This view shows the most common movement at the sacroiliac joint, unilateral reciprocating motion.

It is important to note that nutation and counternutation can occur bilaterally, during which both innominates move together in either nutation or counternutation, or unilaterally, during which one side will be in nutation while the other side is in counternutation. In reality, bilateral motion is rare and it’s deviation from unilateral motion may only be a matter of degree, determined by the power and vector of the inducing force. Please see Bilateral Nutation.

For proper perspective, think of nutation and counternutation as relative movement between the three bones of the pelvis rather than a relationship between the bones and the cardinal planes. In one sense, sacroiliac nutation and counternutation are similar to extension and flexion but also involve simultaneous rotation on an oblique axis, and are not relative to the cardinal planes. Although, traditionally, nutation and counternutation were considered to involve only the sacrum and ilium, they are presented here as the core movement of the entire musculoskeletal system.

In a 2 dimensional (2D) perspective, nutation is defined as an anterior and inferior rotation of the sacral base relative to the ilium. From a 3D perspective, the sacrum and ilium move in accordance with the shape of the joint surfaces.

When nutation occurs unilaterally, e.g. on the right, due to the oblique orientation of the joint surfaces, the right side of the sacrum moves anteriorly and inferiorly, as the sacrum rotates to the left on an oblique axis [1]^P33-51 [2]. At the same time, the right ilium moves posteriorly and inferiorly, and flares inwardly. Simultaneously, counternutation occurs on the left, as the left side of the sacrum moves posteriorly and superiorly and the left ilium moves anteriorly and superiorly, and flares outwardly. The sacral apex moves toward the side of counternutation.

Vleeming [3]^p54&60, refers to Weisl [4], Egund [5], Lavignolle [6], and Sturesson et al. [7] when he notes that the lumbar lordosis increases during nutation and decreases during counternutation.

Examples of nutation and counternutation patterns occurring in an alternating fashion (unilaterally) are gait and cross-crawl patterning.

IMPORTANT POINTS TO REMEMBER

• As nutation and counternutation are defined, when either the sacrum or ilium moves anteriorly, the other moves relatively posteriorly.
• When viewed from the side, during nutation, the cervical and lumbar curves increase and the thoracic curve decreases, while during counternutation, the opposite actions occur.
• As the ilium moves anteriorly, although the ilium carries the sacrum with it, the sacrum counterbalances the anterior pelvic tilt by moving posteriorly relative to the ilium. This movement helps explain Junghanns’ [8] observation that the more the pelvis moves anteriorly, the more the lumbar lordosis decreases.
• Likewise, posterior movement of the ilium will cause the sacrum to move relatively anteriorly.
• The primary direction of trunk movement is defined as the direction of pelvic movement.
• Left rotation of the ilium will be counterbalanced by a relative right rotation of the sacrum and spine (except L4-5). So, while the pelvis carries the trunk to the left, the spine is actually rotating to the right.
• The terms flexion/extension can be very confusing, and lead to misconceptions, especially when considering pelvic movement as a unit of 3 bones in the cardinal planes.  
• Regardless of the position of the pelvis, the relative position of the sacrum and spine to the ilium is defined within the context of nutation/counternutation.
• Muscles acting on the sacrum and ilium generate coordinated action at their other attachments on the spine and extremities.
• All joint & muscular actions involved in shock absorption are functions of nutation.
• All joint & muscular actions involved in rebound are functions of counternutation.
• Ligaments and muscles reciprocate through both phases of nutation/counternutation.
• When ligaments are sprained in nutation, the counternutation muscles will remain contracted during both phases to protect the ligament. 

HISTORY
Kapandji [9]p64-67 defined nutation as “anterior rotation of the sacrum about an axis constituted by the axial ligament…so that the promontory moves inferiorly and anteriorly while the apex of the sacrum and tip of the coccyx move posteriorly…Meanwhile the iliac bones are approximated whereas the ischial tuberosities move apart.” Counternutation is the opposite. He stated that, during counternutation, “The sacrum, pivoting around the axial ligament, rights itself so that the promontory moves superiorly and posteriorly and the apex of the sacrum and the tip of the coccyx move inferiorly and anteriorly.”

According to Kapandji, these movements were first described by Zaglas in 1851 and Duncan in 1854 and first measured by Weisl [4, 10]. Oatis [11]p617 noted that Duncan [12] [13] gave the movement a name by postulating the sacrum rotated, or nutated (from Latin nutare, meaning to nod) around a horizontal axis in the vicinity of the iliac tuberosities. Jungmann & McClure [14]p5 credit Gold 1933, Rubin 1956, Novak 1952, and Antoine 1952 as recognizing this mechanism.

Neumann [15]p306 stated that “Nutation is defined as the relative anterior tilt of the base (top) of the sacrum relative to the ilium. Counternutation is a reverse motion defined as the relative posterior tilt of the base of the sacrum relative to the ilium.” (Note the term “relative” used in both definitions).

References:

1. Mitchell, F.L., Jr. and P.K.G. Mitchell, The Muscle Energy Manual. Vol. 3. 1999, East Lansing: MET Press.
2. DonTigny, R.L., Critical Analysis of the Functional Dynamics of the Sacroiliac Joints as They Pertain to Normal Gait. JOM, 2005. 27(1): p. 3-10.
3. Vleeming, A., et al., eds. Movement, Stability, & Low Back Pain. 1997, Churchill Livingstone.
4. Weisl, H., The movements of the sacroiliac joint. Acta Anatomica (Basel), 1955. 23(1): p. 80-91.
5. Egund, N., et al., Movements in the sacroiliac joints demonstrated with roentgen stereophotogrammetry. Acta Radiologica: Diagnosis (Stockholm), 1978. 19(5): p. 833-46.
6. Lavignolle, B., et al., An approach to the functional anatomy of the sacroiliac joints in vivo. Anatomia Clinica, 1983. 5(3): p. 169-76.
7. Sturesson, B., G. Selvik, and A. Uden, Movements of the sacroiliac joints. A roentgen stereophotogrammetric analysis. Spine, 1989. 14(2): p. 162-5.
8. Junghanns, H., ed. Cinical Implications of Normal Biomechanical Stresses on Spinal Function. English ed., ed. H.J. Hager. 1990, Aspen Publications: Rockville, MD.
9. Kapandji, I.A., The Physiology of the Joints. Vol. 3. 1977: Churchill Livingstone.
10. 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.
11. Oatis, C.A., Kinesiology. The Mechanics and Pathomechanics of Human Movement. 2004: Lippincott Williams & Wilkins.
12. Duncan, J.M., The behavior of the pelvic articulations in the mechanism of parturition. . Dublin Quarterly Journal of Medical Science, 1854. 18(60).
13. Duncan, J.M., Researches in Obstetrics. 1868, New York: W. Wood and Co.
14. Jungmann, M. and C.W. McClure, Backaches, Postural Decline, Aging, and Gravity-Strain. 1963, New York, NY: The Institute for Gravitational Strain Pathology, Inc.
15. Neumann, D., Kinesiology of the Musculoskeletal System. Foundations for Physical Medicine. 2002: Mosby.