Take a typical image from a medical dictionary:
look at ways another artist may depict similar images. I like the work of Peter Randall Page and whilst I don't think this is where he gets his inspiration I can see some similarity to muscle tone:
on Google I found a student designer using Peter Randall Page as inspiration in 2009;
By 2011 Scottacus Anthony was producing this:
and it isn't too far of a stretch of the imagination to look at Peter Randall Page's aquatint etchings to see where it might have come from:
and all this from looking up muscle images...