The Sacrotuberous Ligament runs downwards and laterally outwards from the rear of the sacrum to the Ischial Tuberosity (IT). It can be used to pull downwards on the sacrum or it can resist an upwards pull on the sacrum.
Muscles whose fibers blend with or partially make up the fibers of the sacrotuberous ligament include: Piriformis, Gluteus Maximus and Biceps Femoris (long head).
The Long Dorsal Sacroiliac Ligament connects upwards from the sacrum to the Posterior Superior Iliac Spine (PSIS). It can pull upwards on the sacrum (relative to the hip bones) or resist a downwards pull on the sacrum. It connects to the Paraspinal Rectinacular Sheath. It also connects to the aponeurosis of the Erector Spinae. It also connects to the multifidus.
Both ligaments can be directly affected by muscle tension in such a way that they help to stabilize the SI joint.
With an understanding of the muscles that directly or indirectly add tension to these ligaments a strategy for muscle control can be developed to help keep the SI joint stabilized.
To help in understanding how different muscles can act on the SI joint via the sacrotuberous and long dorsal sacroiliac ligament it can help to understand that both are connected to a connective tissue complex called the Thoracolumbar Complex.
The Thoracolumbar Complex links them both to the spinal erectors and to the transverse abdominus.
Both the piriformis and the gluteus maximus have fibers that blend with the sacrotuberous ligament.
The Piriformis attaches to the front surface of the sacrum and from passes forwards and outwards and slightly downwards to attach to the thigh bone.
Assuming the femurs are fixed then activating the piriformis creates a forward pull on the sacrum. This could induce a slight reduction in tension in the sacrotuberous ligament. However, tension in the fibers common to the ligament could make up for the loss.
Because the Piriformis attaches to the sacrum below the SI joints, activation of the piriformis could pull the sacrum into counter-nutation, a backward nodding of the sacrum relative to the hip bones.
Counter nutation is when the bottom tip of the tailbone moves towards the pubic bone while the Ischial Tuberosities (ITs) move inwards. The net effect is that the opening at the bottom of the pelvis gets smaller. Correspondingly, the opening at the top of the pelvis is widened as the top of the sacrum moves rearwards and the ASISs (Anterior Superior Iliac Spine) move laterally outwards.
Counter nutation may cause a net movement of the sacrum downwards.
To further stabilize the SI joint with the sacrum counter nutated, the pelvic floor muscles could also activate. Quite probably these would activate prior to the piriformis activating. They could be used to initiate the inward movement of the ITs and the forward movement of the tailbone, with the piriformis coming into play once the action has been initiated.
The action of all of these muscles together pull the sacrum forwards and downwards and the Ischial Tuberosities inwards while resisting the opposite movements so that the SI joint is stabilized with the sacrum in the nutated position.
The Gluteus Maximus (GM) has attachments to the rear of the sacrum, and portions of the rear of the pelvis. It also has fibers that attach to (or blend with) those of the sacrotuberous ligament.
Activation of the gluteus maximus could pull the sacrum downwards (caudalwards) with respect to the hip bones, reducing tension in the sacrotuberous ligaments (STls). The fibers of glute maximus that attach to the ligaments could make up for that loss in tension.
One other way in that Gluteus Maximus can help in counter nutation is by pressing the Ischial Tuberosities inwards.
Because the GMs are fairly massive, their bulk, when contracted can press inwards on the ischial tuberosities. The fibers that attach to the sacrum can then pull downwards on the sacrum while the fibers that attach to the sacrotuberous ligament can pull down on the sacrum via those fibers with the net result being a fairly powerful counter nutating force.
The Biceps Femoris (long head) unlike the other hamstrings (the Semi-Tendinosus and Semi-Membranosus) often attaches directly to the sacrotuberous ligament without attaching to the ischial tuberosity (IT).
In a standing forward bend with the pelvis tilted forwards relative to the femurs, the weight of the upper body acting via the spine can create a force that tends to pull the sacrum cranially (towards the head.)
If the hamstrings are activated (even as they are being stretched by the forward tilt of the pelvis), they can add tension to the sacrotuberous ligament so that it resists the cranial (towards the head) pull on the sacrum. The gluteus maximus can be used in the same way.
The lumbar multifidus (and lumbar spinal erectors) can be used nutate the sacrum so that it nods forwards relative to the hip bones.
As the bottom of the sacrum and tailbone moves rearwards, away from the pubic bone, this action adds tension to the sacrotuberous ligament.
The Long Dorsal Sacroiliac Ligament could also play an active roll in nutation since contraction of the Erector Spinae leads to additional tension in this ligament so that it acts in creating an upward pulling force on the rear of the sacrum which leads to nutation.
In this instance the ligament acts more in the manner of a tendon than in the way we normally conceive a ligament would act.
Note that in experiments where traction was used to simulate the action of the latissimus dorsai, resulted in a reduction of tension in the ligament. In this case the simulated force of latissimus dorsai helped to nutate the sacrum. As a result tension was reduced in the long dorsal ligament.
While pressing inwards on the ITs causes counter nutation, a pulling inwards on the ASISs can cause the ITs to move outwards. This action could be driven by caudal fibers of the Transverse Abdominus pulling inwards on the ASICs. This same action creates a tendency for the PSIS, which are behind the SI joints, to move apart. This outward lateral movement of the PSISs is prevented by the Thoracolumbar Composite.
The Thoracolumbar Composite spans the PSIS on both sides as well as attaching to the rear of the sacrum, Long Dorsal Sacroiliac and Sacrotuberous ligaments. It resists any tendency of the PSIS to move outwards and when the Transverse Abdominus are activated it actually helps to compress the SI Joints.
This is relevant to the focus of this article in that tension added to the TLC by the action of the Transverse Abdominus could help to amplify the action of the multifidus when they act to nutate the sacrum.
Hip muscles that can cause nutation include the Iliacus and Obturator Internus.
The Iliacus, could help to pull the ASICs inwards, acting in the same way as the Transverse abdominus. Lack of leverage might be made up by an increased contact area of the iliacus.
Obturator internus, like the iliacus, attaches to a large surface area on the inner surface of the innominate bone.
It's tendon passes out through the lesser sciatic notch, wraps around the pelvis and attaches, like the piriformis, to the inner surface of the greater trochanter just above the neck of the thigh bone.
Assuming that the femurs are stabilized against external rotation (and perhaps even slightly internally rotated) the obturator internus could activate in such a way as to help pull apart the ITs, adding tension to the Sacrotuberous Ligaments.
Actual nutation of the sacrum could then be carried out by the sacral multifidus.
Note that one possible consequence of using the obturator internus to add tension to the sacrotuberous ligament is that it could give the fibers of the Gluteus Maximus that attach to it a firm foundation from which to act on the thigh bone.
This could be important in extreme hip flexion. The gluteus maximus could then have extra fibers to recruit to help pull the body out of hip flexion if needed.
In addition this could also give the biceps femoris a more stable anchor to assist the gluteus maximus in pulling the hips out of flexion.
As shown above, there are a number of options for creating nutation and counter-nutation both.
While some papers suggest that nutation is the more stable position I would suggest that the muscles of the pelvis and hip (and back) can function in such a way to stabilize the SI joint through a full spectrum of positions. So rather than just being stable when fully nutated or counter nutated, the muscles that create these actions can be used against each other (or against body weight) as an option for creating stability in any of the in-between positions of the SI Joint.
The muscular control of the Sacrotuberous and Long Dorsal Sacroiliac Ligaments offers a few possible mechanisms for creating this stability.
One possible sequence is to first activate transverse abdominus and pull inwards on the ASISs. Then gluteus minimus and/or TFL have a stable foundation from which to act on the femurs, internally rotating them or stabilizing them against external rotation. The Obturator Internus could act from the femurs to pull outwards on the ITs.