Obturator internus can be used to widen the sitting bones, pulling them away from each other and also away from the sacrum and tailbone.
This possibly adds tension to the sacrotuberous ligament (and the sacrospinous ligament)
The sacrotuberous ligament connects the sitting bones to the sacrum.
The sacrospinous ligament attaches from the ischial spine (just above the ischial tuberosities, and in between the upper and lower sciatic notches) to the sacrum.
Pulling the sitting bones apart, and flicking the tailbone backwards (nutation) can add tension to these ligaments.
The gluteus maximus has fibers that attach from the sacral tuberous ligament to the thigh. Adding tension to this ligament, by spreading the sitting bones, potentially gives these fibers of the gluteus maximus a more solid foundation from which to act.
The hamstrings connect to the sitting bones and so potentially, adding tension to the sacrotuberous ligament can create a line of tension all the way from below the knees to the sacrum, and potentially from there up the back of the spine.
And so spreading the sitting bones using the internal obturators can be a key action in forward bends since it integrates the back line of the body.
Now why the obturator internus?
The obturator internus has a large area of attachment within the bottom back half of the pelvis.
If you draw a line from the top rear of the pelvis (just above the upper surface of the sacrum) to the pubic bone, the obturator internus attaches to a large area below and this line.
It attaches to connective tissue covering the obturator foramen, the openings at either side of the bottom of the pelvis, and it is this that gives it its name.
(The obturator externus attaches to the outside of the connective tissue covering this opening.)
The obturator internus wraps around the lower sciatic notch, which is just above the sitting bone. From there, assuming a standing upright position, it passes forwards and upwards to attach to the inner surface of the greater trochanter, near where it connects to the neck of the thigh.
Acting from thighs that are fixed in place, stabilized, the obturator internus can be used to spread the sitting bones apart.
After passing around the lower sciatic notch it unites with the two gemelli muscles which attach to the ischium just above and below the sciatic notch.
These attach to the same point on the greater trochanter as the obturator internus.
And so these muscles can also aid in spreading the sitting bones.
I believe that the large surface area to which obturator internus attaches gives it a broad foundation from which to act, and so perhaps a greater ability to act in spreading the sitting bones.
This actually results in a shape change of the pelvis, slight but perceptible. This shape change can affect tension in the pelvic floor as well as the hips joints as well as the lumbar spine…
The pelvic floor muscles are one set of muscles that can be used to draw the sitting bones inwards and the tailbone forwards, towards the pubic bone, closing the bottom of the pelvis.
This action causes the sacrum to tilt backwards at the SI joint. Some call this action anti-nutation. It could also be thought of as nodding the sacrum backwards.
With the two halves of the pelvis pivoting at the SI Joints and the pubic bone, when the sitting bones move inwards, (and the bottom of the sacrum forwards) the upper front parts of the pelvis (the "eye bones" of the pelvis) spread apart and the top of the sacrum moves backwards opening the top of the pelvis.
Spreading the sitting bones, and perhaps pulling the tailbone back with the aid of the spinal erectors and deeper rear spine muscles, has the opposite effect, opening the bottom of the pelvis and closing the top by nodding the sacrum forwards (nutation) and spreading the upper front part of the wings of the pelvis.
Why might the pelvis be built with this ability?
The shoulder blades move relatively freely with respect to the ribcage. They can be positioned so that the shoulder joint always has room to move and so that the muscles that cross the shoulder joint are at optimum length for optimum tension.
As an example, when lifting the arm bones the bottom tips of the shoulder blades rotate outwards so that the arm bone can clear the accromion and coracoid processes.
Faulty shoulder blade positioning can lead to shoulder impingement or shoulder pain or shoulders that operate poorly.
The pelvis is a more stable structure but it may change shape to allow optimum hip muscle tension and positioning of each half of the pelvis relative to the thigh bones.
Where pulling the sitting bones together can be helpful in backbending the hips, spreading the sitting bones can be helpful in forward bending postures.
It may also be helpful in side-to-side splits, giving the thighs room to abduct or move out to the sides.
The obturator internus can act as an external rotator.
In a standing position, the natural tendency is for the arches to collapse rolling the shins and thighs inwards.
(If while standing you relax your arches, and then relax your hips, while staying upright, you may be able to experience this for yourself.)
With both the hip and the foot working together, torsional stresses on the knee may be reduced.
I had a knee injury from a motorcycle accident about twelve years ago. I never had it checked but the inner knee was damaged, to the point where it would dislocated slightly when I was walking.
Later on the knee did get better but then I found I had problems balancing on that one foot.
I believe that one of the ways that my body may have compensated for the injury, or acted to protect my knee, was to keep the obturator internus activated so that my thigh wouldn't roll inwards.
Rolling inwards perhaps places the most stress on the inner knee, and so when damaged, the body acts to protect the damaged area.
My own experience was that I couldn't ground through the base of my big toe while trying to balance on that foot
My hip isn't a hundred percent now. As well as injuring the knee I've also bruised my tailbone but it is getting better and I think the obturator internus is one of the key components in standing on one leg and in some types of seated forward bends.
One of the things that I notice now is that I tend to sit with my left sitting bone slightly lifted. The left knee was the knee I injured. This is accompanied by a slight tension in my pelvis around the region of my left sitting bone.
Conscious of it I am getting better at relaxing that tension and sitting evenly on both sitting bones.
As mentioned, the obturator internus may be useful in spreading the sitting bones.
This adds tension to the sacrotuberous ligament so that fibers of the glute max that attach to that ligament can also be used. And it creates a continuous line of tension from hamstrings to sacrum to the spinal erectors.
It also can cause the front of the pelvis to narrow.
This may create tension in the tensor fascae latae, or give them room to contract.
Also it may affect the gluteus minimus in the same way. Gluteus minimus is an internal rotator of the thigh.
Tensor fascae latae (TFL) attaches to the it band running down the side of the thigh and may also act in internal rotation. Remember the obturator internus is an external rotator.
With TFL active, and glute minimus, the external rotation can be nullified or balanced.
One of my favorite exercises for stabilizing the hip is the one legged standing forward bend.
The key difference between this and a pose like warrior 3 or the one legged dead lift is that the knee of the free leg is bent and pulled forwards and kept pulled forwards.
The weight of the body is then more infront of the supporting leg meaning that the hamstrings and butt have to be strong to stand back up.
This means that the back line of the leg has to be integrated. And based on my experience so far that means activating the obturator internus to add tension to the sacro-tuberous ligament.
Spreading the sitting bones is easy when both feet are on the floor so that the legs are stabile.
How do you spread the sitting bones while standing on one leg?
It's a little like flexing the biceps. (You keep the arm bones still while trying to close the front of the elbow joint.)
To activate the obturator internus while standing on one leg, you keep the pelvis stationary relative to the standing leg while "spreading" the sitting bone of that leg.
That means keeping the pelvis stable relative to the leg while spreading the sitting bone.
While tilting the pelvis forwards, limit the amount that the pelvis turns left or right so that the obturator internus can continue to stay active.
If I can't spread the sitting bone, then I'll adjust the position of my pelvis, often turning it to the lifted leg side so that I increase the space between the back of the thigh and the back of the pelvis.
This is a very slight movement.
From there I try to keep obturator internus active as I bend forwards and as I stand up.
This same feeling can then be applied in standing forward bends where both feet are on the floor.
On my less strong side I find that I get a pulling sensation (uncomfortable) which means that my hip isn't working properly. And so I am now working on maintaining the ideal alignment of my pelvis while bending forwards so that my obturator internus can activate.
Ideal alignment is something that I focus on feeling so that I don't have to think about it.
If there is pain then I haven't got ideal alignment. If bending forwards and standing up is pain free, or sharp tension free, then I have got ideal alignment, or perhaps not ideal, but good enough that my body can function effectively.
Ideal, or "functional" alignment means positioning the pelvis relative to the thigh bone so that hip muscles like the obturator internus have room to activate.
If the hip muscles as a whole have room to activate then they can work effectively together to maintain the position of the pelvis or change it with a minimum of effort or sharp pain.