations) in the equations of motion and simply compute the frontal plane moment of the ground reaction force with respect to the knee joint center. Because the inertial terms are ignored, you would not have to deal with the complexities of 3D kinematics that Ross Miller mentioned. See references below where they did such an analysis for real-time feedback. These computations are not that complicated. If the frontal plane ground reaction force is (Fx,Fy), and the knee joint center is at position (x,y), you could use:

        moment = Fx*y - (Fy-mg)*(x - COPx)

        COPx is the x-coordinate of the center of pressure which is measured by the force plate. mg is the weight of the limb segments below the knee, see Winter's Biomechanics and Motor Control of Human Movement for estimates.

        If it is a sports movement with a lot of lateral deceleration, the inertial terms may be too important for the ab-adduction moment to ignore them. You could try estimating the error. But remember that the lateral acceleration/deceleration of limb segments is very hard to measure anyhow. The kinematic data needs to be low pass filtered and that causes you to underestimate the inertial terms. This is, unfortunately, inevitable.

        Winter's book Biomechanics and Motor Control of Human Movement is a good reference for the methods.

        Ton van den Bogert

        Shull PB, Lurie KL, Cutkosky MR, Besier TF. Training multi-parameter gaits to reduce the knee adduction moment with data-driven models and haptic feedback. J Biomech. 2011; 44(8):1605-9.

        Wheeler JW, Shull PB, Besier TF. Real-time knee adduction moment feedback for gait retraining through visual and tactile displays. J Biomech Eng. 2011;133(4):041007.

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        • #5
          Re: Calculating joint moments using Optotrak

          Hi Joanne,

          The easiest approach is probably to calculate external moments (e.g. http://www.ncbi.nlm.nih.gov/pubmed/16168997) but they assume contributions from forces other than the GRF (e.g. inertia, gravity) are negligible. I don't study landing, but I would think this is probably a bad assumption for landing because the body segment accelerations are high. Hopefully others with more experience can weigh in.

          Learning how to do full-blown 3D inverse dynamics (i.e. calculate internal moments) is undoubtably a good thing, but it isn't easy and will probably extend the time course of your project by a bit. I'd recommend starting with Winter's book (Biomechanics & Motor Control of Human Movement) and the Robertson text (Research Methods in Biomechanics). Visual3D is great software for doing inverse dynamics if you have access to that. There are probably some freely available Matlab codes out there as well.

          Good luck,
          Ross

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          • #6
            Re: Calculating joint moments using Optotrak

            Thanks Ross and Ton for your responses. I have the two recommended books on order and have contacted a few researchers with experience using Optotrak data. I agree Visual3D would be great, but I think I will be using MatLab as I have inexpensive access to it through our university.

            If anyone else would like to weigh in on this topic, I'm all ears!
            thanks
            Joanne

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