I am running simulations with C3D motion capture data. Therefore, I turned OFF the Scapulahumeral rhythm in Anybody in order to drive the scapula and humerus with the motion capture data. Everything worked fine, however I am a little bit surprised about the output angles of the glenohumeral and scapulothoracic contribution to shoulder abduction in scapular plane.
I took the output values from Main.HumanModel.BodyModel.Interface.Rightand calculated the angles in degrees with *180/pi.
I got following starting values for one patients for example:
ThoraxHumerus.Elevation.Pos : 23.75709863°
ThoraxScapula.MedialRotation.Pos : -24.71730105° (means lateral rotation)
ScapulaHumerus.Elevation.Pos : 18.55574373°
How is it possible that the angle for scapulothoracic medialrotation is even bigger than the angle of the overall shoulder abduction (thoraxhumerus.elevation)?
Is it possible that I have to consider an offset somehow? Or is it possible that the coordinate systems are wrong or something like that?
Well spotted. But I think there is good explanation for this. The two measures ThoraxHumerus/ScapulaHumerus are conceptually different from the ThoraxScapula measures.
The two first measures are of the rotation sequence type y-x-y, meaning "plane-of-elevation"-"elevation"-"external-rotation". So the first and second rotation are both around the same axis with respect to the moving humerus. Hence "Elevation" doesn't necessarily mean "clinical" abduction, it could also mean 'clinical' flexion depending on the "plane-of-elevation".
So you can expect your three variables to roughly 'add' up if you models does abduction. But as soon as the model does any kind of 'clinical' flexion then the variables will not add up. Since the elevation planes doesn't align with the medial rotation plane of the scapula.
Make sense, as my patients abduct the humerus in the scapular plane (around 30° ventrally). I guess this would also explain another observation I have made: sometimes the ScapulaHumerus angle is bigger than the ThoraxHumerus angles in the beginning of the abduction. This is then propably because the patient has in the beginning a more medial rotated scapula, which increases the ScapulaHumerus angle, but not the ThoraxHumerus angle. Or am I wrong?
OveralI, do you think I can assume that these are the best measurements to calculate the GH (ScapulaHumerus) /ST (scapulathoracic medial rotation) ratio in AnyBody for simplicity? Or do you know if there are some better outputs/approaches to calculate the ratio?