Observing higher spinal load with Exoskeleton support than without Exoskeleton in some frames of MMH tasks

Main Forum
1

I am using AnyBody v8.1 for an inverse dynamics study on back exoskeletons. I am running a counterfactual simulation using MoCap data recorded while the subject was wearing the exoskeleton. I am comparing two conditions on the exact same motion kinematics:

  1. Exo Condition: Assistive torque applied.
  2. No-Exo Condition: Assistive torque set to zero (simulating no-exoskeleton)
    The problem I am facing is, in specific frames (particularly at the start of flexion), the "Exoskeleton Condition" yields significantly higher L5/S1 compression than the No-Exo condition. Theoretically, the assistive torque should reduce the load, but here it seems to increase it. I would want to know -
  3. Has anyone else experienced this result where adding assistance increases the joint load in specific phases?
  4. What are the likely causes for this in the AnyBody solver?
  5. Are these transient spikes considered acceptable, or does this indicate any modeling error that needs to be changed? Thank you so much!
2

Hi Ishmam,

I am not too surprised to read that a back exoskeleton results in higher loads at the start of trunk flexion. If I suppose it's a passive exoskeleton, it seems normal that the user has to overcome the resistance in the flexion phase to receive support in the extension phase. But I would also expect to see a consistently higher load in the flexion phase.

You mentioned, you see spikes in a few frames. I wonder if it is caused by the MoCap data. Do you see some artefacts/spikes in the L5/S1 compression force at the same frames in the No-Exo condition? I also wonder if it's the assistive torque and the human-exoskeleton interface. How have you modelled the exoskeleton? Can you please share some details about how have you calculated and applied the assistive torque?

Best regards,
Dave

3

Hi Dave, it turned out that those spikes were indeed due to jerky MoCap data at those specific frames. thank you for the reply, it helped a lot!

4

Hi Ishmam,

It's good to know you have identified the problem. You could try to play around with the filter settings in LabSpecificData.any/C3DSettings.any. You could consider lowering the marker filter cut off frequencies, but the default values are already on the lower side. There is a risk that you lose information if the movement is too fast for the cut off frequencies.

Another option is to go back to the mocap system and see if you selectively fix those few frames before exporting to c3d/bvh.

Best regards,
Dave