Edit: Took the advice given and went with the muscle under the arm. I remove the muscle that laid over the shoulder. The simple solution almost always seems to be the best.
Rather than creating one muscle I tried two. The lighter area is thicker, the darker area is thinner. Because I don't want the darker area (directly under the pectorals) to suffocate the muscles that raise the rib cage. That would probably be bad for someone trying to fly, haha, or doing anything physically intense.
I didn't cover the abdominal muscles or partially cover them. I tried it and it just looked weird. XD
Any additional feedback?
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This is a WIP sketch that's a long ways from finished, but I wanted to post it primarily to get feedback. XD I know I have watchers that know a lot more about anatomy than I do.
Yeeees I know that a human's chest would have to be massive to realistically fly, there's also the fact that Gethin is way too heavy to probably even glide, XD but this is fantasy. I'm only going for a certain amount of realistic.
Any input on the arrangement of muscles?
I've added extra bones to the wing in the hope of getting around the shoulder being in the way (and also to make his wings a little more flexible, for other purposes).