Psoas Major Anatomy
Psoas side view with hip straight.
Psoas side view with hip flexed.
The psoas major attaches to the sides of the lumbar vertebrae and the intervening vertebral discs as well as to the transverse processes of the lumbar vertebrae. (The psoas minor is a smaller muscle that attaches between the lumbar spine and the front of the pelvis.)
Viewed from the side with the body upright, the psoas angles forwards and downwards from the lumbar spine to the front lip of the pelvis. From there it wraps around the edge of the pelvis just in front of the hip socket, and then passes rearwards and downwards to attach to the inside edge of the femur.
It attaches to the rear of the inside of the femur just below the neck to a bump of bone called the lesser trochanter. (The greater trochanter is on the outer surface of the thigh bone and is the large bump of the femur that you can feel about a hands width below the hip crest.)
The psoas as a toggle joint actuator
Bending forwards at the hips , the psoas "unwraps" and straightens. Its fibers then connect the lumbar vertebrae to the greater trochanter in more-or-less of a straight line.
With the hips straight, as when standing upright, any contraction of the psoas will create a force that pushes rearwards on the pelvis. At the same time it creates a forwards pull on the lumbar spine and the femurs. This could be thought of as a toggle-release like action.
With the legs in line with the spine, if the psoas didn't bend around the front of the pelvis, then it wouldn't be able to bend the hips forwards. Engaging the psoas with the hips straight, forward bending can be initiated by a combination of the psoas pushing back on the pelvis and pulling forwards on the lumbars and the femurs.
Shortening the psoas by bending the hips
When the angle between the femurs and the lumbar spine is small enough, the psoas no longer bends around the pelvis.
This could allow the psoas to shorten considerably.
In a seated position where the hips are at 90 degrees the psoas is shortened considerably. Whether it is relaxed or active may depend on the way one sits. A slouch with the back against the back rest of a chair may actually stretch the psoas compared to sitting with the spine upright. Sitting near the edge of a chair with the knees close to the floor (almost as if kneeling) can also lengthen the psoas since it opens the front angle of the hip. But probably what is most detrimental to the operating length of the psoas is sitting still for long periods of time without any change in posture.
Allowing the psoas to lengthen or shorten
Standing upright, if the knees are straight and the legs vertical, the distance the psoas spans is greater that if the knees are slightly bent. Bending the knees slightly (while keeping the tilt of the pelvis constant) causes a forward bend at the hips. This reduces the distance that the psoas spans.
Assuming a constant thigh angle, tilting the pelvis forwards so that the lumbar lordosis is increased allows the psoas to shorten slightly while tilting the pelvis rearwards so that the lumbar lordosis is decreased or flattened actually lengthens the psoas.
To keep the psoas the same length, it is possible to bend the knees slightly and at the same time decrease the lumbar lordosis and then straighten the knees so that the femurs become more vertical and increase the lumbar lordosis. In this case the psoas as a whole remains the same length but because of the change in lumbar lordosis, some fibers may actually lengthen while others contract.
And so one very simple way to stretch the psoas is to stand with knees straight, and tilt the pelvis rearwards so that the lumbar lordosis is reduced or flattened.
Psoas posterior and anterior attachments
The psoas attaches, as mentioned to the transverse processes as well as the bodies and intervening discs from the top of the lumbar spine to the bottom. It does not however attach to the sacrolumbar disc, the disc positioned between the sacrum and the lowest lumbar vertebrae.
The fibers that attach to the transverse processes are slightly longer and are called the posterior attachments while the fibers that attach to the lumbar vertebral bodies and discs are called the anterior attachments. Interestingly, the muscle fiber length for the anterior attachments are longer on average than those for the posterior attachments (3 to 8 cm verses 3 to 5 cm)
Psoas Major: a case report and review of its anatomy, biomechanics, and clinical implications
Sandy Sajko, BPHE, DC, MSc1 and Kent Stuber, BSc, DC, MSc2
On writing the previous paragraph it occurs to me that if you keep the knees straight you can stretch the psoas by tilting the pelvis rearwards. This can be done by activating the gluteus maximus and possible the hamstrings and using them to pull downwards on the rear of the pelvis. Or it can be done by using the abs (abdominus rectus and external obliques) to pull up (and in the case of external oblique) rearwards on the front of the pelvis.
Another way to do this is to shape and position the spine, ribcage and skull so that the weight of all of these elements push downwards on the rear of the pelvis and sacrum causing the pelvis to roll backwards.
If this is difficult with knees straight it could be done while keeping the knees slightly bent to begin with and then gradually straightening the knees.
When doing the relaxed version of this exercise it may be helpful to pull inwards on the belly using the transverse abdominus.
While the psoas can affect lumbar stability I would suggest here that it doesn't shape the lumbar spine (increase lordosis or decrease it) in isolation. If it did it would probably require individual control of each set of fascicles so that we would have an L1 psoas, L2 psoas etc and possibly even subdivisions i.e. L1 anterior and L1 posterior.
The paraspinalis muscles at the rear of the spine are much more varied in architecture with muscles that span single joints, two joints and more. And so increasing lordosis could be achieved by these muscles shortening while decrease lordosis could result from these muscles lengthening under control.
Because the joint between the sacrum and L5 does not have any psoas attachments, and it susceptible to injury, it would seem that any action of the psoas would have to be coordinated with SI joint and hip joint muscle action to reduce wear and tear (undue stress) on the lumbosacral intervertebral disc.
And so to this end it could help to conceptualize the sacrum and lumbar spine as a unit with the two halves of the pelvis (the innonimate or hip bones) moving relative to the sacrum at the SI joints.
To this end, when I teach spinal back bending and forward bending while standing, I'll often start with movements of the pelvis with a focus on moving the sacrum. The idea then can be to expand awareness upwards to also feel the lumbar spine as it bends forwards and backwards.
I teach this action usually while standing on both feet first, and then with weight on one leg but the other foot still on the floor and then with the other foot slightly lifted.
Again, during the flexion stage (decreased lordosis) it may be helpful to pull in the belly using the transverse abdominus.
Active hip flexion to activate and strengthen the psoas
One way to activate and possible strengthen the psoas is with active hip flexion. With weight on one leg, knee on the standing leg slightly bent, start of by reducing lordosis (drop the tailbone). Next, as lordoses is reduced gradually lift the knee of the unweighted leg. If you lift heel first keeping toes on the floor and then gradually lift the toes, then the decrease lordosis phase could end just when the toes are about to leave the floor. The decreased lordosis position can be maintained as the knee is then lifted higher keeping the lifted knee bent. I'd suggest doing this without activating the glute of the standing leg.
If you've decreased lordosis enough the ribcage will be far enough back that the weight of the upper body pressing through the sacrum is enough to maintain this position.
Lower and repeat and experiment with doing it with lower belly pulled in versus not pulled in.
This exercise can also be done with knee straight and a good test to see whether psoas is active or not is if the rectus femoris cramps up, chances are the psoas is not active.
Using hip extension to stretch the psoas
A complimentary movement, and one that potentially stretches the psoas is hip extension. For this exercise stand with one foot back and weight on the forward foot. lift the back leg and as you do so feel the gluteus maximus and hamstrings of that leg activate. Lift your chest and bend your thoracic spine backwards. Then lift the back ribs. The reasons for doing so are to activate the spinal erectors and to make it easier to activate the spinal erectors.
The psoas for the lifted leg may not relax in this position but it will be lengthened. Try to keep your torso as upright as possible. In this pose the hips bends backwards as well as the lumbar spine and thoracic spine.
Using a variation of pigeon pose to stretch the psoas
A variation of this stretch is intense pigeon hip stretch. In the pigeon position with the front leg hip on the floor use the arms to lift the chest. Lift the chest further and bend the thoracic spine backwards. Lift the back ribs. Press the front knee down and use the knee and the arms to push the ribcage towards the back-leg-side. Point the back knee down. Activate the buttock and hamstring of the back leg (knee can lift.)
Here again the focus is on bending the hip and spine backwards.
The more you push your upper body to the back-leg side the more downward pressure on that side of the pelvis and the deeper the backbend for the hip.
Movements of the lumbar spine that can affect the psoas
Note that some lumbar spine positions or actions that are affected by or that affect the psoas include side bending, twisting and "sliding" of the ribcage sideways relative to the pelvis. A slight sideways displacement of the ribcage (or a slightly twist) may indicated that the psoas on one side is either not activating (inhibited) or over active (facilitated.)
How thigh rotation can affect the psoas
Rotation of the thigh relative to the pelvis can also affect or be affected by the psoas.
In an upright position the psoas pulls forwards (or can create a forward pull) on the femur relative to the pelvis. The iliacus and pectineus can also create a similiar forward pull and so any one of these muscles may substitute for the other with various affects.
The psoas in flexed hip positions
In a flexed hip position (bent forwards at the hip) past the point where the psoas tendon no longer folds across the front of the pelvis, say while sitting, then the psoas can create a rearward (and upward) pull on the femur. The obturator externus can create a forward pull on the femur. Gluteus minimus and adductor brevis may also create a forward pull on the femur in this position.
How joint capsule tension (or its lack) can affect the psoas
Assuming the hip joint capsule to be a tensegrity structure, in any position of the hip it would be desirable to maintain tension on the hip capsule so that it in turn pressurizes the fluid within the hip capsule to keep the hip joint centered. That can mean that the hip bone doesn't actually contact the hip socket but instead is kept in a particular relationship with it.
The interplay of fluid pressure and capsule tension is ideally one such that tension throughout the fabric of the joint capsule is distributed in such a way to keep the capsule healthy and functional. To this end it can help to understand that if some muscle of the hip, or lumbar spine, or knee isn't functioning ideally some other muscle may be turned on in specific actions to take its place and the ripple effect can be of a psoas that becomes seemingly tight because of over use.
And so when dealing with a tight psoas it can be helpful to be aware of the hip, the lumbar spine, the knees and even the feet.Where is there excessive tightness, "strange" movements or a lack of feel and/or control.
Exploring the psoas with the standing twist
One final posture for exploring the psoas, gluteus maximus and even the latissimus dorsai is the standing twist.
With feet about hip width activate the left gluteus maximus. Do it in such a way that your left hip is pushed forwards so that your pelvis turns to the right. Twist your ribcage to the right and reach the right hand back. Keep the squeeze on the left butt cheek and in addition create a downward pull on your right arm to activate the latissimus dorsai. Imagine squeezing the lat and the glute towards each other to deepen the twist. In this instance the left psoas may be stretched by the glute since the hip is extended but it can also be active in twisting the lumbar spine. the psoas could be active on the right side if you focus on sucking in and up on the front of the right hip. It in turn is stretched by the right side of the lumbar spine pulling spiralling rearwards into the twist.