Muscle Via Point Location when not in Coordinate Range

[Nathan Knodel]

Hello,

I'm working on extracting muscle unit vectors from OpenSim models for given motions collected by motion capture. I recently noticed that some of my unit vectors do not look correct at certain points in time and traced it back to via points (specifically the quadriceps muscles). I understand from this page (https://simtk-confluence.stanford.edu/d ... nd+Muscles) that via points are only used in the muscle path when a specific coordinate is within a specific range of values.

However, when I extract the path points from the muscles in MATLAB, I still receive position information for the via point when they are not technically being used in the muscle path. Is there a "default" positional value for these points when they're not used? I tried plotting the three attachment points of the rectus femoris through time in MATLAB and I cannot see an obvious answer to this question as the via point is clearly not lying along the muscle path when the leg is straight (they are correct, however, when within the designated knee angle range). This is counter to what OpenSim shows in the visualizer window, it always shows the via point essentially right "on top of" the first attachment point on the pelvis when the knee angle is not within the designated range. In summary, I'm really just trying to figure out the best way to move forward so that I'm always using the correct attachment points to calculate the unit vectors at any point in time.

Thank you,
Nathan