Often the knee can be thought of as a hinge joint. However, when it flexes, the knee permits the lower leg to rotate. So it is not a pure hinge joint in absolute sense because it permits more range of motion then a pure hinge joint.
Since the hip joint is a ball and socket joint, this means that, relative to the pelvis, the femur can also rotate.
In short, both have the capacity to rotate, to some extent independently.
Each leg has one femur and one lower leg. Each can be in one of three states.
- Inwardly rotated
- Outwardly rotated
Basic math tells me that means 3^2 = 9 states
This seems like a small number, but if you then consider both legs that gives you 81 possible states.
I presume not all of these are possible. I assume that, if we exclude deformities, birth defects, or growth defects, in general there must be some kind of consistent patterns that humans tend towards.
What are the most common relative orientations (as defined above) for the femur and lower leg?
For example, if the femur inwardly rotates, does the lower leg automatically rotate inwards?