The piriformis and obturator externus, both external hip rotators, act like pliers at the SI Joint, pulling the sacral base posteriorly and the ilia anteriorly, into counternutation.

Piriformis

Origin: Anterior and lateral aspect of the sacrum from S2 to S4 [1]p81. Some fibers attach to the sacrotuberous ligament and the anterior capsule of the sacroiliac joint.

Insertion: It goes through the sciatic notch to insert into the superior medial aspect of the greater trochanter.

Functions:

• Pulls the sacral apex anteriorly and laterally, rotating the sacral base posteriorly [2], into counternutation
• Rotates the leg externally
• Observation reveals that by pulling on the sacrotuberous ligament, and limiting nutation, it provides a counternutation moment.
• The line of action is similar to that of a sacroiliac belt in providing a compressive self-bracing effect on the sacroiliac joints [3].

Vleeming [2] dissected 12 cadavers and found that the piriformis is either directly, or indirectly, attached to the ventral part of the sacrotuberous ligament. Therefore, the pull of the piriformis would cause anterior rotation of the sacral apex, rotating the sacral base posteriorly into counternutation.

Obturator Externus

Origin:  External surface of the obturator membrane and surrounding external surfaces of the inferior pubic ramus and ischial ramus

Insertion: Medial surface of the greater trochanter at the trochanteric fossa

Functions:

• Rotates the leg externally
• Observation reveals that it pulls the lower pelvis posteriorly, rotating the iliac crest anteriorly, into counternutation.

Piriformis/Obturator Externus Counternutation Pairing
As noted above, the piriformis pulls the sacral apex anteriorly and laterally, which results in the ipsilateral base moving posteriorly and superiorly. The obturator externus pulls the lower pelvis posteriorly, resulting in the ilia crest moving anteriorly. Together, these two muscles, acting separately on the sacrum and pelvis, exert a pliers-like action that induces counternutation and reduces the sacroiliac joint nutation lesion. Additionally, the leg is rotated laterally by both muscles, indicating a relationship between external femoral rotation and counternutation.

References:

1. McMinn, R.M.H. and R.T. Hutchings, Color Atlas of Human Anatomy. 1977, Chicago: Medical Publishers, Inc.
2. Vleeming, A., R. Stoeckart, and C. Snidjers. The Sacrotuberous Ligament: A Conceptual Approach to its Dynamic Role in Stabilizing the Sacroiliac Joint. in Proceedings of the 1st Interdisciplinary World Congress on Low Back Pain and its Relation to the Sacroiliac Joint. 1992. San Diego: ECO.
3.  Snijders, C.J., Transfer of Lumbosacral Load to Iliac Bones and Legs: Part 2 – Loading of the Sacroiliac Joints when Lifting in a Stooped Position. Clinical Biomechanics, 1993b. 8: p. 295-301.