Applying forces in non-gait simulations

[Amy Stalker]

Hi Dan,
I will need to try your recommendation of assigning the force under the "actuators and external loads" tab. Is this how you applied the force in your pedaling model?
Did you successfully build your pedaling model with OpenSim or did you need to use another type of modeling software?
Thanks again!
Amy

[Daniel Leib]

Hi Amy,

I didn't actually end up applying forces in my cycling model; I didn't have a way to reliably collect them. We do have instrumented cranks now, so that's something I'll be doing in the future. Depending on where OpenSim development is at the time we do some preliminary research it may not be the best tool for the job as we'll want to model some aspects of the bike as well. I imagine you could get away with using a lower body model with an essentially rigid joint between the pelvis and ground placed at a certain coordinate and then applying forces to the foot, but I'm not sure about the particulars in that scenario. The software we'd probably use allows for more streamlined person-world and person-object interactions in this type of situation, though when OpenSim allows application of forces in the ways we've been discussing, it'd be a strong candidate as well.

I used OpenSim to look at musculotendon length (and lengthening velocity) between conditions in my master's thesis, as the kinematic tracking and muscle geometry aspects in OpenSim are quite good.

-Dan

[Ayman Habib]

Dear Amy & Daniel,

Please check OpenSim's User Guide
https://simtk.org/docman/?group_id=91

Chapter 21 discusses how to get your data including ground reaction forces into OpenSim. Hope you find it useful, and please let us know if you have questions.

Best,
Ayman

[Amy Stalker]

Ayman & Dan,

After looking at chapter 21 as Ayman recommended, I am still unclear as to how I can apply a force (not a ground reaction force) to a certain joint or location.

I have also looked at the analyze tool under the "actuators and external loads" tab. There, I do not know whether I need to create a separate motion file of only forces (not joint angles), or if I include the forces in the same motion file with the joint angles and reference the same file under the "external loads" check box. I've not been successful with that method so far.

Dan, what program are you using for your cycling simulations now?

Thanks,
Amy

[Daniel Leib]

Hi Amy,

We're currently not doing a lot with cycling due to some other projects, but we're looking at using the MD Adams engineering suite along with the LifeMOD plug-in to do some work as we also want to look at stresses/strains on certain parts of the bike itself. It doesn't do neuromuscular stuff quite as well as OpenSim does at this point, but it does allow for a smoother workflow for analyses that include more than just the human body such as bicycles, chairs, etc. as it's a pretty strong general engineering platform.

-Dan

[Andrew Dressel]

Has anything changed since 2008? Is OpenSim any better at person-object interactions? Are there any examples available?

[Chris Zirker]

OpenSim 2.0 can do this much better. It's in the works and hopefully it won't be too much longer before it's available.

[Andrew Dressel]

That is great news. I missed the OpenSim 2.0 Developers Workshop, so is there anywhere else can I read about OpenSim 2.0 schedules, features, etc.?

[Chris Zirker]

Yes there is: https://simtk.org/home/jamboree_oct09

Things posted include OpenSim 2.0 betas (VERY beta, most things must be done via command line or code), a link to the developer's guide, videos of some of the presentations made, and so on.