When nutation occurs bilaterally, due to the oblique orientation of the joint surfaces, the sacral base moves on a sagittal axis, anteriorly/inferiorly/rotating contralaterally, while the ilia move posteriorly/medially/inferiorly, flaring inwardly. The ilia move towards each other posteriorly while the ischia move apart anteriorly. Counternutation is the opposite movement; the sacral base moves posteriorly/superiorly/rotating ipsilaterally and both ilia move anteriorly/laterally/superiorly, flaring outwardly. The ilia separate posteriorly and the ischia approximate anteriorly [1-6].

NOTE: The normal motion of the sacrum and ilium at the sacroiliac joint is reciprocating unilateral motion (See Reciprocating Unilateral Nutation). However, bilateral nutation and counternutation, as described here, occur in some situations but, probably, only to the degree determined by the power and vector of the inducing force. That being said, the examples below discuss the forces that induce bilateral nutation.

In a pelvis with normal function, lifting a heavy object with even weight distribution over both hands and feet will promote bilateral nutation, as does the act of childbirth when the fetus is in the pelvic outlet. Another example of bilateral nutation occurs during deep exhalation while rotating the arms and legs internally; the cervical and lumbar curves increase, while the thoracic curve decreases. Conversely, bilateral counternutation occurs while taking a deep breath while externally rotating both arms and legs; the cervical and lumbar spinal curves decrease and the thoracic curve increases. Our first breath (inhalation) is counternutation and our last breath (exhalation) is nutation.

 

References:

1. Weisl, H., The articular surfaces of the sacro-iliac joint and their relation to the movements of the sacrum. Acta Anatomica (Basel), 1954b. 22(1): p. 1-14.
2. Weisl, H., The movements of the sacroiliac joint. Acta Anatomica (Basel), 1955. 23(1): p. 80-91.
3. Kapandji, I.A., The Physiology of the Joints. Vol. 3. 1977: Churchill Livingstone.
4. Neumann, D., Kinesiology of the Musculoskeletal System. Foundations for Physical Medicine. 2002: Mosby.
5. Oatis, C.A., Kinesiology. The Mechanics and Pathomechanics of Human Movement. 2004: Lippincott Williams & Wilkins.
6. Vukicevic, S., et al., Holographic analysis of the human pelvis. Spine, 1991. 16(2): p. 209-14.