Re: Fiber Velocity Outputs
Posted: Mon Nov 16, 2020 9:52 pm
Hi Nick and Ross,
I've run a series of other optimizations with a few different settings and I think I've gotten some better results for my model. After getting better velocity outputs with the minimum activation bound set to 0.01, I then tried changing the fiber damping coefficient. First I increased it from the default of 0.01, to 0.05. Turns out, this is actually worse for the fiber velocity output. This was a little counterintuitive to me, but maybe makes more sense to others.
Then I decreased the fiber damping coefficient to 0.005 and it is way better:
I think these differences between the fiber length derivative and fiber velocity (~0.1 m/s) are more in the range of what Ross was finding with his results. As Ross suggested, it does seem to often happen around toe off, but not always. Also, I have yet to notice any negative fiber force outputs from my optimizations, at least nothing that I saw from warnings after convergence.
So it seems that adjusting the minimum activation bound and fiber damping coefficient help resolve this issue - more or less. I have not tried these muscle model settings out with other speeds or conditions yet. I may try to build out some code to calculate the fiber velocity analytically from the muscle states, as Ross suggested earlier, to see if there are any obvious pinch points with these peaks. I can report back if I notice anything interesting from that.
For now, I'm still thinking that using the derivative of fiber length will get me more reliable results for input into my fiber work calculations, and so I may stick with that approach going forward.
Thanks for the help and input with this issue.
Russell
I've run a series of other optimizations with a few different settings and I think I've gotten some better results for my model. After getting better velocity outputs with the minimum activation bound set to 0.01, I then tried changing the fiber damping coefficient. First I increased it from the default of 0.01, to 0.05. Turns out, this is actually worse for the fiber velocity output. This was a little counterintuitive to me, but maybe makes more sense to others.
Then I decreased the fiber damping coefficient to 0.005 and it is way better:
I think these differences between the fiber length derivative and fiber velocity (~0.1 m/s) are more in the range of what Ross was finding with his results. As Ross suggested, it does seem to often happen around toe off, but not always. Also, I have yet to notice any negative fiber force outputs from my optimizations, at least nothing that I saw from warnings after convergence.
So it seems that adjusting the minimum activation bound and fiber damping coefficient help resolve this issue - more or less. I have not tried these muscle model settings out with other speeds or conditions yet. I may try to build out some code to calculate the fiber velocity analytically from the muscle states, as Ross suggested earlier, to see if there are any obvious pinch points with these peaks. I can report back if I notice anything interesting from that.
For now, I'm still thinking that using the derivative of fiber length will get me more reliable results for input into my fiber work calculations, and so I may stick with that approach going forward.
Thanks for the help and input with this issue.
Russell