Spreading of the ala of ilium
- The ala of ilium is the upper part of the ilium that spreads out like a fan, but the degree of fan opening differs among animals. In quadrupeds, the fan opening is smaller than in humans (elongated ilium). In humans, which are characterized by upright bipedal locomotion, the fan opening is large (broad ilium). Specifically, human ilium is relatively smaller than that of other primates (monkeys) in terms of ilium height (vertical diameter) compared to ilium width (anteroposterior diameter).
- The human ilium is more elongated and bent posteriorly than in other primates. This results in a relatively deep greater sciatic notch. This posterior elongation of the ilium repositions the gluteus maximus and gluteus medius muscles in relation to the hip joint, which is advantageous for bipedal walking.