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Hi I am doing a piece of research on gait with loaded backpacks (up to 20% weight) and exercises with barbells (eg squats). I am just starting out in OpenSim so pardon some novice language. All of which the external devices are not modelled (no markers attached). Naturally, when I ran RRA on my trials it failed. I did manage to get it working but was hoping to get some further input on further refinement. I have a similar post (viewtopicPhpbb.php?f=91&t=5862&p=15770&start=0&view=) but feel a need for a more step wise assistance.

What I did for my gait with backpack trial?

1) Changed RRA_Actuators.xml file, specifically residuals optimal force or torque by a factor of 5 (arbitrary). So optimal torque residual was 10 for example (default was 2).

Results:

Curiously it asked me to reduce my mass ("Total mass change: -0.60114") which appeared strange.
* After torso COM and Kinematics adjustments:
* FX=-0.521321 FY=30.6895 FZ=-4.57665
* MX=3.61648 MY=-1.49324 MZ=11.4203

2) I understand from the user guide increase optimal force for residuals is cheap, and one should increase excitation. When I did not change optimal residual force, but increased excitation up to 1000 (just trialing first), my model still falls slightly into the ground (indicating residuals are insufficient).

Question:

1) Why did it ask me to reduce my mass, when I actually added an external load?
2) In experiments with unmodelled loads, what should be the first approach to ensuring RRA runs, and how should we work backwards? Example, should I find a minimum residual optimal force that ensures the model runs and residuals low? Is there a simple calculation to estimate residuals needed? Eg. if I carry a 10kg (100N) bag, that is 100 N in residuals.
3) Should I increase excitation or optimal force or both of the joint actuators?


I really do appreciate the help.

Regards,
Bernard

Hi Bernard,

RRA is trying to reduce the residuals by either i) altering segment COM, or ii) adjusting kinematics. It does not use muscles, just joint torques (purely a torque generated simulation). Does each foot have it's own force (two forceplates were used)? Have you added the additional mass to the trunk model? Have you tried adding a block body to the torso with a weld joint and giving it the mass of the backpack?

-j

Hi James,

This is a ridiculously late reply from me. So sorry. But I only had time to play with it now. I have modified the 2392 model, creating a weld joint of the backpack to the torso. Funny thing now is when scaling, there is no error message, just that it keeps scaling and not actually getting scaled. I am unsure what is wrong. I can scale it when just using the normal 2392 model. I am wondering if you could give me some pointers.

I have attached the necessary files just in case. Many thanks for the help.
Kind Regards,
Bernard