A clarification for others wanting a neck marker - the offset was actually:
(-0.05, 0.45, 0)
I was off by a factor of ten in what I wrote earlier for that first coordinate.

<r>That helped a lot (and completely resolved my problem). I'm providing a link to a screencast that might help others who wonder how this works as, even with your spot on instructions, it took me a while to figure out how to do this.<br/> <br/> <URL url="http://screencast.com/t/7NXxPq3L">http://scr...

<t>Maybe I'm misunderstanding the trc file, but it seems that to add the marker to that file, I'd actually need to do motion capture (or write some code to infer the marker position from other markers in that data).<br/> <br/> I tried using the Analyze Tool and adding the PointKinematics analysis, b...

<t>I've added a marker to the Gait2354_Simbody example data that approximates the position of the base of the neck, and after generating the Inverse Kinematics data, I see that the marker follows the neck around just like I want. My problem is that I now want to plot the data from the new marker, an...