Projection angles script (Vicon BodyLanguage)

Click here to download.

This is a short PlugInModeller script (for use with Vicon software) that calculates what I have called projection angles in my book (page 138). The angles represent the alignment of the principal axis of each segment and the rotation about this as projected onto the principal planes of the global coordinate system (see precise definitions below). You can read about how these angles might be useful either in the book or in  recent blog article.

It works on the output of Plug-in Gait which must thus have been run on the data before running this model. The angles are written to the .c3d file along with all the other gait variables and can be handled in Polygon or other software in exactly the same way as conventional joint angles.

The software assumes a  conventional Vicon global coordinate system with z-up, x-forwards and y- to the side (it will give incorrect angles if the global coordinate system is set up differently). It should detect which way the person is walking in the x-direction (up or down the lab) and correct for this.

It is some time since I used this software routinely. I can’t see any reason why it should not still work (other than the limitations I’ve stated above) but users should check that output makes sense particularly on the first few occasions that it is used.

Variable Component Segment axis Projected onto Relative to Positive1
PelvisProjections 1 anterior Sagittal (y=0) x-axis “anterior tilt”
2 medio-lateral Coronal (x=0) y-axis up
3 medio-lateral Transverse (z=0) y-axis forward
FemurProjections 1 proximal Sagittal (y=0) z-axis “flexion”
2 proximal Coronal (x=0) z-axis “adduction”
3 anterior Transverse (z=0) x-axis internal
TibiaProjections 1 proximal Sagittal (y=0) z-axis “flexion”
2 proximal Coronal (x=0) z-axis “adduction”
3 anterior Transverse (z=0) x-axis internal
FootProjections 1 anterior Sagittal (y=0) x-axis “dorsiflexion”
2 set to zero
3 anterior Transverse (y=0) x-axis internal

1     Quotation marks are used as these are not joint angles and anatomical terms are thus, strictly speaking, inappropriate.

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