The free and open-source MuscleMap extension may be helpful: New extension: MuscleMap -- Automated Whole-Body Muscle Segmentation
We are working towards automated whole-body contrast-agnostic muscle segmentation for MRI and CT. Please post any questions on the MuscleMap extension in the new topic via the above link.
Also, feel free to suggest any new features that may be helpful for your needs. We are here to help!