Warning: I am no expert when it comes to the human body so it is likely I have made some errors. But that is fine seeing as how I am not studying to become a doctor. I am just trying to make a doll with as natural an articulation as possible.
Although you can find drawings of joints people used for their own dolls, I think it is a good idea to understand how they work. Plus, learning how the joints work allows me to select the joint style that best suits my needs instead of just whatever works. While learning about how the ankle and knee joints worked I realized that it is not essential I understand joints for my working drawing. Below are my rough sketches.
I think it will be a challenge to design the knee joints because of the angle. A seiza position requires an angle of 155 degrees. Sitting in a chair requires 90 degrees. After some consideration I have decided that my doll, which is a child, should be able to sit in a 'w' position but not a seiza position. This makes the angle I am working with less and hopefully less of a challenge. But I won't know until it comes time to make the joints.