Seconding MysteryMeat. With art this small, you're inherently simplifying and distorting things just to make them work at this size, which isn't great for learning anatomy. You should give yourself enough resolution/space that you can express everything as you see it, without forced stylization. That way, you'll learn anatomy rather than a version of it distorted in ways you weren't even aware of, and when you apply your anatomy knowledge to pixel art, you will consciously make the stylization decisions. Your art will look better for it.