I would like to compute the force of a muscle (e.g. the infraspinatus muscle) that is modelled by more than one fibre (6 for the infraspinatus) in the anybody upper-extremity model.
Should I sum the forces (Fm in the model output) across the 6 fibres? However, to compute a resultant force, I have to have force vectors. Does Fm provided in the output represent a projection of the force developped by each fibre on a commun direction then?
I think the method depends a little on what you want to find out. Do you want to know the resulting force vector from the point-of-view of the bone the muscle is acting on, or are you interested in the inner workings of the muscle. In the former case, the right tool to use is the AnyForceMomentMeasure2 class, which allows you to automatically sum up several forces acting on a reference frame in the coordinate system of your choice.