The spinal column of a human body must support the weight of the upper half of the body including the head and the upper limbs, befitting to the bipedal upright posture. Vertebral bodies of the lower lumbar are larger than the upper one for this support. Since the ventral side of the spinal column has a thorax, cervical and lumbar lordoses are indispensable to bring the center of gravity close to a centroidal line. Therefore, the thickness of the inferior lumbar vertebrae are greater ventrally, and an intervertebral disk is thick, and wedge-shaped. Since the weight shifts forward and backward as to the standing and the sitting position, the angle of a pelvis must be changed, and a sacrum and lumbar vertebrae cannot be unified. The lumbar vertebrae at the time of a walk should take the rotational shear, between an upper-limb-thorax block and a lower-limb-pelvis block, since they can hardly rotate due to the dissociation of the center of the rotary axis and the actual center of the vertebral bodies. The situation results in propensity for disk herniation or compressive fractures.