forces in IK/ID?

[Rahul Goel]

Even if I just run Inverse kinematics or/and Inverse Dynamics and go to the plotter, then I could plot muscle fiber forces (total, active, fiber) and tendon force etc, I wonder from where those forces are coming from? Because as per my present understanding, its only after I do the static optimization or CMC than I could get muscle forces.
Please tell, seems I am missing something very basic
Thanks
Rahul

[Kathleen Reilly]

Hi Rahul-After IK, are you plotting something from the *_walk_ik.mot file? If so, the values you see are the joint coordinates/angles computed by IK. After ID, if you plot something from the *_force.sto file, you are plotting torques/moments. You are correct in your understanding that the muscle forces are only computed with CMC.

Does this help?

[Rahul Goel]

But, I select 'Tendon Force' and other 3 fiber forces that I mentioned as my Y-Quantity, so do you think, it still just picks up torques/moments, even though I do not select that?

[Lisa MacFadden]

Rahul,

When you go and plot the tendon and fiber forces I believe that you are plotting the maximum isometric forces computed for that muscle. Essentially the muscle is being maximally activated and the force is being computed with calculations from the hill force length curve assuming an isometric contraction. Therefore this is not an optimized submaximal contraction which is contributing to your motion, but rather an independent computation being done by OpenSim in the plotting tool.

[Tim Dorn]

Hi Rahul & Kathleen,

I enquired about the same thing a little while ago (https://simtk.org/forum/message.php?msg_id=2041) but about fibre lengths. Since they are a state of static optimization and CMC, they are determined by optimization. However I tried plotting the fibre lengths before CMC (plot from the GUI) and after CMC (plot from states file) and they were close but not the same. This led me to believe that the fibre lengths from the plotter are some sort of approximation to the optimized fibre length for purposes of simulation debugging (i.e. we can see a rough trajectory of the fibre length before optimization to see if it could be relating to insufficient muscle force production during the simulation).

I went a bit off track there because I'm still waiting for clarification on that. However in my opinion, the answer to the total fibre/active/passive fibre force is as follows:

e.g. med_gas_r about ankle_angle_r

Passive Fibre Force: activation = 0.01. So as fibre length > LoM, passive force kicks in (this still requires a relationship between joint angle and lom - I'm a bit unsure how that is calculated)

Active Fibre Force: activation = 1, no passive component

Total = active+passive

These parameters basically plot the strength boundaries of the model.

[Rahul Goel]

Isnt maximum isometric forces a constant value (though we can change it) for each muscle in the muscle editor?
I am not sure if it is computed?

[Lisa MacFadden]

What is your x value that you are plotting against? For example if you were to plot active force vs knee joint angle at each point it would compute the maximum isometric force at that point. You can use this as Tim mentioned to look at your strength boundaries or your maximum isometric force or torque capacities for a range of joint angles or generalized coordinates.

[Rahul Goel]

Thanks everyone.
Lisa, I used x value as states (essentially time).
So just to clarify based on the above discussion, so is it like the plotter just gives an approximate fiber force, based on the fiber length and hill's curve, and it will not take into account the external loads that I would apply to run ID in the first place?
Rahul

[Ayman Habib]

Hi all,

Please check User Guide section 13.6 on plotting.

Hope that helps,