Obtain Fiber Lengths Directly from Joint Angles

[Stephanie Hachem]

Hi all,

Given a model, is it possible to obtain muscle fiber lengths directly from joint angles? I know the Computed Muscle Control and Forward Dynamics tools produce a states files containing the muscle fiber length for each muscle, but if users can load the joint angles file (produced by Inverse Kinematics, input to Inverse Dynamics) and a model into the GUI and see the model play out the motion, aren't muscle lengths necessarily computed just for watching that simulation? If muscle lengths are computed just for watching that simulation, how can they be obtained?
(I'm asking as I want to obtain muscle fiber lengths, given joint angles or excitations, in a faster way than Computed Muscle Control and Forward Dynamics.)

Thank you,

Stephanie

[Thomas Uchida]

is it possible to obtain muscle fiber lengths directly from joint angles?

It depends what you mean by "directly" and what assumptions you are making. If you know the joint angles, you can pose the model and compute the muscle--tendon lengths (under certain assumptions). You could then compute muscle fiber lengths if you assume the tendons are rigid; otherwise, if you assume the fiber and tendon are in equilibrium, you would compute the fiber and tendon lengths at which they generate the same force. You may want to refer to Zajac and Millard for more information:

- Zajac, F.E. (1989) Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control. Critical Reviews in Biomedical Engineering, 17(4):359--411.
- Millard, M., Uchida, T., Seth, A., Delp, S.L. (2013) Flexing computational muscle: modeling and simulation of musculotendon dynamics. ASME Journal of Biomechanical Engineering, 135(2):021005.

[Stephanie Hachem]

Hello,

Thank you so much! I will take a detailed look.

Stephanie