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Summary: Force covariation with speed

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  • Summary: Force covariation with speed

    Dear list members,
    I posted a question concerning Force covariation with speed.

    >We are working with force measuring horse shoes and conducted several
    studies on the changes of the >force pattern/magnitude with speed of running
    (trotting). There are may reports on the changes of >magnitude in the human
    litterature, but I have not found any describing covariation/cross
    correlation of the >vertical, fore-aft or medial-lateral. Can anyone help
    me? A follow-up question is whether the cross-talk >between the transducers
    of the forceplate (as most commonly used) changes with increasing force. I
    am >aware of the problem with plate deformation and the effect on the point
    of application, and thus more >interested in the behavior of the transducers.

    I received several responses and wish to thank all.

    A short summary will follow here.

    In doing some work with our force measuring horse shoe on a variable speed
    treadmill, we found that the vertical and horizontal forces tend to
    correlate (though not to a large extent) with increasing speed. The horse
    shoe which is a composite of may materials, among of which is polyurethane
    (80 degree Shore A) between the two 3 mm shoes. This is to keep the weight
    and height of the shoe to be equivalent to a normal horse shoe. Static and
    dynamic test (though at a low frequency) demonstrate linearity. Cross talk
    can then easily corrected for. The question arises concerning more rapid
    loading to the shoe. We thought about buying or constructing a force plate
    to in vivo calibrate the horse shoe when mounted on the horse's hoof.
    Before making such an investment though we were interested in knowing if the
    force plate is that much better in similar circumstances. Horses weighing
    say 600 kg can easily exert a vertical force of 10 kN in a 200 ms period at
    a slow trot. Force plates used in equine studies are usually large, and it
    would be expected to deform to this loading. Would we expect a linear
    deformation of the plate with speed and different rates of loading during
    the stance? Are the transducers linear at high rates of loading?

    Several responses referred to the need to control the elements of the
    measuring system (from the strain gages to the multiplexing..). These
    controls have been done and accounted for. Others commented on the need of
    an accurate calibration matrix, which also has been developed (Instron
    machine). Thirdly, the question of natural frequency also arose, which in
    this construction is above 1000 Hz. A problem with calibrating the force
    shoe with a force plate may also be due to the dynamical organ which it is
    attached, the hoof as pointed out by Young Hui Chang.

    Assuming the force plate/shoe is sufficiently rigid at low loading rates
    can one also assume the same is true at higher rates, such as heel strike in
    humans?

    No comments were offered on the true correlation of the various forces. I
    would however assume that the geometry and movements of the limb and body
    would result in periods of high and low correlation. These periods would be
    at higher rates of loading, thus returning full circle to the question of
    calibration of force plates and our shoe and loading rates.

    If my assumptions are incorrect or my questions are not properly formulated
    please feel free to comment. Secondly if anyone has any further comment
    please contact me. If further comment is of interest I can continue this
    summary at a latter date, hopefully addressing the question of calibration
    and loading rates.

    Thank you again for your interest

    Chris

    Chris Johnston, DVM
    Equine Biomechanics Lab.
    School of Veterinary Medicine
    P O Box 7011
    750 07 Uppsala, Sweden
    Christopher.Johnston@ah.slu.se
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