Definition of "reaction force" in MocoJointReactionGoal

[Ross Miller]

Hi all,

I am wondering about the definition of "reaction force" (common problem in biomechanics!) in MocoJointReactionGoal. Is this force the "resultant force" from inverse dynamics (i.e. the force labeled Fk in the knee diagram below) or is it the "contact force" (i.e. the sum of the two forces labeled fc1 and fc2 below, calculated from the resultant and muscle forces)?

Ross

Screen Shot 2020-11-21 at 8.35.32 AM.png (505.42 KiB) Viewed 220 times

[Karthick Ganesan]

Hi Ross Miller,
I believe that Moco uses same definition as that of opensim. In opensim joint reaction includes muscle forces and resultant from inverse dynamics. The reaction force Ri is calculated from the below free body diagram.

Reaction.png (12.88 KiB) Viewed 210 times

The following links may be helpful.
https://simtk-confluence.stanford.edu/d ... seDynamics
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3319529/#

[Ross Miller]

Thanks Karthick

[Nicholas Bianco]

Hi Ross,

MocoJointReactionGoal uses the values returned by Joint::calcReactionOn*ExpressedInGround() (where * is either Parent or Child depending on the setting you choose). These values are re-expressed in a different frame if specified by the property on the goal.

As Karthick says, these reaction forces include both the muscle forces and resultant from inverse dynamics.

Best,
-Nick