About negative muscle activation during simulation

[Jong In Han]

Hello, I'm using Opensim 4.0 and working with Lowerlimb model that has 3DOF(hip, knee, ankle) and 9 muscles.
When I hit the 'simulate' button from default model poses, the opensim generates 'Results' file that consists of angles, velocities, fiber length, and muscle activation.
The muscle activation values during the simulation period are nearly zero including negative values. I'm curious about the negative value of the muscle activation. By definition, the muscle activation is "Activation level of the muscle" and 1.0 is full activation; 0.0 is no activation.
Could I know why this muscle activation range violation occurs?

[Dimitar Stanev]

It is strange because the activation is clamped between a_min and 1. Can you post the simulation results (e.g. state.sto)?

[Jong In Han]

Dimitar Stanev,

Attached is my result file for the simulation.
8, 9, 11 ... columns are muscle activation.

Thank you for your comment.

[Dimitar Stanev]

This could be a bug. Please raise an issue on github with details on how to reproduce this behavior:

https://github.com/opensim-org/opensim-core/issues

This is important since it is not clear from your post how to reproduce these results.

Thanks!

[Jong In Han]

It seems like to use 'CMC Tool' to compute exact muscle activation from the attached states file.
I could get all muscle actuation values between 0 and 1 by using CMC Tool.
Thanks.