FSTM » Gait Lab Users Forum 2017
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The latest posts to FSTM » Gait Lab Users Forum 2017en-GB(C) 2020 University of WarwickWed, 11 Oct 2017 15:46:28 GMThttp://blogs.law.harvard.edu/tech/rssSiteBuilder2, University of Warwick, http://go.warwick.ac.uk/sitebuilderStrange force readings
https://warwick.ac.uk/fac/sci/fstm/gait_lab/forum2017/?post=8a17841b5ffd0dc3015ffe9b0c2a4e1b
<p>Repost from previous years:</p>
<p>In previous versions of Nexus there was a bug in how it presented force readings in the Model Outputs. We had hoped that the upgrade to version 1.8.5 would solve this problem but apparently it hasn't. So here is the issue:</p>
<p>Consider three ways of plotting the GRF: (i) From the force plate (ie from System tab in Resources pane using Graph -> Components in Workspace); from Model Outputs tree under SubjectName in the Subjects tab in Resources (and Graph -> Model Outputs) - either using (ii) XGroundReactionForce or (iii) XNormalisedGRF (where X is either R or L depending on which foot made contact with the force plate).</p>
<p>Suppose the subject is 100 kg. Then (i) should give the correct weight in N (albeit with a minus sign). The graphs for (ii) and (iii) will seem strange and, although they're listed with N as units they aren't.</p>
<p>For (ii): This number needs to be multiplied by the body mass to convert to N. i.e., the correct units are Newtons per kg body mass.</p>
<p>For (iii): This is actually the percentage (expressed as a decimal, so 0.5 = 50% etc) of body weight (in N). So to convert to absolute force you would need to multiply by mass (in kg) and then acceleration due to gravity.</p>
<p>Thanks to Abi for spotting this one and confirming with Vicon, who I'm guessing corrected it in their latest version of Nexus (Nexus 2). Hopefully this clarifies things.</p>
<p>**Note: If (i) is wrong then most likely someone has changed a setting for the force plate that they shouldn't have - NO ONE should change settings for the force plate!</p>
<p>We believe that all forces and moments might be scaled by body weight/mass in the model outputs, so be wary of this (ES97E students check the module forum).</p>Mon, 27 Nov 2017 17:51:34 GMTNeil Evans8a17841b5ffd0dc3015ffe9b0c2a4e1b