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I would like to ask if someone can tell me whether I can modify the amount of lower limb muscle mass in the Gait2354_Simbody file?
Or tell me any method that can change the muscle mass of the lower limbs, thank you

Hi, both Thelen2003Muscle and Millard2012EquilibriumMuscle models assume that the mass of the muscles are negligible. They are only applicable and accessible through useProvidedMass in Umberger2010MuscleMetabolics and Bhargava2004MuscleMetabolics probes.

Hello, I am new to opensim and not very good at using it.
Could you please tell me if the information you uploaded is related to how to change muscle mass in the lower limbs?

As stated in the answer above, the muscle models do not account for muscle mass directly: the mass of the muscles would be lumped into the mass of the body segment. However, if the mass of a muscle were to increase due to strength training, for example, that would also increase the muscle's maximum isometric force. The maximum isometric force is a property of the muscle models in OpenSim and can be changed in the GUI (see the "Property Editor and Outputs List" page in the documentation: https://opensimconfluence.atlassian.net ... tputs+List).

Hi
When I directly select a muscle in the navigator and then make adjustments in the properties such as max_isometric_force, optimal_fiber_length, tendon_slack_length, change the parameters, use the plot of tools to open Inverse Dynamics and set the muscle part, the curve that comes out is the same as when there is no change. What went wrong?

Changing muscle properties will not change the output from the Inverse Dynamics Tool. You may wish to read the "Overview of OpenSim Workflows" page in the User's Guide (https://opensimconfluence.atlassian.net ... +Workflows) to determine which OpenSim Tool would be most appropriate for answering your research question. There are also some examples and tutorials (https://opensimconfluence.atlassian.net ... +Tutorials) that demonstrate common use cases.

Thank you very much for your reply, which helped me understand that changing muscle properties does not alter the output of inverse dynamics. Additionally, I would like to ask if there is a simple model or method available to simulate the amount of torque required for a patient with lower limb muscle atrophy to walk.

There are several possibilities. It depends on your research question. For example, after choosing a model, you can decrease the maximal isometric force in the target muscles and use static optimization to study muscle force distribution (https://opensimconfluence.atlassian.net ... timization).
If you want to gain a deeper understanding of muscle-tendon dynamics, you can use Computed Muscle Control (https://opensimconfluence.atlassian.net ... le+Control).