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Hello,

I would like to obtain the joint reactions of my model during human gait. I am running the JointReaction Analysis but I obtain forces and moments that are far away from the inverse dynamic loads.

I have a model of one leg (pelvis, femur, patella, tibia, talus, calcaneus and toes). I use the Analyze Tool using as inputs:

- Motion from file (the Inverse kinematics results) / Filter coordinates (6 Hz)
- Actuator and External Loads --> External Loads --> External loads spec. file (the same .xml file that I used for the inverse dynamics analysis).
- Analyses, JointReaction: Joint Names (all), Apply on bodies (child), express in frame (child).

As far as I understand, the reaction loads are those that are necessary to produce the prescribed motion. For a DOF like the knee flexion I think that the Reaction Loads should be the same that the calculated by Inverse Dynamics. But I obtain a flexion moment that is completely different both in magnitude and shape. Using inverse Dynamics Analysis, it seems reasonable, but using Joint Reaction Analysis it doesn't.

I also tried running JointReaction Analysis and Inverse Dynamics Analysis using a simpler model (the Arm26 model). The inverse dynamics loads seem reasonable but the Joint Reaction loads are completely different.

What am I doing wrong?

Thank you very much in advance,

Gil Serrancolí

Hi Gil,

Please consult the documentation page below for what to expect from InverseDynamics and from JointReaction analysis and let me know if you continue to have questions.

http://simtk-confluence.stanford.edu:80 ... seDynamics

Best regards,
-Ayman

Thanks Ayman,

I had muscles in my model and I needed the joint reaction forces using just the skeleton, so when I run JointReaction Analysis with no muscles, what I obtained makes sense. It seems they actuated passively.

I also ran it using a simple pendulum and I could understand the difference with JointReaction Analysis and Inverse Dynamics.

Thank you very much,

Gil