Teach-in '99 : Optical versus Electromagnetic Motion Analysis
by Raymond Lee & Chris Kirtley, The Hong Kong Polytechnic University

This experiment compared the output of Vicon video-based motion analysis with an electromagnetic tracking device (Polhemus Tracker). As shown in the figures, the standard Vicon Clinical Manager marker set was used and the two Tracker sensors were attached (using double-sided adhesive tape and tight velcro straps) to the thigh and shank segments. The subject then performed a series of flexion-extension movements of the knee, as seen in the animation.

The VCM analysis is shown below:

The sagittal plane flexion-extension of the knee is well recorded. However, note two things:

The Tracker output shows a similar artifact:

In this case, there is a slight hyperextension offset, and an abduction/adduction of around 40 degrees peak to peak.

With the Tracker, it is quite a straightforward matter to realign the measurement axes of the sensor, and this was done in order to bring the two sensors into alignment defined from a static trial. After this correction, the following results were achieved:

Note that the artifact is now much reduced (about 20 degrees peak-to-peak), and is now of similar magnitude to the Vicon data.


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