Dear OpenSim users,
I am doing a static optimization of my sit to stand data and I am using the full body model by Rajagopal. For many of my participants the static optimization fails when the hip if flexed more than 80 degrees and when ground reaction force increases on the foot, but not all of the hip flexors are activated yet if I look at the results. I did make a graph plotting the moment arm and moment of these muscles against the movement and this shows that the muscles are able to produces torque in these hip flexion angles.
Also for the trials that do not fail, some of the muscles that I see activated in my EMG measurements are not during the static optimization. The vastus lateralis, which certainly activates during a sit to stand does not activate for example. Is there anything I can do to be sure that these muscles do activate? Other than increasing the minimum activation?
I really appreciate any help.
Best,
Maud
Static Optimization
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Maud Hendriksen
- Posts: 20
- Joined: Thu May 14, 2020 12:00 am
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Carmichael Ong
- Posts: 401
- Joined: Fri Feb 24, 2012 11:50 am
Re: Static Optimization
It can be hard to pin-point the specific issues in your optimization problem since there are many variables involved, but here are a few ideas:
1. It's great that you looked at moment arms to start with. Another key piece is whether the muscle itself has enough force capacity. You can get this information by adding a MuscleAnalysis and looking at the normalized fiber lengths and fiber velocities. One thing to note about OpenSim's Static Optimization implementation: it does not consider passive muscle forces, so only the active force contribution is used.
Link to Thelen Muscle model documentation: https://simtk-confluence.stanford.edu/d ... scle+Model
Link to Millard Muscle model documentation: https://simtk-confluence.stanford.edu/d ... cle+Models
2. I don't think static optimization has this, but CMC has a way to add control constraints, which you can use to make sure muscles are on at certain times of your simulation. https://simtk-confluence.stanford.edu/d ... efinitions
3. The model you're using was validated on gait, and thus wasn't tested on high hip flexions. You may want to find another model validated for those movements. https://simtk-confluence.stanford.edu/d ... tal+Models
1. It's great that you looked at moment arms to start with. Another key piece is whether the muscle itself has enough force capacity. You can get this information by adding a MuscleAnalysis and looking at the normalized fiber lengths and fiber velocities. One thing to note about OpenSim's Static Optimization implementation: it does not consider passive muscle forces, so only the active force contribution is used.
Link to Thelen Muscle model documentation: https://simtk-confluence.stanford.edu/d ... scle+Model
Link to Millard Muscle model documentation: https://simtk-confluence.stanford.edu/d ... cle+Models
2. I don't think static optimization has this, but CMC has a way to add control constraints, which you can use to make sure muscles are on at certain times of your simulation. https://simtk-confluence.stanford.edu/d ... efinitions
3. The model you're using was validated on gait, and thus wasn't tested on high hip flexions. You may want to find another model validated for those movements. https://simtk-confluence.stanford.edu/d ... tal+Models