The usual way of explaining hip joint anatomy is to focus on muscles according to region and/or function. I thought it would be helpful in this article to offer a slightly different point of view of the muscles of the hip.
With hip joint anatomy it may be helpful to compare the hip to a bicycle wheel with the understanding that the muscles of the hip work on the hip joint from different angles to both centralize (or centrate) the hip and stabilize it. The spokes of a bicycle wheel center the hub relative to the rim and help to keep this relationship stable, even during the stresses of being ridden.
With balanced tension among the hip muscles the ball of the thigh is centered in the socket of the hip bone and the hip joint is kept stable. During changes in orientation of the pelvis relative to the leg or the leg relative to the pelvis, or changes to whether the leg is acting as a foundation or not, the muscles of the hip maintain this balanced tension.
Because the hip joint supports the weight of the upper body when standing, the muscles of the hip also work against the weight of the body. In particular when standing upright with the ribcage stacked over the pelvis, the tendency may be for the weight of the body to press down on the rear of the pelvis causing it to want to tilt backwards.
And so for hip joint stability while standing the muscles of the hip joint can exert tension in such a way to counter this tendency of the pelvis to tilt back.
Another important point to note is that hip muscle tension may directly affect tension of the joint capsule envelope. With balanced tension throughout the tissue of the joint capsule, the synovial fluid withing the joint capsule may be pressurized enough to create an outward push on the thigh bone. The effect could be like squeezing a balloon in the middle. When you do so the ends of the balloon expand outwards, away from each other.
Viewing the hip joint as being like a bicycle wheel we can study spokes (muscles) individually and understand their function as tensioning devices, but we get a better idea of how the spoke works when we view them all in situe, as part of a bicycle wheel. No one single spoke is more important.
Instead, for correct functioning of the hip, they all are.
When looking at the muscles of the hip joint, rather than thinking in terms of adduction and abduction, flexion and extension, we are going to look at the hip muscles as a complete set of spokes on a wheel so that we can get a sense of them balancing each other. Rather than one muscle being more or less important, they are all important for a "naturally" functioning hip joint.
(That being said it is handy to know which muscles do what.)
In a bicycle wheel, spokes have enough tension when the wheel is built that they hold the hub at the center of the rim and not only that, they have enough tension to maintain that relationship when the wheel is part of a bike and the bike is being ridden.
As the bike is ridden and the wheels roll, the spokes at the top of the wheel have more tension and the spokes at the bottom less.
However, the spokes at the bottom don't completely relax. They experience a lessening of tension. Then as the wheel rolls, those spokes experience a gradual increase in tension which probably peaks when a spoke is at 12 o'clock (or somewhere in the region since that riders' weight is actually in front of or behind the wheel depending on whether we are looking at the front wheel or back wheel.)
Once passed the peak point, tension lessens and then the cycle continues.
However, at all times, all spokes have tension, it's just that some have more tension and others have less.
The hip joint could be viewed similiarly. Standing upright the muscles in a position to pull the pelvis upwards with respect to the thigh will have more tension. Tilting the pelvis forwards or backwards then the muscles best positions to suspend the pelvis will then have the greater amount of tension. In each case the tension is enough to balance the weight that it is working against.
Instead of spokes the wheel will have fabric attaching the hub to the rim. This fabric will attach, like spokes, to specific points on the wheel and rim. The fabric is like connective tissue.
And to add tension, there will be muscles which act like spokes, embedded within the fabric of the wheel. When the muscles of the hip are completely relaxed, the connective tissue loses tension so that thigh and pelvis can move freely with respect to each other.
By tensing muscles in particular ways the hip can be made stable in various positions or moved into various positions. The muscles can not only be used to create hip stabilty and movement but they can also be used as feedback mechanisms to help know the position the hip is in. Effort exerted by muscles can help us figure out the disposition of the hip joint. In addition tension in the connective tissue can also give us clues as to the disposition of the hip joint.
While it helps to start with an overall big picture of hip joint anatomy, in particular viewing the hip muscles as being similiar in layout to the spokes of a bicycle wheel, now we can look at the muscles of the hip in groups to see how muscles might work against each other to create hip joint stability. For this look at hip joint anatomy we'll focus in the main on the single joint muscles of the hip, just for simplicity.
For reference here are some of the main landmarks of the pelvis.
The two most obvious shapes at the back of the pelvis are the upper and lower sciatic notches which are separated by the ischial spine and are located above the ischial tuberosities.
Two hip muscles that could be paired are the iliacus and the obturator internus. One reason I would suggest that these muscles are "functional opposites" or "compliments" is that they both cover a large portion of the inner surface of each hip bone.
These muscles both wrap around the pelvis before attaching to the thigh bone and both attach to the inner surface of the thigh bone close to the hip socket.
The obturator internus covers a large portion of the lower rear inner surface of the hip bone. It wraps around the back of the pelvis at the lower sciatic notch and from there runs slightly upwards to attach to the inner surface of the greater trochanter.
In contrast the iliacus covers a large portion of the lower forward inner surface of the hip bone. It wraps around the front of the pelvis at the span of bone between the pubic bone and hip socket and from there passes down and back to attach to the lesser trochanter, on the inner surface of the shaft of the thigh bone which is just below the neck of the thigh bone.
From the pictures below you can see how these two muscles could be thought of as directly opposing "spokes".
Note that the obturator internus, when active, may cause the sitting bones to flare or protract slightly. Meanwhile the iliacus may cause the upper front of the pelvis to widen or flare. This assumes that the thigh bones are stable.
Muscles with similiar lines of action are the pectineus and the gemellus superior and inferior.
The pectineus runs from near the pubis, to the lesser trochanter and so has a similiar line of pull to the iliacus.
The gemellus superior and gemellus inferior connect above and below the lesser sciatic notch and attach to the thigh bone at the same point as the obturator internus.
Another set of muscles that share similiar points of attachment to the thigh bone include the piriformis and the psoas.
The psoas attaches to the vertebrae of the lumbar spine and like the iliacus wraps around the front of the pelvis to attach to the lesser trochanter.
The piriformis attaches to the front of the sacrum and passes through the upper sciatic notch to attach to the thigh near the same place as the obturator internus and the gemelli muscles.
Because the hip muscles need to be able to support the hip joint through a range of possible movements I'd suggest that the obturator externus may share a similiar job to the psoas/iliacus/pectineus set while the quadratus femoris may share or substitute for the obturator internus/gemelli/piriformis group.
The obturator externus attaches to the side of the pelvis below the hip socket and wraps under the neck of the thigh bone to attach to the inner surface of the greater trochanter.
The quadratus femoris attaches towards the bottom rear of the pelvis, above the sitting bone and from there attaches to the rear of the thigh bone. This may be particularly true in a "bent forwards" (or flexed) position.
The Glute Maximus should be mentioned here because it opposes the psoas in a couple of ways. The gluteus is massive enough that in a standing position it can be contracted in such a way to push the pelvis forwards. The psoas, because of the way it folds around the front of the pelvis can be used to push the hips back. Both actions can have a momentus (filled with momentum) feel to them.
The gluteus maximus attaches to the back of the sacrum, the sacrotuberous ligament, and portions of the rear of the pelvis. It attaches to the rear of the thigh. (It also has fibers that act on the iliotibial band but we'll discount those for now.)
One thing I didn't mention with respect to a bicycle wheel is how it would change shape. In particular how could the relationship between the hub and the rim be changed or varied? Quite simply by varying spoke tension. On a real bicycle wheel spokes can be tightened or loosened. I've had experience of this trying to fix a bent rim. By changing spoke tension it's possible to use the tension of the spokes to straightent the rim. A possible consequence is that the rim might be less round or the hub shifted slightly relative to the rim. It could be possible to vary spoke tension to shift the rim relative to the hub, rotate one relative to the other or even tilt one relative to the other.
The interesting thing about tightening spoke (or loosening them) is that when you turn the key you get an idea of how much tension the spoke already has. This is the same when tuning a guitar. The amount of effort required to turn the tuning pegs tells you how much tension the string already has.
Replacing spokes with muscle tissue, muscles could then be used to maintain the relationship, make the relationship super stable, or change the relationship. Muscle tension and some indicator or muscle length could then be used to help us figure out what the current relationship is between hub and rim.
The following group of hip muscle could be thought of as creating space in the hip joint. They don't really create space in the hip joint, though it can be a useful way to think of them. Instead they simply help to keep the head of the femur centered in the hip socket. When we are standing upright, they resist the weight of the body and prevent all that weight from resting on top of the ball of the hip joint.
Gemelli means twins and the gemellus inferior and superiorare like twins both above and below the obturator internus muscle.
These muscles originate just above the sitting bone and reach forward and up (and outwards) to attach to the top of the thigh bone.
The gap between where they originate both is called the "lesser sciatic notch." The obturator internus passes through this notch. Meanwhile the gap in the pelvis above this is called the "greater sciatic notch." (The piriformis passes through this notch.)
Openings in the body are called Foramen. The pelvis has two openings on each side that look like eye holes when the pelvis is viewed from the front.
An obturator is a soft structure that closes off an opening in the body. The obturator internus and externus muscles attach to connective tissue that covers the foramen of the pelvis.
The obturator internus is located within the pelvis, exiting the back of the pelvis. The obturator externus runs along the outside of the lower pelvis.
If you look at a picture of the side view of a pelvis without the thigh bone it can be easy to confuse the hip socket and the obturator foramen. The obturator foramen is located below the hip socket (acetabulum.)
The internal obturator covers the inside of the obturator foramen. It wraps around the back of the pelvis and then passes through the lower sciatic notch between the gemelli. It blends with the gemelli to attach to the top of the thigh bone.
(Note that the internal obturator covers a large portion of the inside of the pelvis.)
The external obturator covers the outside of the obturator foramen. It hooks back and up passing under and behind the neck of the thigh bone to attach to the back of the femur.
All of these muscles (gemellii and obturators both) pass upwards from the pelvis to the thigh bone and can be thought of as lower spokes on the wheel of the hip joint.
From a back view (or front view) of the pelvis, these muscles pass outwards and upwards to the thigh. Because of this they can be used to pull the pelvis upwards relative to the thigh bone.
And that was the point of mentioning the importance of the lower spokes of a bicycle wheel.
While for now I'm refering to the pelvis in an upright position, it might be helpful to understand that just as a wheel can function in any orientation (it can roll) so too can the pelvis.
Muscles that might oppose or counterbalance the tension created by the obturators and gemelli are the gluteal muscles, in particular gluteus minimus and medius.
These muscles attach to the side of the "blade" of the pelvis (ilium).
The attachments of the gluteals, obturators and gemelli have some overlap, just like the spokes of a wheel.
This overlapping helps these muscles to stabilize the hip (in the same way that spokes on a bicycle wheel overlap for stability).
Gluteus Medius has more "bulk" towards the back of the side of the pelvis. It tends to originate higher up on the side of the pelvis than the gluteus minimus muscle. It attaches towards the back of the greater trochanter of the femur.
The obturators both attach to the inner aspect of the thigh bone, but at the back and top. The internus wraps around the pelvis while the externus wraps under the neck of the thigh bone.
Another single joint muscle of the hip that wraps around the pelvis to attach to the thigh is the iliacus.
This muscle attaches lower to the inside of the thigh bone (to the lesser trochanter, the little bump of bone that sticks out towards the back of the thigh.)
Note that the illiacus and obturator internus both cover a large portion of the inner surface of the pelvis. This may give them both "better grip." Also because they fold around the pelvis to attach to the thigh, this fold may create a pulley affect, giving these muscles extra leverage.
Because of their angle of attachment to the femur, the iliacus and obturator internus (and externus) could be thought of as "hip flexors."
They tend to tilt the pelvis forwards relative to the thigh bone.
It may be that because of the way the spine is at the back of the body, when standing our upper body exerts a downward pressure on the back of the pelvis. These muscles can help to resist that downward pressure.
In a similar vein it may be that while standing on one leg the natural tendency is for the non weighted side of the pelvis to move forwards (causing internal rotation on the standing leg side.)
And so most of the single joint muslces of the hip help to oppose this action, using external rotation to resist the natural tendency of the pelvis to "internally rotate."
The pectineus and quadratus femoris work from opposite sides of the pelvis. Pectineus angles back and down front the front of the pelvis attaching to the inner part of the thigh bone.
Quadratus femoris angles forwards and down from just in front of the sitting bones, but also to the inside of the back of the thigh.
These muscles can help to stabilize the "forward/backward" tilt of the pelvis (or fine tune it.)
All of the muscles that have been mentioned so far attach between the pelvis proper and the thigh bone. They act like the spokes of the wheel to control the relationship between pelvis and thigh.
The next set of muscles are multijoint muscles.
The piriformis attaches to the sacrum and cross the SI Joint to attach to the inner surface of the greater trochanter (close to where the gemelli and obturator internus attach.) While the SI Joint doesn't have a lot of movement, it does allow the pelvis to "flex" or change shape. And so changes in shape of the pelvis may affect the piriformis. What may happen is that changes in tension in the connective tissue that connects the sacrum to the pelvis acts as a signalling mechanism, helping to tell the piriformis when to activate and when to relax.
The psoas muscle attaches the lumbar vertebrae. It's fibers reach down towards the front of the pelvis, on either side of the pubic bone. They pass underneath the inguinal ligament (the line that separates your belly from your thighs) and from there reach back to attach to the inner thigh at the lesser trochanter.
This muscle is affected by changes in angle between the pelvis and thigh bone. But it is also affected by sideways bending, front or back bending and even twisting of the lumbar spine. With respect to the psoas I often think of the lumbar spine as a tensioning device for the psoas. To this end, if balancing on one leg and lifting one knee then I allow my tailbone to drop, my pelvis to tilt backwards and my lumbar spine to curve forwards (so that the back of the lumbar spine is more "open" than the front) so that the psoas has "more room" to contract and lift the thigh.
The gluteus maximus is actually a complex muscle. It has attachments to the back of the sacrum, as well as to the back and rear of the pelvis and it also attaches to connective tissue covering the gluteus medius. At the other end it attaches to the back of the thigh bone close to the top. But it also attaches to the Fascia Latae, a band of connective tissue which connects to the top of the outside of the fibula, the smaller of the two lower leg bones.
I've yet to find an anatomy book or reference that clearly delineates which fibers attach where, but it may be that the fibers that attach to the pelvis and to the outside of the gluteus medius are the ones that also attach to the Fascae Latae while the fibers that attach mainly to the sacrum are the ones that also attach to the back of the thigh.
For this reason it could be helpful to think of the gluteus maximus as two separate muscles, one that works on the lower leg bone and one that works directly on the thigh.
A big deal used to be made about how the gluteus maximus is an external rotator of the hip, which it is. But it is also the major hip extensor. And I'd suggest that it is actually pretty easy to learn to activate the glute max in such a way that it causes external rotation and also to activate it in such a way that it extends the hip without externally rotating the thigh.
Obviously... or not so obviously, other hip muscles will come into play to help neutralize the external rotation component, or it may be that there are fibers that are more apt to cause external rotation, and those fibers can be "turned of" when "pure" hip extension is intended.
The adductors include the adductor brevis, longus, and magnus. (I've already mentioned pectineus and obturator externus.)
Adduct means "pull inwards" or towards the centerline.
Where the glutes attach to the blade of the pelvis from front to back, and while the iliacus and obturator internus cover a large portion of the inner surface of the pelvis, the adductor magnus attaches to the most of the length of the ischio pubo ramus.
You can think of the ischio-pubo ramus as the "rockers" of the pelvis.
The fibers of adductor magnus that attach towards the front of the ischio puboramus attach higher up on the thigh. The fibers that attach towards the back of the ischio pubo ramus attach lower down on the thigh.
Adductor Brevis attaches towards the front of the ischio pubo ramus. It also attaches to a short portion of the upper thigh.
Adductor Longus attaches even further forwards but attaches lower on the thigh.
The knee affects the hip and vice versa. I've recently experienced this first hand getting over some knee problems. One way to think of the hamstrings is that with the knees straight the hamstrings have some "pre tension" making it easier for them to be used in extending the hip. If the pelvis is tilted forwards, opening the back of the hip joint, then this can add tension to the hamstrings even if the knees are bent.
I go over the hamstrings (and how they interact with the glutes) in more detail in hamstring anatomy.
The Sartorius, Gracilis and Semitendinosus all attach to the inside of the tibia just below the knee joint. They pass behind the thigh bone and then extend up the thigh to attach to the sitting bone (semi-tendinosus, which is also a "hamstring muscle") the pubic bone (gracilis) and the front peak of the pelvis or ASIC (the sartorius.)
My impression of these muscles is that with the knee straight they help to create a forward push on the bottom of the inner thigh bone while at the same time pulling backwards on the top of the inner tibia. This could be a way of stabilizing the knee. Rather than acting together, these muscles may be positioned so that no matter what position the pelvis is in, it is possible to "stabilize the knee." With the knee bent any of these muscles could be used to rotate the shin internally relative to the thigh.
In a standing position with the lower leg stable these muscles may also be used to help stabilize the pelvis, creating a downward pull on the sitting bone, pubic bone and ASIC.
Note while the focus here is on the hip joint, I'd suggest that if you have painful knees, you may find it helpful while sitting to pull up and back on the ASICs. In my limited experience, this activate the external obliques, which then may give the sartorius a stable foundation from which to contract to help stabilize the knee. I found that when my knee was newly injured, it seemed to go out of joint when standing after a long period of sitting. If I pulled up and back on the ASICs while sitting, I seemed to be able to avoid this problem.
I'd suggest that while controlling the hip muscles in isolation is a helpful exercise, particularly when you are trying to figure out problems with your hips, (and I go over some simple isolation exercises in the hip control guide) another approach is simply to "balance tension" in and around the hip joint.
For hip control with respect to improving hip joint flexiblity in forward bends and backward bends, I've included step by step exercises in Active Stretching.