You need to remove the macroscript header from the script in the startup folder as well. Otherwise you are defining a Macro instead of running a script. Remove these lines:
Sorry for the necro, but, depending on your render engine you could use vertex colour to define a transition between a "red muscle" and "white muscle" material.
This is definately most interesting, but I believe most people will not think this far, and even wont go as far as to properly set their hard edges even for baking :)
take note of that, the problem of the second one is that is much more noisy and less defined, but might work. Trying to bake amb occlusion on vertex right now
I solved the issue by defining two MImages, one to read from, one to write to. Before, I was simply using the same stream, and it was causing the issue :|