In addition to the individual muscle morphs, there is also a “All muscle volume” morph and a “flexion on off” setting (controls the amount of automatic muscle flex when bending limbs). It is a great set that adds lost of value to Genesis in my opinion.
I agree—I use it all the time, although it adds quite a lot of Latin to your parameters library (remembering the names can be a pain in the glutei maximi!). I have something like this in my pictures file to refer to, although I am starting to learn some of them. It adds a lot of realism to your figures, especially male where muscles are more visible (or very visible for body builders!).