The image of a strong upper back sticks in my mind. Probably because I remember there was a time when I consciously realized that I didn't know the muscles of the back that well. I started looking at classical sculpture and paintings where backs were shown. In most pictures no one wants to see backs and backs of heads! So now when I go to the gym I can't help looking at backs and drawing them in my head.
This is a sketch in Photoshop on my Cintiq. No special processes.