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Figure 1 - Animation
Nutation
Nutation is anterior/inferior movement of the sacrum with relative posterior/inferior movement of the ilia. As the ilia move backwards, the pelvis narrows and the ilia approach each other posteriorly...
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Figure 12 - Animation
The medial thigh counternutation muscles function together
The pectineus, adductor brevis, adductor longus, anterior head of adductor magnus, and gracilis, as a group, rotate the ilia anteriorly to induce counternutation.
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Figure 2 - Animation
Reciprocating side-to-side motion
Here is the more common pattern of reciprocating side-to-side motion. When one side is in nutation, the other side is in counternutation.
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Figure 13
The internal and external obliques function together
The fascia of the right internal & left external obliques interdigitate at the linea alba to function as a unit in inducing counternutation on the right side and, simultaneously, nutation on the left side.
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Figure 3 - Animation
Gravitational Line
The gravitational line passes anterior to the sacral pivot and posterior to the innominate pivot, resulting in anterior rotation of the sacrum and posterior rotation of the ilium....
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Figure 14 - Animation
Internal oblique
The internal oblique works with the contralateral external oblique to rotate the rib cage down and toward the pelvis, flattening the ipsilateral lumbar spine, while pulling the pelvis anteriorly into counternutation.
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Figure 4
Two and three dimensional views of SIJ surfaces
The articular part appears to be "L" shaped. Weight bearing occurs at the posterior syndesmosis, which is 1.57 times as large as the synovial part.
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Figure 15 - Animation
External oblique
The external oblique causes ipsilateral nutation by pulling the ilium posteriorly, while rotating the rib cage down and towards the contralateral pelvis, increasing the ipsilateral lordosis. It also assists the contralateral internal oblique in inducing contralateral counternutation.
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Figure 5 - Animation
Piriformis and obturator externus function together
The piriformis and obturator externus, both external hip rotators, act like pliers at the SI Joint, pulling the sacral base posterior and the ilia anterior, into counternutation.
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Figure 16
Transverse abdominis
The transverse abdominis is active in both nutation and counternutation but primarily in nutation. In nutation, it increases lumbar lordosis and pulls the ilia posteriorly.
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Figure 6 - Animation
Illiopsoas
The psoas combines with the iliacus to create another pliers type pairing, pulling the SIJ into counternutation.
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Figure 17 - Animation
Gluteus Maximus
The upper part of the gluteus maximus, by its attachment to the ilium, pulls the ilium posteriorly into nutation. The lower part, by its attachment to the sacrum, pulls the sacrum into counternutation. Acting together, they pull the entire pelvis posteriorly to extend the hips and raise the trunk.
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Figure 7 - Animation
Superficial and deep erector spinae function separately
The superficial group on the left causes nutation by flattening the thoracic spine and increasing the lumbar lordosis...
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Figure 18
Gluteus maximus and contralateral latissimus dorsi function together
Together, the latissimus dorsi and opposite gluteus maximus muscles create a force that induces nutation on the gluteal side and counternutation on the latissimus dorsi side. In a right SI lesion, the left GM and right LD will contract and the right GM and left LD will be inhibited.
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Figure 8 - Animation
Spinalis Thoracis
The thoracis spinalis narrows the space between the ribs, flattening the ipsilateral anterior/posterior curve, & it rotates the vertebral bodies contralaterally, toward the concavity of the lateral curve.
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Figure 19
The front and back muscle slings function together
In a counternutation response to a right sacroiliac nutation lesion, the right latissimus dorsi and left gluteus maximus pull the right shoulder inferiorly and posteriorly, while the right internal oblique and left external oblique pull the left rib cage inferiorly and anteriorly.
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Figure 9
Multifidus
Although it spans the entire spine, the lumbosacral area is most commonly referenced, where it induces nutation.
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Figure 20 - Animation
Superior and inferior muscles function together
The rectus abdominis, hamstrings, and posterior head of the adductor magnus pull the ilia posteriorly into nutation. These muscles would be inhibited on the side of a nutation lesion.
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Figure 10 - Animation
Quadratus lumborum
The quadratus lumborum lifts the ilium superiorly and anteriorly, bends the spine laterally and, through coupled motion, rotates the spine ipsilaterally into counternutation.
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Figure 21
Structural compensation patterns
In a state of over-compensation, the lumbosacral spine will have an increased lordosis, but the lumbar curve above L4 will flatten and the thoracic spine (more so the upper) will increase its curvature. Not shown are lateral flexion and rotational imbalances.
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Figure 11 - Animation
The anterior thigh counternutation muscles function together
The anterior fibers of the gluteus medius & gluteus minimus, along with the tensor fascia lata, rectus femoris, and sartorius, act together to pull the pelvis anteriorly into counternutation.
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PART 2: illustrations
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