I really need to study muscle groups more, especially for when I draw big burly comic men (or turtles). A friend of mine lent me his Anatomy for Artists book, so I set to it right away, in my contrast sketchbook.
In this sketchbook, the left page is always in black ink, and the right page in watercolour. This is so that I can practice both techniques in one sitting.
The photograph I based this on is by John Davis.