Lordosis: the inward curvature of the lower spine.
Pelvis: the bony structure you sit on, made of your hip bones, tail bone, and other bones.
Our entire body is adapted to walking on two legs, an ability called bipedality. Bones of the human body have specific shapes that improve their function during bipedality. The hip bone, or pelvis, connects the legs to the rest of the trunk. The top of the pelvic bone, or ilium, is short and broad. It is shaped almost like a bowl, which holds up your guts. This bowl-like shape better supports the weight of your upper body. It also repositions the trunk muscles to both improve balance and leg movement. Repositioning these muscles creates better leverage, making them more efficient.
The lower spine curves inward. This curvature is called a lordosis. The lordosis brings your center of gravity over the pelvis, improving your balance when you walk upright. If we didn’t have curved backs, it would be much easier to fall over when walking.