Theta angle of arm flexion/extension

Is it possible to measure theta angle of flexion extension as you can see on the image without affected by abduction/circumduction?

First of all i'm using quaternions for all of my operations to avoid gimbal lock, and then i convert to euler angles to display Θ angle(of flexion/extension) to the user. I also set an initial quaternion (that can be my inertial coordinate frame) and compare all quaternions with initial. Last i get yaw, pitch, roll from result_q. If i don't make any other move except of flexion/extension i get the correct Θ. The problem occurs when i also rotate my wrist(circumduction). Then if the sensor is not in the initial position i get wrong angles.

result_q = Conj(initial) * q

Any ideas?


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