Pelvic Height and Iliac Flare
The pelvis is a structure comprised of three articulating bones, the right and left hip bones (singular: os coxa; plural: ossa coxae) and the sacrum. These bones differ markedly from those of apes in a number of features. Relative to apes, humans have ossa coxae that are short superoinferiorly and wide mediolaterally, that have a marked curvature to the iliac blades (the upper portion of the hip bone), and that have a prominent angle between the ischium and ilium (the lower and upper portions of the hip bone, respectively). In addition, humans have a sacrum that is superoinferiorly short and mediolaterally broad compared to the sacra of apes. These morphological differences in the pelvis reflect differences in the habitual locomotor and postural repertoires of humans (orthograde posture and bipedal locomotion) and other apes (pronograde posture and quadrupedal locomotion). The reduced height of the human pelvis serves to bring the sacroliliac joint (the articulation between the sacrum and os coxa) closer to the hip joint, and thus to reduce bending forces on the portion of the ilium that transmits body weight from the vertebral column to the hip joint during bipedal standing and walking. The curvature of the iliac blades may serve dual functions: the convexity of the posterolateral surface orients the two smaller gluteal muscles (Mm. gluteus medius and minimus) such that they can operate as femoral abductors rather than extensors (this then allows these muscles to counteract the pull of gravity that would drop the opposite side hip when body weight is being supported on one leg during bipedal walking), and the concave anteromedial surface may provide some support of abdominal viscera (and also of the fetus, uterus and placenta in a pregnant female) during bipedal locomotion and stance. Finally, the greater angulation between the ilium and ischium allows humans to maintain an orthograde (upright) trunk without having to flex the hip joint, thus allowing us to balance body weight over one or both limbs with a minimum of muscular effort during standing and walking.
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