Whenever an artist is having difficulty with something a detailed study of the subject is called for. In no area is this more true than human anatomy.
The pelvis is a complicated shape and is an important factor in the width of the hips as it contains the socket into which the thigh bone (femur) fits.
Back Muscles are very difficult to draw properly. The largest muscles in the body are there and can easily be confused with the scapula (shoulder blade). Study them every chance you get.