Humans are unique among mammals in their exclusive use of a bipedal gait. While many other mammals may use a bipedal posture on occasion, humans and their ancestors have restructured their postcranium so drastically that they must use this posture permanently. Many of the most striking differences between bipeds and quadrupeds reside in the pelvis.
In a bipedal animal the quadriceps muscles become the primary muscles involved in propulsion. These muscles act to straighten, or extend the knee while the limb is still touching the ground. The quadriceps is divided into four different heads. Rectus femoris is the most superficial of these, and originates at the front and bottom of the ilium as well as from a groove situated above the acetabulum. It attaches to the patella.
The hamstrings acts in direct antagonization to the quadriceps by acting as a knee flexor. The most important activity of the hamstrings is to control the deceleration of the limb as it reaches the final stages of the swing phase of walking.
Gluteus minimus and medius act as pelvic stabilizers while the body is being supported by only one limb, and Gluteus maximus stabilizes the trunk upon that same limb. It also controls forward rotation of the trunk when making ground contact by drawing the pelvis backward. Gluteus maximus originates at the back of the superior-most spine of the ilium, as well as from the posterior and inferior portions of the sacrum. It inserts into the iliotibial band, which is a tract of fascia which runs down the leg, as well as onto the gluteal tuberosity of the femur, which runs from the linea aspera to the base of the greater trochanter. Gluteus minimus arises from the outer surface of the ilium and the sciatic notch and inserts onto the anterior surface of the greater trochanter of the femur, and Gluteus medius arises from the outer surface of the ilium and inserts onto the lateral surface of the greater trochanter.
We can look to the fossil record to see how the locomotion of our ancestors compares to our own and to that of modern apes. The “Lucy” specimen of Australopithecus afarensis bears an almost completely intact left innominate bone. Lucy exhibits a broad, low, and flared ilium, which shows that the anterior abductor muscles (Gluteus maximus, medius, and minimus) had a very active role in supporting the torso while supported on one leg. The position of the ilium allowed the gluteus maximus to exert control over trunk rotation as the heel was hitting the ground. The shortened shape of the pelvis also allowed the spine to curve enough so that the center of gravity of the animal was now positioned directly above the hip.
The upper pelvis of A. afarensis is almost certainly indicative of a modern bipedal locomotion, but the lower pelvis retains some ape-like features. This is because many of the changes that occured in this part of the pelvis reflect the contraints and demands of giving birth to large-brained infants rather than of locomotion. A. afarensis was still giving birth to relatively small-brained infants, so this selective pressure was not yet acting. Thus, the pelvis of A. afarensis doesn’t represent “functional intermediacy,” but rather, it shows that mechanical advantage in the human lineage has actually decreased as the result of parturition.
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