Modelling 4 External Forces

[Alexandra Dunning]

Hey everyone!

I am currently working on modelling eccentric cycling and there are 4 forces to be modelled in my inverse dynamics (backrest force from bike setup, seat force from bike setup, and two foot forces from the pedals). When running the ID with just the back and seat forces, the external forces file works and the forces can be visualised (can confirm they are of the right magnitude and in the right position). I have also done this for just the feet and it works again (expected magnitude and position). However, when combining the four forces into the same mot, all synced etc, the ID will run the external force mot, yet only the two foot forces are showing when associating the motion data. When running static optimisation, I checked the residual forces and it confirms that the back and seat forces are not being read properly from the external forces mot file (even though they work if only processing the two forces).
Does anybody know if you can do 4 forces or only 2 (i.e. for gait)? If not, have any suggestions?

I have attached a screenshot of the reaction forces I will be attempting to model.

I am using opensim 4.4

Thanks,
Ali