Ah, that explains the accelerations we're getting when running BodyKinematics with only a motion imposed on the model. It's still strange though that the positions and velocities correspond to the loaded motion whereas the accelerations do not. If the accelerations are calculated from F=m*a, I'd exp...

I'm facing the same problem with several OpenSim models, as well as the examples included with OpenSim. I'm trying to determine body segment COM accelerations based on a motion file alone (so, no muscle excitations). The results are unrealistic. For example, the vertical acceleration for the pelvis ...