Hip and Knee Kinematics
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Humans have a distinctive pattern of hip and knee movements during walking bipedally. Human hips and knees contact the substrate at the touchdown phase of gait in nearly fully extended posture, with the knee straighetening before the hip. The dorsally expanded ilium and ischium increase the mechanical advantage of hip extensors in nearly fully extended postures, unlike in nonhuman primates. Ape knees are flexed even when bipedal, roughly 15 degrees or so. The straight-kneed gait of humans results in a flattening of the distal contour of the femoral condyles, increasing joint surface contact area during weight bearing allowing protection of the articular cartilage. Apes have rounded condyles, which are adapted to maximizing articular contact areas in a wider variety of postures, and allowing minimal rapid alterations in quadriceps muscle length-tensions relationships during movements of the knee. There has been debate about how much hip and knee flexion characterized Australopithecus, with some arguing that a reduced anterior horn of the acetabulum and small hindlimb articular surfaces indicates bent knee gait, but the presence of distinctly flattened femoral condyles and a pronounced femoral bicondylar angle, along with well developed spinal curvatures that are an integral part of fully upright posture, demonstrates that australopiths had a human-like pattern of hip and knee postures. Kinematic analyses have demonstrated that African apes have different patterns of hip and knee motion even when bipedal than do orangutans and humans, who are similar, which may reflect specialization for terrestrial quadrupedality in these species, with humans retaining a more general, primitive kinematic pattern.
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