Human Pelvis - Evolution

Evolution

The shape of the pelvis, most notably the orientation of the iliac crests and shape and depth of the acetabula, reflects the style of locomotion and body mass of an animal. In bipedal mammals, the iliac crests are parallel to the vertically oriented sacroiliac joints, where in quadrupedal mammals they are parallel to the horizontally oriented sacroiliac joints. In heavy mammals, especially in quadrupeds, the pelvis tend to be more vertically oriented because this allows the pelvis to support greater weight without dislocating the sacroiliac joints or adding torsion to the vertebral column. In ambulatory mammals the acetabula are shallow and open to allow a wider range of hip movements, including significant abduction, than in cursorial mammals. The lengths of the ilium and ischium and their angles relative to the acetabulum are functionally important as they determine the moment arms for the hip extensor muscles that provide momentum during locomotion. In addition to this, the relatively wide shape (front to back) of the pelvis provides greater leverage for the gluteus medius and minimus. These muscles are responsible for hip abduction which plays an integral role in upright balance.