Sketch Practice

In my spare time, which isnt a lot, I've been trying to increase my knowledge of anatomy. This is just a sketch just to test my current understanding without any reference. I can often use various references for my artworks, but Im wanting to start really understanding what Im painting on a much better level. 

Here's a gif to the progression of this artwork. - img.photobucket.com/albums/v70…

Painted in Photoshop CS6 with a Wacom Intuos 3.