FIGURE 5 The spine and pelvis during standing and sitting. Compared with the chimpanzee (left), the human lower spine (the lumbar region) has a curvature (a lordosis) that positions our center of mass (circle) above our hips when we stand. When we squat on the ground (the way people often sat for millions of years) or slouch when sitting in a chair with a backrest, we tend to rotate the pelvis backward and flatten the lower spine, reducing this lordosis. (Note that I have shown just a few of the many postures people adopt when sitting.)
