Love the way you shaded your forms, the simplicity of it makes it look so clean. Not too mention the energy that you created with your forms, which is really nice. I like the observation with the heads, It's something that seems to get glossed over a lot. Nothing wrong with drawing it as a perfect circle by any means, but its good to distinguish the observational reality too.
Really great work, your portrait studies are looking great! I'm excited to see how much you grow in a month, as you're current studies are looking good, can't imagine how much better you'll be by then! Keep up the awesome work!