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Bicondylar Angle of the Femur
The bicondylar angle of the femur is defined as the angle between the long axis of the femoral shaft and a line tangent to the distal extent both femoral condyles, in frontal view. It is also referred to as the ‘carrying angle.’ It usually measures between about 8 and 14 degrees. This angle is unique to humans, and serves to place the knee and foot under the body’s center of gravity during single support phase of gait. Hip joints are set lateral to the body’s midline, so this angle allows the lower limb to be aligned with the center of gravity. The bicondylar angle is greater in females than males, given the wider spacing between their hip joints. Nonhuman primates do not exhibit a bicondylar angle, because they lack a single support phase of gait. This is true even for monkeys trained to walk bipedally. The bicondylar angle develops during early childhood, and is in place by roughly seven years of age. It is a plastic response to bipedal walking in humans, and does not develop in people who are never ambulatory. As such, the presence of a femur with clear bicondylar angle in a fossil hominid indicates that this individual walked bipedally, and was also the product of a prolonged history of selection for human-like bipedality because this plastic response was able to occur.