Nutation refers to a significant movement of the sacrum within the pelvis. It involves the forward tilting of the sacrum relative to the iliac bones. This motion is fundamental in biomechanics, playing a role in maintaining the stability of the spine and the entire pelvic structure.
The Sacrum and Its Movement
The sacrum is a triangular bone located at the base of the spine, between the two large iliac bones of the pelvis. These bones articulate at the paired sacroiliac (SI) joints, connecting the upper body and lower limbs. The SI joints are the largest axial joints, serving as a central point for motion and shock absorption.
Nutation describes the forward and downward tilting of the sacrum’s upper portion (promontory) relative to the ilium. This motion causes the angle between the sacrum and the hip bones to increase. Conversely, counter-nutation is the opposite movement, where the sacrum tilts backward and upward relative to the ilium. These motions involve a minimal range, approximately 2 to 4 mm, guided and limited by strong ligaments surrounding the SI joint. The posterior sacroiliac, sacrotuberous, and sacrospinous ligaments are important in resisting excessive nutation.
Nutation’s Role in Spinal Health
Sacrum nutation is important for managing forces and maintaining stability. This movement helps in shock absorption during various activities, such as walking and standing. The sacroiliac joint absorbs forces transmitted through the body, such as from the ground up through the legs or from the upper body down.
As the sacrum moves into nutation, it helps distribute weight effectively from the upper body to the lower limbs. This balanced weight distribution is maintained by the coordinated action of nutation on one side of the body and counter-nutation on the other during movement. The slight movement at the SI joints during nutation enhances pelvic girdle stability, preventing excessive spinal stress by acting as a “central spring” in the body’s shock absorption system.
When Sacral Movement is Impaired
Impaired sacral movement, whether restricted (hypomobility) or excessive (hypermobility), can lead to discomfort and pain. Individuals may experience localized pain in the lower back or buttocks, often on one side. This pain can sometimes radiate down the leg, mimicking sciatica, though it usually stays above the knee.
Other common symptoms include stiffness and a reduced range of motion in the lower back, hips, pelvis, and groin, which can make activities like climbing stairs or bending difficult. Factors contributing to impaired sacral movement include prolonged static postures, repetitive movements, or previous injuries like falls or car accidents. Pregnancy can also lead to increased ligament laxity and altered sacral movement due to hormonal changes and increased weight.