Unexpected output from a given loading scenario during inverse simulation

[Erica Montefiori]

Dear OpenSimers,

We have a lower limb personalised MSK model and we are trying to impose some desired contacting loads at the interface talus/tibia.

To do that we are modifying our external loads altering the mot file and adding three force components applied to a certain point in the talus and balanced by the same force applied at the same location in the tibia.

Being an MRI-based model, talus and tibia bodies have their ref sys located somewhere in the space coincident with the origin of the MRI they were segmented from. We are therefore defining the components of our forces and application points in these ref sys, and referencing them accordingly in our xml external loads file.

We are confident that we are doing the right calculations because when we display our external loads in the OpenSim GUI they look correctly located in the talus and the talus and tibia forces are equal in magnitude and point in opposite directions (Pic 1). Also the magnitude is reasonable when visually compared to the GRF.

To further test the process and understand what is happening when we run the ID routine we have designed these two ideal loading scenario:

1) a constant torque, directed as the ankle rotational axis and equal in amplitude to the patient mass
2) a constant force applied on a fixed point on the talus, equal in amplitude to the patient weight

In both cases loads are applied only to the talus body, connected to the tibia by a revolute pair. We then run ID, SO and JRA in the absence of GRF and removing the muscles from the model.

In the two cases, given that the accelerations should be small, we would expect to see:

1) a constant joint torque, with a normalized amplitude of 1Nm/kg; inertia may introduce small oscillation about this value
2) a constant force acting on the ankle joint, with a normalized amplitude of 1BW, and a constant joint torque whose amplitude depends on the force direction and lever arm; inertia may introduce small oscillation about this value

We obtain instead the attached results: Pic 2 and 3 for loading scenario 1, and Pic 4 and 5 for loading scenario 2.

Are we missing something here?

Any suggestion is more than welcome!

Thank you in advance,

Erica