They wanted to emphasize how the tendons of the lower bicep connect to the upper forearm. When approaching anatomy I never rely on a single source of information. I'll usually cross reference at least three different sources. Without exception you'll find three slightly different interpretations which can make it difficult to know which is right. What I do in a case like this is make an approximation of the three, unless one is clear photographic reference which is hard to find.
You'd think Bodyworlds would be an incredible source of information for this kind of thing and it certainly does help. While on the whole it is truly amazing, my beef with Bodyworlds is that I feel the muscle is so desiccated by the process of plastination that the true volumetric relationships between muscles and tendons/ ligaments are distorted. Between that and the strict no photography policy at their exhibitions it leaves me with the catalogue as the only reference for what could be a much better source of information.
There's a thousand books out there on anatomy for drawing and a good portion of the one's I've seen are horrible. I've encountered a few that really stood out. One being a book which I unfortunately don't know the name of because it was in German. A large volume with a beige cover , the illustrations are incredible. Unfortunately I don't have any more information on it than that, but if it rings a bell with anyone please let me know.
When I first started to get into animation back in the late eighties I found two books at a local art store. Similar in size and format to the Walter Foster series, these are still to this day the very best of all the anatomy and figure drawing books I've ever seen. The illustrations are fantastic and the labeling of the different body parts is thorough.
But at the end of the day, unless we have access to cadavers we are all making educated guesses to some extent with this kind of work.
I assume that there must be a resource out there that would give accurate photographic insight to what lays just beneath the skin, but I have yet to find it.
I have a second anatomy illustration that will be in the July edition of the magazine. I'll post that once it's published.