Hey forrest! Nice studies.
Hope you don't mind the paintover, it's something small to keep in mind.
Highlights are VERY important to the way you read the form. So manipulating well your highlights helps a ton to give the effect of volume So here:
1- Beak the highest light on the elbow. The muscles and shifting inwards there, so they aren't catching as much light. Just stop the straight white stripe of the highlight there or you'll get spaghetti arms.
2- The pectorals aren't one flat solid form over the chest; If you can get the feeling that it connects to the sternum there and is not perfect smooth and round, it will also help with the idea it's organic. Just break the big form so it's doesn't look too round - like a doll.
3- Another highlight break for end of the muscle/part of the bone. That is a straighter part of the arm where the skin is almost direct on the bone - so we indicate the muscle is ending, again, but lowering the highest light there. It can be subtle, but just improving the shape your highlight have gives a ton of information about what you're painting.
4- Pushed a bit the highlight/shadow for the ribcage. It is a major form and help giving roundness to the torso, and it was looking flat on that area.
5- I know you don't see if like that in the photo, but I just added a subtle shadow to separate the muscle from the clavicle. I think it's the tattoo that messed up the value reading there. Again, this is just a change to the highlight to make the form read better. Remember photos kills the midtones, so we use our knowledge to bring it back. Its more important to the form to read better than to be 100% accurate to the values to the photo.