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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.

  1. Neutral
  2. Inwardly rotated
  3. 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?

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  • $\begingroup$ You have probably meant 18, not 81 states for both legs. $\endgroup$ – Jan May 28 at 11:01
  • $\begingroup$ @Jan Stan mean 9^2, that is 81 possible combinations using his argument. $\endgroup$ – Alex May 28 at 13:49
  • $\begingroup$ If one leg can be in 9 positions, wouldn't you say that both legs can be in 9 additional positions, so 18 in total? Anyway, if you say that one leg in one position and the other leg in another position make a new combined position, you can say 81. $\endgroup$ – Jan May 28 at 13:54
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In anatomy, the term movement usually applies to a specific bone in a specific joint, not to the limb. It is said "the flexion of the femur in the hip," and not "the flexion of the leg." So, if you rotate the femur in the hip inwards, your lower leg will also rotate inwards, but this does not "count" as an additional movement.

Possible movements in the hip joint include (TeachMeAnatomy):

  • Flexion
  • Extension
  • Inward rotation
  • Outward rotation
  • Abduction
  • Adduction

There are known possible movements in the knee joint and in the ankle. After all possible movements in these joints, your feet will be in different positions. It then depend on you what you consider a different position. You can have a flexed hip and flexed foot, or an extended hip and flexed foot - is your foot in the same or different position? From the anatomical viewpoint it is in one (flexed) position in both cases.

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