Nice work Arts~ and I look forward to seeing your stuff Inky!
Also, in a bit of a "why didn't I realize this earlier?" moment, I finally found out why that gap in the sternum and clavicle is rarely pronounced. There's a muscle layer covering the lot of it. Might make a whole day out of it later, but introducing the Platysma: https://en.wikipedia.org/wiki/Platysma_muscle
This thin layer from the chin to the clavicle bone near completely covers the neck muscles. While it's job is mostly just moving the jaw, it and the skin on top of it smooth out a lot of the bumps and lines in the average neck. Obviously, bodybuilders who workout those muscles have definition that shows through this, but it's presence helps explain why that gap between the sternum and clavicle connection of the Sternocleidomastoid is usually only visible in the most extreme flexes, and instead looks more like a smoothed bump.
I tried to show how that functioned on your piece Arts, hope you don't mind the Redline.
The Sternocleidomastoid is a muscle I've learned and re-learned each time I tried to it seems. But the Platysma covering it is often not shown in diagrams when you're learning these muscles. Since it'd cover them all up and be hard to see, it makes sense, but it shows I've got more to learn here too.
Time to start on the Triceps~