The steps of creating 'em could be like this.
1. Select from a list the joint wich should be the root of the muscle.
2. Select the joint that should be the end of the muscle.
3. Hit a "muscle setup" button (I know it sounds like magic!)
A locator for the start, mid and end of the musle pops up, along with a spilne that runs though all 3.
4. Move these locators to where they should be positioned.
5. Set a grow atribute for the mid segment of the muscle.
6. Hit the "finalize muscle" button.
Now it should be a enlongated sphere, named after the joint it's parented to, that flexes as the joint moves
Simply add the "muscle" as a sculpt deformer or a influence object to get it to give some muscle effect.
This might be to much work. I have no idea. Never actually created a muscle that would be setup in a similar way, and I have no experience of writing MEL scripts or any other script...