How can I learn anatomy to start from an arm or a leg?

Start with the mechanics, then the whole figure, then particular areas.

Learn the Latin names of the major muscle groups. Learn why a muscle only works through being pulled, never pushed. Learn all the insertions and origins and processes that are visible to the eye, the bony processes and why they are important. Learn how the whole body relates to the center line of the figure. Learn about the center of gravity and the supporting leg, the dynamic pose.

Because you are talking about artistic anatomy, it’s okay to only focus on the muscles that are visible. If you want to know the deeper, hidden muscle groups of course that’s good, but it’s not essential for drawing the figure.

After understanding these general principles and practicing them by drawing the whole figure many, many times, you will start to find what parts of the body you are strong at and where your weaknesses are. Then you can isolate your weakness and build it up. I found that I struggled to draw the back and the foot, so I spent a couple of semesters of art school using class time with the model to concentrate on paintings and drawings of feet and of the back. I also borrowed casts of the foot from the sculpture department to have a long time to study it. When I struggled with hands I got a copy of George Bridgman’s “Book of a Hundred Hands” and copied each and every drawing in my notebook while also reading his notes.

If you aren’t sure where to start, begin with George Bridgman’s “The Human Machine” and copy every drawing on every page, and read what he has to say. Bridgman taught literally thousands of figure drawing and anatomy students during his career, and he knew how to communicate this information to students.

I also recommend that you go to YouTube and find the anatomy lectures of Deane G. Keller, who also taught thousands of anatomy students in his lifetime. (Me among them.) You have to put up with weird production values because he only had a video camera, but all the same his anatomy drawing classes will give you a great start in understanding human anatomy and how to draw the figure based on solid anatomical principles.

Learn more about one leg squat anatomy with proper guidelines