Animator Bill Justice drew this scene with the character of John from Peter Pan.
Without thorough knowledge of the human body via life drawing this type of animation would fall apart. Spine, shoulders, rip cage, hip bones etc. are things that you need to know about.
You need to know how they connect, and of course how they look in motion. Tracing live action reference is simply not an option. Your knowledge of human anatomy is key.
Bill Justice did his homework. He might be known as the animator who brought Chip and Dale to life, but he was also capable of handling complex realism.
I don't know who drew these pose of Pluto, but I really like them. Here you find anatomical structure as well, just applied in a much looser manner. Pluto is a very cartoony dog, but dog anatomy is evident nevertheless. There are definitive elbows, knees, shoulders and hips.
This earlier style of Disney drawing allowed for greater flexibility and cheats.
Disney's animators always knew how to handle a character based on the degree of realism. They went far out with Pluto and Donald Duck, but were holding back effectively on characters like Bambi or Lady and the Tramp.
John drawings/Howard Lowery