The tibia and femur bones of the leg work in conjunction with the foot as it is pronating and supinating through normal gait. This is due to the fact that the tibia is “locked in” to the talus in the talus’ abduction (causing external rotation of the tibia) and the talus’ adduction (causing internal rotation of the tibia). It has been demonstrated that a foot that is 3° pronated causes the tibia to be 5° internally rotated. This is illustrated best by working with the skeletal model in three dimensions and in Figure 2.8. It has also been shown that a foot, which is 2° supinated causes the tibia to be 10° externally rotated. Therefore, as the foot is going through its normal pronation and supination in the stance phase of gait, the tibia rotates a full 15°. The range of motion is from an initial contact phase position of 10° externally rotated (when the foot is 2° supinated)) to 5° internally rotated at the end of contact phase (when the foot is 3° pronated). Figure 2.8 shows these various movements.
In normal gait, the tibia and femur rotate simultaneously and in the same direction. In other words, the femur is internally rotating while the tibia rotates internally (during pronation) and the femur is externally rotating while the tibia externally rotates (during supination). In the case of abnormal pronation, the femur and tibia can counter-rotate causing the patella (knee cap) to track improperly. See Pathology section under “Chondromalacia Patella”.