People in the upright or sitting when the spine feels straight, in fact, the vertebrae wrapped around our body is not straight, it has a certain degree of curvature will protrude forward or backward, this curvature is called the physiological curvature of the spine. The human spine has four physiological curvature is four curvatures, including cervical, thoracic, lumbar and caudal curvatures, the cervical vertebrae protrude forward, the thoracic vertebrae protrude backward, the lumbar vertebrae protrude forward, the sacrococcygeal vertebrae are protruding backward, the side view of the spine or in the X-line look like a snake, each vertebrae is under different stresses, the curvature is also different, suitable for the physiological structure, adapted to the function of each vertebra, so there will be physiological curvature.