Explanation of active fiber force

[Chris Zirker]

Hi all,

I'm looking at fiber forces from inverse kinematics simulations where there is no dynamic or muscle activation data present. While passive fiber forces can easily be determined from force-length relationships from the muscle, I'm not sure what is meant by "active fiber force" in the model given there is nothing more than kinematic data. I'm getting positive numbers which are, in general, much larger than passive fiber forces. This is not clearly addressed in OpenSim documentation (as of the 2/10/2010 guide).

Can someone explain what active fiber force is supposed to represent when all your muscle activations are 0 and you only have kinematic data? How is this metric calculated?

Thanks!

[Sam Hamner]

Hi Chris,

Someone (e.g., Ayman) might need to correct me, but I am pretty sure that when you do not specify an activation, the analysis the reports active fiber force with an activation of 1 (or fully activated). Might be a bit misleading, but this is intended to give you the max capacity of the muscle with force-length relationship in a give posture.

[Sam Hamner]

Chris, just a follow up. How are you getting these forces? With the plotter?

[Chris Zirker]

Yes, with the plotter.

I think you're right about the 100% activation. I thought it was a sort of "max theoretical force production" but wanted a 3rd party verification.

Thanks!

[Ayman Habib]

Hi Chris,

Sam is right indeed, I'll just add that you can override this behavior by providing a different value in the "Activation" box on the "Advanced.." panel of the Plotter.

Best regards,
-Ayman

[Chris Zirker]

No, this is good. I just wanted to make sure I understand my data correctly.

Thanks guys!

[Chris Zirker]

Is force-length the only relationship being considered, or does force-velocity also affect the calculation?