I LOVE to draw anatomy! I grew up reading comic books so large chests (all genders) are a must-have in my art... most of the time :) . When I first started out really trying to learn anatomy I found it hard to make sense of how the chest moved as the arms and muscles beneath the skin moved.
After countless hours of uh... looking at reference... I started to finally understand some of the subtle changes in the chest as it moves and shifts. Here are some of the things I've learned along the way.
First off to get an idea of what the muscles do under the skin let's check out this fun little VIDEO! (Don't forget to give them a little love on YouTube if you liked the video!)
Next, when looking at the chest it's comprised of several muscles connected at the
clavicle, the sternum, and the humerus. The humerus!? you might ask, yes the pectoral muscles slide under the shoulder muscles and connect to the bone in your upper arm! SHOCKING!
Let's look at REALLY rough sketch... have you ever wondered why a bodybuilder or a very lean person with defined pectorals has striations running from the center of their chest and over to their arm? Those are caused by the muscles of the pec.
When you break down the muscles it looks something like this...
The muscles are a tad stylized here but you get the idea, there is a muscle at the bottom that gives the pec the shape (yellow) this muscle like the ones above it stretch to the humerus and do a little twist as they connect. The top muscles connect to the clavicle. The grey muscles are the shoulder muscles which cover the connection between the pectoral muscles and the humerus.
NOW that being said, this is how I remember how to draw muscles. I don't know actual names of muscles or groups of muscles so take the science side with a grain of salt.
When drawing those with a more feminine build the pectoral muscles are usually covered with a layer of fat. The biggest mistake I personally made and have seen made on numerous occasions is when breasts are drawn too high or just as circles on a frame.
Breasts are tear-shaped, the top makes up the pectorals, and the bottom the piece that gives the breast it's shape is fat. That's the round piece. REMEMBER breasts can be so many shapes and sizes the main thing you have to remember is they are tear-shaped, they hang unless in some sort of garment and they separate on the chest when not pushed together.
ALSO breasts are siblings not twins, one is more often than not larger than the other they may fall slightly different and so on.
I've gone on for quite a bit so I leave you with this, links to my favorite anatomy books!
I also use the App called Anatomy 3D it can be found on both android and apple.