The spine is balanced on an undulating base that is known as the pelvic ring or pelvic girdle. This is the body's chief weight transmitting structure, connecting the upper body to the legs. It consists of the sacrum at the back and two innominate hip bones, which are connected to the femurs thigh bones.

Together, these bones form five joints, two sacroiliac joints at the base of the spine, two hip joints where the hip bones connect with the legs and the symphysis pubis where the two hip bones join in front. During normal standing or sitting, the ligaments of the sacroiliac joints and the pelvic ring are somewhat loose.

When weight bearing occurs, however, the pressure extended down through the spinal column causes the sacroiliac ligaments to tighten, changing the pelvic ring from a loose, or neutral, structure to one that makes for greater stability. A second key characteristic of the pelvic ring is that its degree of tilt affects the curvature of the spinal column above.

Any change in the angle of the sacral portion of the pelvis will thus determine posture. If the pelvic ring is in a balanced position, the spinal curves will be proportionately balanced and the posture safe. If, however, the pelvic ring is abnormally tilted, poor posture will result and the spine will consequently be more vulnerable to injury and pain.

The basin shaped pelvis at the base of the spine supports the upper half of the body and protects the abdominal organs. The tilt of the pelvis affects the curvature of the spine and thus influences posture. Discs cushion vertebrae. They are firmly attached to those above and below and can withstand much compression.

They do not actually slip but they can bulge or rupture. Semitendinosus is also known as the hamstrings. It runs down the middle of the back of the thigh and is used to extend the thigh and flex the knee.