The Sacroiliac Joint: Anatomy and Function
The sacroiliac joint (SI joint) is involved in literally every yoga pose we practice. It is the hub for the transference of force between the legs and torso. Although it is designed to be a stable joint with limited mobility, some yoga practices can destabilize the joint, causing dysfunction. Understanding the anatomy of the SI joint can help us understand how to move in ways that promote stability.
The SI joint is the articulation between the ilium and the sacrum on each side of the pelvis. As with other joints, it is comprised of the bony stabilizers, the static soft tissue or ligamentous stabilizers, and the dynamic muscular stabilizers. On the surface of the bone is the articular cartilage.
The SI joint depends primarily on the stout ligaments that cross it for stability. The bones also have shallow interdigitations that correspond on each side, thus conferring some bony stability. Finally, there are the muscles (dynamic stability) and fascia—especially the thoracolumbar fascia.
Bones that Comprise the SI Joint
Figure 1 below illustrates the bones that comprise the SI joint.
( Figure 1: The bones of the sacroiliac joint. )
The Stable Sacroiliac Joint
Figure 2 below illustrates the stout ligamentous stabilizers of the joint. These include:
The anterior (front) and posterior (back) sacroiliac ligaments running from the sacrum to the ilium
The sacrotuberous ligaments running from the sacrum to the ischial tuberosity
The sacrospinous ligaments running from the sacrum to the posterior iliac spine
(Figure 2: The ligaments of the sacroiliac joint.)
Movement is very limited for this joint, but includes nutation or anterior tilt (flexion) of the sacrum between the ilia, counternutation or posterior tilt (extension) and small movements of the ilia themselves. The stable SI joint thus functions for shock absorption and transfer of torque during ambulation. Muscles and fascia also confer stability to the joint.
The Erector Spinae and the Pelvic Floor
Figure 3 illustrates the relationship between the erector spinae muscles of the back and the muscles of the pelvic floor. You can see that the erector spinae muscles draw the sacrum into flexion (nutation) and the muscles of the pelvic floor (especially the pubococcygeus) draw the bone into extension (counternutation). Simultaneously engaging these muscles creates opposing forces that stabilize the joint.
(Figure 3 left: The interaction between the erector spinae and pelvic floor muscles for stabilizing the SI joint.)
How to Strengthen the SI Joint Stabilizers
Figure 4 below illustrates the relationship of the latissimus dorsi and gluteus maximus muscles on opposite sides of the body. In between is the thoracolumbar fascia. Note how the fibers of these structures run perpendicular to the joint. Thus, working with core exercises and yoga poses such as Bird Dog Pose can help to strengthen the dynamic stabilizers of the SI joint. These muscles, along with the fascia comprise the “posterior oblique subsystem.”
(Figure 4: The posterior oblique subsystem for stabilizing the SI joint.)
Reprinted with permission from The Daily Bandha.