The gluteus maximus is a large muscle and a complex one.
It may have fibers that function solely to help close or stabilize the SI joint, attaching from the sacrum to the ilium.
It has fibers, the "superficial fibers of the gluteus maximus", that attach from the top back corner of the hip bone to the IT band and from there to the tibia (with a few strands to the top of the fibula). In this guise it is part of the long thigh muscle (or long hip muscle) group and there it can aid in controlling knee rotation and/or hip bone positioning.
Then there are the fibers that can act to extend the hip and/or externally rotate the hip. And in addition there are fibers that can be used to abduct the hip.
In this article the focus is on how the gluteus maximus in combination with the iliacus, obturator internus, obturator externus and other hip muscles, can help to stabilize the hip joint and SI joint, while standing upright.
As mentioned in the introduction, the purpose of this article is to explore interactions of the gluteus maximus and other hip region muscles while standing. And so for the most part the following discussion assumes an upright posture, with the body either still and on one or both legs or locomoting forwards.
When and if needed, reference positions will be (re)specified as we go along.
Since the bulk of this article assumes a standing position and since the Gluteus maximus both affects and can be affected by the SI joints, the first thing we should talk about are three basic positions of the SI joint while standing.
If the pelvis is tilted forwards, and the ribcage slides forwards relative to the pelvis to accomodate this movement, this shifts the gravitational center of the upper body forwards of the sacrum.
(The hips may push back slightly so that the center of gravity of the whole body stays somewhere over the feet. For now, lets say that the weight of the whole body is kept even between forefeet and heels so that forefeet and heels press down with even pressure.)
If the pelvis is tilted rearwards, and the ribcage slides rearwards to accomodate, then the lower back can flatten. If in addition we also slide the head back (or keep it over the ribcage as it slides back) this can shift the center of gravity of the upper body rearwards.
This can be a bit difficult with knees straight and so an alternative is to do it with knees slightly bent.
If the pelvis is neutral, somewhere between tilted back and forwards, then we can position the ribcage (and the center of gravity of the upper body) in such a way that it supports this neutral position. Another way to put it is that in this position the ribcage, and mass of the upper body, is balanced over the sacrum so that it neither tilts forwards or backwards.
Nutation is where the sacrum tilts forwards relative to the hip bones. Rather than just being a movement of the whole pelvis tilting forwards, it is actually a movement of the pelvis as a whole plus movement of the sacrum relative to each sitting bone at the SI joints. It causes the bottom tip of the sacrum to move rearwards, away from the pubic bone while at the same time causing the ischial tuberosities, or sitting bones, to move outwards. The ASICs correspondingly move inwards as if the hip bones are hinging at the pubic bone and SI joints.
Tilting the pelvis forwards, with a shift of the center of gravity forwards of the sacrum tends to cause the sacrum to nutate.
Counter nutation is the opposite movement. The sacrum tilts back relative to the hip bones. The pelvis as a whole tends to tilt back, but the effect is that the bottom tip of the sacrum moves towards the pubic bone, the sitting bones move inwards, and the ASICs move outwards, away from each other.
Tilting the pelvis back, while at the same time shifting the center of gravity back can tend to counter-nutate the sacrum.
Relative to the SI joints, a neutrally positioned sacrum is somewhere in-between these two positions.
For SI joint integrity, i.e. so that the SI joint isn't damaged, the assumption is that nutation and counter nutation are both supported by muscles working against each other and/or other opposing forces (body weight, externally applied forces) to stabilize the SI joints in any of these positions. With muscles like the gluteus maximus that affect the SI joint, the hip joint and the knee joint, the assumption is that SI joint stability and hip joint stability are closely tied together.
In general it is possible to stand upright with minimal perceivable muscle activation. If someone presses against us we can brace the body to maintain our position. Thus, we can use muscles to stabilize joints and maintain a particular position. By the same token, we could activate one set of muscles, and to maintain position, i.e. not change the shape of our body, we can activate opposing muscles.
With the above three positions, it's possible to do them with minimal muscle activation. However for joint integrity in the face of greater loads it's also possible to reinforce these positions with activation of opposing muscles. Muscle activation in this case not only creates stability, it also creates feelability.
One idea that is important with muscles in general is understanding that while muscles can cause particular movements, they can also be used to resist movements. And when exerting against opposing muscles they can be used to stabilize joints around the axis of opposition.
What that means is that external rotators can work with internal rotators to stabilize against rotation. Flexors can work with extensors to stabilize against flexing and bending. adductors can work against abductors to stabilize against side-bending of the hip.
Working with hip flexors, the gluteus maximus in its hip extension capacity can help control or stabilize flexion/extension movements of the hip.
Working with hip internal rotators, the gluteus maximus can help control or stabilize rotation of the hip.
Working with hip adductors, the gluteus maximus can help control lateral tilting of the pelvis.
Stabilizing the SI joint is a little more complicated, but it helps to understand the above points, that muscles can work against each other to help create joint stability in particular directions. But first it may help to understand basic hip stability while standing and with the pelvis and SI joints neutral.
When standing upright it's fairly easy to relax the hips, or make them feel fairly relaxed. They may not be totally relaxed though. Smaller muscles like the pectineus, gemellus superior and inferior, and even the quadratus femoris may be active. Pectineus is hip flexor, but also an internal rotator. The gemelli and the quadratus femoris and external rotators. The gemelli may be hip flexors while the quadratus femoris is a hip extensor.
Controlling pitch of the pelvis or hip flexion/extension, the hip flexors that the gluteus maximus can work in concert with can include the iliacus, the obturator internus and also the obturator externus.
Working with these muscles, the gluteus maximus can help to stabilize the SI joints while the sacrum is nutated.
The iliacus and obturator internus both attach to and cover a large portion of the inner surface of the hip bone.
Iliacus and obturator internus, can also nutate the sacrum at the SI Joints, nodding it forwards relative to the hip bones and thus causing the ASICS of each hip bone to move inwards and the ischial tuberosities (the sitting bones) to move outwards.
This movement of the hip bones relative to the sacrum occurs at the SI joints and causes the pelvis to slightly change shape.
The sacrotuberous ligament runs from the sacrum to the ischial tuberosity or sitting bone.
The gluteus maximus has fibers that attach from the sacrotuberous ligament to the femur.
Because nutation of the SI joints draws the bottom of the sacrum rearwards and the ischial tuberosities outwards, this action adds tension to the sacrotuberous ligament.
With the sacrotuberous ligament tensioned by the activation of iliacus and obturator internus, the fibers of the gluteus maximus that attach to the sacrotuberous ligament have a stable foundation from which to act. They can thus create a hip extending force that opposes the hip flexing force of the iliacus and obturator internus.
Working together, these muscles can stabilize or control flexion/extension of the hip.
When the gluteus maximus and iliacus activate in concert they help to create a downwards pull on the hip bone relative to the femur. This can cause the top of the hip socket to press against the top of the ball of the hip joint.
Obturator internus and obturator externus are muscles that can counter this downward force due to their postioning and angle of attachment.
Obturator externus is positioned along the outside of the bottom of the pelvis, below the level of the hip socket. From it's attachment to the hip bone, it reaches back, under the neck of the femur to attach to the back of the femur along the inner surface of the greater trochanter.
The obturator externus is a hip flexor. In addition, it can also exert a rearwards pull on the hip bone relative to the femur.
The obturators attach to either side of a layer of connective tissue that covers the foramen the hole at the bottom of each hip bone. As such, activation of one, say the obturator internus, adds tension to this connective tissue layer. The fibers of the opposing muscle that attach to this connective tissue layer then have a foundation from which to active. Note that as well as providing anchoring, this also provides a simple means of communicating. The anchoring of the connective tissue layer by one muscle signals the other muscle to activate.
As a rough analogy, it's equivalent to charging an ipod or iphone or other electronic device and using the same wire to upload or download information, whether songs, photos, documents, new software.
Because the iliacus and obturator externus are internal rotators and the obturator internus and gemelli are external hip rotators, their co-contraction can help to provide the force components necessary for stabilizing the hip joints against rotation. Since these are all also hip flexors, the main contribution of the gluteus maximus in this instance is the extension force.
Co-activation of the iliacus, obturators and gluteus maximus not only has the effect of stabilizing the hip for flexion/extension and rotation. It also stabilizes the SI joint.
If the aforementioned muscles are activated for both hips then both SI joints are stabilized. If they are activated for one leg then they stabilize a single SI joint, the SI joint of the leg in question.
If iliacus and obturator internus (and then externus) are activated first and gluteus maximus is then activated, this may tend to cause nutation of the SI joints. Thus, co-activation stabilizes the SI joints while they are nutated.
While standing upright with the ribcage upright, nutation tends to accentuate the natural curve of the lumbar spine while counter-nutation flattens it.
Also note that nutation tends to accompany internal rotation of the hips while counter-nutation does the opposite.
Gluteus minimus attaches to the outside of the hip bone. It runs from the ASIC down and back to attach to the top of the femur at the greater trochanter.
If iliacus is activated, it tends to create an inwards pull on the ASIC and this inwards pull can anchor the hip-bone attachment of the gluteus minimus. As a result it can activate to internally rotate the femur and flex the hip.
This may be one reason why nutation tends to accompany internal rotation of the hips.
While standing, our glutes tend not to be always active, particularly with weight on both legs. What might a minimum hip muscle activation consist of in this case?
Pectineus runs from near the pubic bone to the lesser trochanter. Its path and action is similiar to iliacus, but not quite as well rooted.
Pectineus can exert a rearwards pull on the hip one. It can also generate an internal rotation force as well as a hip flexion force.
The gemelli attach from above and below the lower sciatic notch and have paths that duplicate that of the obturator internus, but for a shorter distance.
The gemelli can create a forward pull on the hip bone. They can also externally rotate the hip. And like the pectineus they may help to flex the hip.
Because of the weight of the upper body pressing down on the back of the pelvis, body weight could be sufficient to resist the hip flexing action of these muscles. However, if it isn't, then the quadratus femoris could be called into play.
This muscle attaches from the outside of the ischial tuberosity to the back of the femur. It can externally rotate the hip and also extend it. And it can create a rearward pull on the hip bone.
These muscles could work together as a minimal force for controlling and or stabilizing the hip joint while the body is standing upright.
If the sacrum is nutated while the body is upright, it tends to cause the pelvis to tilt forwards, accentuating the curvature of the lumbar spine.
Standing with knees straight, or reasonably so, this tilting forwards lifts the sitting bones (while causing the ASICs to move downwards.)
One possible side effect is that this lengthens the adductor magnus long head muscle. This is an internal rotator that attaches from the sitting bone, running down the thigh to attach to the bottom of the inside of the femur, just above the knee joint.
This muscle is a hip extensor but also an internal hip rotator. If it does activate, the external rotation fibers of the gluteus maximus could activate to oppose it.
If adductor magnus long head activates, it can help to anchor the bottom of the femur in such a way that the knee rotators have a stable foundation from which to stabilize or control knee rotation.
Knee rotators includes the vastus medalis obliquus (VMO), popliteus, biceps femoris short head, as well as vastus medialis proper and vastus lateralis.
For a simple set of exercises for how to activate the adductor magnus and subsequently the vastus medialis, check out the Learning to activate adductor magnus long head and VMO course.
It's possible to stand easily without nutating (or counter nutating) the sacrum. While standing upright, our feet and ankles tend to act as anchors.
There are seven or so muscles that attach from the inside and outside of the lower leg bones to the corner points of the hip bones. If the lower legs are stabilized against rotation, these muscles have an anchor so that they can effectively act to help control the hip bones. If a hip bone is stabilized from the ground up, then the bone can be used as a foundation for stabilizing The SI joint.
Whether nutating the sacrum via the iliacus and obturator internus or controlling the hip bone using the long thigh muscles, both types of action are easy to learn to feel and control. For a taste of both, check out the Improve Hip Control, Feel and Control Your Pelvis course.
With the hip bones stabilized with respect to the femurs, each hip bones can act as a foundation for any muscles that attach to it. Likewise the femur.
Because the femur and hip bone are stabilized they provide a larger effective mass than they would if not stabilized. Thus the hip bone can act as an anchor for the long thigh muscles, the muscles that can help control shin rotation relative to the femur and hip bone. And the femur can act as an anchor for muscles that act to directly control knee rotation.
Because the SI joints are also stabilized by this action, the hip bones and sacrum can then act as an anchor for the spinal erectors and paraspinal muscles.