Sketching an illustration for my old brother's birthday. He is a big fan of Alien.

In human he has cereal box body type. And it's more challenging to draw without making the poses look stiff.

One of the basis to understand the anatomy is to learn the main groups of muscle and how to simplify them in order to apply them over the gesture. I specially recommend the collection books of Michel Lauricella they are the cheaper and more effective ones I found for beginners.

For animals I recommend: - An Atlas of Animal Anatomy. - The Art of Animal Drawing. Contrusction , Action Analysis, Caricature. They are the best to understand the different anatomies and shapes of different animals.

Remember that gesture>anatomy accuracy. You can draw perfectly all the muscles but if your pose hasn't got any gesture or rhythm. I highly recommend read every post of Griz and Norm. They explains this perfectly and thanks to them I have improved a lot.