Thanks for the explanation, it really shines the light the reason behind your suggestion. Perhaps, I should explain my research background a little bit.
My PhD research is to investigate the wear of the cervical disc implant under the loads and motions that represent activities of daily living. This is a two-part project where the first part is to gather subject-derived data of cervical spine biomechanics using the motion capture technique where the loads and motions will be calculated from those activities. These data will be fed into the second part of the project where it will be used as input parameters for the spine wear simulators.
From the MoCap, I plan to gather information on:-
- joint reaction force between C3 to C7 vertebrae (extracted from 'Selected Output' folder),
- joint moment between C3 to C7 (but this is impossible with AnyBody model, according to my reading from the forum/tutorials),
- the linear displacements of C3 to C7 (the issue that is currently being discussed), and
- the angular displacements of C3 to C7 (answered in the second post of this thread).
As the spine wear simulator has 5 DoF, I decided to prioritise AP and ML displacement (not that superior/inferior displacement does not happen) on top of the 3 rotational movements as the input for the machine. Plus, I will have the superior/inferior joint reaction force as the main input for the spinal loading on the spine wear simulator which would render superior/inferior displacement insignificant (I think).
And I also totally understand when you say that the translational movement is co-dependent on rotational too. I am, however, more interested in the quantity of the translation movement that is happening (albeit at a minuscule distant) than the quality of movement. Given the kinematic rhythm is robust enough to simulate the rotational displacement of the vertebrae, if the rotational movement values from FDK will only have a small discrepancy with a negligible margin of error from rotational movement values from kinematics rhythm, I'd rather stick to kinematic rhythm. Let me know of your view on this as I am still new to rigid-body modelling.
As for joint stiffness, I plan to incorporate this numerically (instead of geometrically), taking into account the disc stiffness, ligaments, and facet joints as per thoracic/ribcage modelling from Ignasiak et al. (doi: 10.1016/j.jbiomech.2015.10.010.).