The sacroiliac joint is at the center of the body’s shock absorption system. When force is transmitted superiorly or inferiorly through the body, a spring-like action takes place. Below the sacrum, the feet pronate, the fibulae move inferiorly , and the knees twist as the tibia goes farther into internal rotation than the femur. As the innominate rotates posteriorly, the sacrum rotates anteriorly. At the sacroiliac joint, this movement winds the interosseous ligament, drawing the sacrum and ilium closer  p55, but not together . Above the sacrum, the lumbar curve increases like a compressing spring. The sacroiliac joint is forced into nutation. Once the energy is absorbed and the end of the range of motion is reached, the body reacts by going into counternutation, and all the above actions are reversed as the spring rebounds. For more, see Shock Absorption.
Wilder et al.  was of the opinion that, because the sacroiliac ligaments pull the sacroiliac joint surfaces closer in limiting translation, they may absorb energy and function in shock absorption. Grieve  stated that, if the sacroiliac joint does act as a central shock absorber, as suggested by Wilder, then, with sacroiliac joint dysfunction, degeneration of the lumbar spine and ipsilateral hip would be inevitable (which indicates that the SIJ is an underlying cause of lumbar disc herniation).
The weight of the entire upper body rests on the sacrum. From the sacrum, the weight is transferred through the sacroiliac joints to the innominate bones and legs. Likewise, force is transferred up through the legs through the sacroiliac joints to the spine, torso, and upper extremities.
Balanced Weight Distribution
The underlying pattern of nutation/counternutation provides a balance in weight distribution during movement. When one side of the body is in nutation, the other side is in counternutation. Within nutation, anterior sacral movement is countered by the posterior innominate movement. Within counternutation, anterior innominate movement is countered by the posterior sacral movement. Axial rotation of the pelvis is countered by sacral and spinal rotation in the contralateral direction.
The sacroiliac joints serve as proprioceptive balance centers that control and orient the body’s posture through muscle contraction and inhibition patterns. Snijders  stated that “The SI joint can be compared to a multidirectional force transducer, for it can be expected that the ligaments surrounding the SI joints possess mechanoreceptors. In that case, the SI joints can be assumed to be large sensors located in the middle of considerable force streams being transferred by the pelvis from the upper part of the body to the legs. In this transfer, the largest muscles of the body are involved.”
The sacroiliac joints enable the sacrum to function as a hub, suspended between the ilia by ligaments . Bipedal motion is transferred from the legs, through the pelvic bones, to the spine and cranium in a unilateral, alternating fashion.
Movements of the sacroiliac and pubic joints allow the spreading of the pelvis to accommodate the birth process. To accomplish this, the pelvis goes into nutation. Posterior displacement of the sacral apex increases the pelvic outlet p64-67.
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