Adjust IMU orientation according to anotther IMU as reference

[Arthur Matta]

Hello,

I'm analysing an individual's movement using an upperbody model adapted from the scapulotoracic model. Using Inverse Kinematics everything works fine if the individual moves only the arms and shoulders, however, if the individual moves the torso, by inclining or rotating it, the movements becomes erratic with orientations all messed up. I have one IMU sensor on each arm, one on each shoulder and one on the cervical (C7) that measures the torso movements. How can I adjust the arms and shoulders IMU orientations according to the C7 IMU orientation so that when the individual moves the torso the movements adjust accordingly? Can I specify this orientation of reference on the model?

For reference, I'm using OpenSim 4.4, python 3.8 and Linux 22.04.

Best regards,
Arthur Matta

[Ayman Habib]

Hi Arthur,

This depends on the model you have and how is the torso allowed to move relative to ground. Ideally you have enough data and degrees of freedom in the model to orient the torso and arm to match the data. If the model is fixed relative to ground or if you don't have enough IMUs that the IK solver has the freedom to either move the arm or torso and to still match the data then you need to fix this, possibly by placing bounds on torso coordinates and freeing up coordinates to allow the solver to orient both the torso and arms.

-Ayman