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Is Fx a good measure of shear

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Phil Rees, Feb 25, 2011.

  1. Phil Rees

    Phil Rees Active Member

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    Is the Fx ground reaction force plot from a force plate a good measure of shear?. If so, how do you interpret the magnitude and duration of the shear from the graph. Do you measure the area under the anterior and posterior sections, or the peak values, or the angulation and duration of the slope from anterior to posterior.
    Shear indicates that there is linea displacement of one body of material relative to another paralell to the force, is it possible to extrapulate actual shear values (mm/n) from the Fx plot.
    Any comments would be appreciated

    Phil Rees
  2. David Smith

    David Smith Well-Known Member

    Need some clarification here, what direction is the Fx plot - vertical or one of the horizontal axes? I suspect you are looking at the vertical GRF as Fx so if this is the case then the simple answer is no you can't calculate shear from the vertical GRF. However when examining normal walking then anterior posterior forces are about 25 - 33% of vertical GRF and mediolateral about 5 - 10%. When you mention mm/n are you referring to strain displacement coefficient?

    Regards Dave
    Last edited: Feb 26, 2011
  3. Phil Rees

    Phil Rees Active Member

    Sorry David,
    I should have clarified the direction of Fx. The Fx component is the horizontal GRF in the direction of the progression of the centre of mass.
  4. efuller

    efuller MVP

    Then Fy is the medial lateral shear and Fz is the vertical component. So, if you only look at Fx then you are ignoring Fy. For walking straight ahead, Fy is probably so small that it is safe to ignore. For tasks other than walking you need to check Fy. If you are able to measure Fx, you are probably going to be able to measure Fy.

  5. efuller

    efuller MVP

    Shear that is being measured by a force plate is forces between the shoe and the plate. Are you interested in something different?
  6. Phil:

    The force plate is able to measure the force vector of ground reaction force in three dimensions which means there will always be three components to the resultant ground reaction force vector: Fz -the vertical force component, Fx- the medial-lateral shearing force component, and Fy-the anterior-posterior shearing force component. Fz is not necessarily proportional to Fx or Fy and one should not assume that Fz can be used to extrapolate the magnitudes of Fx of Fy at any time during gait.

    Sounds like a little review of the physics of force plates and the resolution of the ground reaction force vector into its three components are in order.



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