Anatomical Adaptations for Bipedalism: Pelvis

An illustration of gluteus medius originating dorsally from the illium and inserting on the greater trochanter in a modern human.

An illustration of gluteus medius originating dorsally from the illium and inserting on the greater trochanter in a modern human.

A diagram that outlines the lever system of the hip joint, shown on a reproduction of an australopithecine.

In humans and Australopithecus the iliac ala flares toward the front of the body, and the pelvic outlet is increased in size''.

The gluteus medius and gluteus minimus muscles originate on the dorsal side of the ilium and insert on the greater trochanter of the femur. Their actions are critical to propulsion and stability while walking. Since bipedalism requires special adaptations the orientation, and thus the function, of the gluteal muscles, is different in bipedal humans and quadrupedal apes. In apes, the flat portion of the iliac ala is roughly parallel with the plane of the back, while in humans it is shifted laterally and flares more on the sides. The more lateral orientation of the alae in humans abducts (i.e., move away from the body) the hip joint and the gluteal muscles stabilizes the area by preventing the hip on the supported side (the standing leg) from collapsing toward the unsupported side (the swinging leg). In apes, these muscles are attached more dorsally (i.e., more toward the back and less on the sides) and act as hip extensors, which move the leg backward when the primate takes take a step⁹.

The australopithecine pelvis has widely flaring iliac ala. This flare is a critical component of the lever system of the hip and acts to increase the mechanical advantage of the lesser gluteals by increasing their lever arm. However, the lateral flare of the australopithecine ala is more pronounced than typically seen in modern humans. The fact that the australopithecine pelvis appears more similar to humans than to apes suggests that Australopithecus was fully bipedal. It is thought that the australopithecine unique morphology suggests the species did not utilize the modern gait seen in later Homo.⁹

The modern human pelvis has relatively larger hip joints and larger pelvic outlet relative to australopithecines or modern apes. These differences appear to be a compromise between two functional needs: 1) efficient bipedalism; and 2) allowing enough space for wide shouldered, large brained infants to pass through the birth canal.

As the size of the pelvic outlet increased, the hip joints were repositioned relatively further from the center line of the body. The hips have to support and balance the weight of the torso during locomotion. More force is exerted on the hip joint as the joint (acetabulum and femoral head) moves further away from the body's center of gravity, and thus affects stability as the weight of the torso presses downward toward the middle of the body. This issue is resolved through several adaptations in the pelvis and femur. In the pelvis, the hip joint is enlarged allowing more stress to be absorbed and to accommodate a larger femoral head. See later discussions for information on adaptations of the femur^9,16.