Unit of the total-fiber force in OpenSim

[Phong Phan]

Hi everyone,

I'm a new user of OpenSim and have a question that I hope to hear your opinions.

While I'm testing with the gait2354 model in the tutorial, I realize that I can use the Plot tool to plot the total-fiber-force (and other equivalent forces) of the musculoskeletal model after scaling and importing the IK motion from the tutorial. However, the values showed in the plot were a bit high (around 2000-5000), without any unit presented in both x-y axes. (Plot is attached below)

Hence, I wonder what is the unit for the total-fiber-force and (x-axis) in this case? And is this a correct way to extract these forces just by importing a motion to it using IK tool? If not, could you please suggest some possible method to extract force from motion capture data (of a normal gait for example) in OpenSim?

I hope everyone can help me clarify this question. Thank you very much for sharing your time.

Best,
Phong Phan

[Carmichael Ong]

My guess is that if you are choosing total fiber force, it's giving you each muscle's total capacity of force it can give you at each frame of the IK solution, rather than the actual force generated by the movement. This is because IK doesn't solve for the muscle forces to generate the movement, it just solves for the joint angles given the marker data.

To find out about some methods to estimate muscle forces, please see our guide on the inverse problem: https://simtk-confluence.stanford.edu/d ... rseProblem

[Phong Phan]

Hi Dr. Ong,

Thank you very much for your reply.

If I interpret it correctly, I need to use Static Optimization tool to extract the muscle force from a given gait movement. However, I'm not really sure about the unit of the force results. I tried the SO tool a few time on the gait data/reaction force that I had but couldn't got a practical muscle force magnitude (10-100 times higher than the value in the literature).

Hence, I hope you could help me clarify the unit of the muscle force obtained from SO tool.

Thank you for sharing your time.

Best,
Phong Phan