The psoas muscles are your deepest core muscles. They run on either side of your lumbar spinal column, pass through your pelvis across the pubis and then attach to the inner thigh bones.
The important physical functions of the psoas muscles are hip flexion (lifting your thigh bone up), hip abduction (takes the thigh bone away from the midline of the body), lumbar flexion (draws the lumbar spine forward) and external rotation (rotates the thigh bone outward).
Equally important is that the psoas is one of the first muscle responders for the flight, flight or freeze response in the body. When one feels threatened and adrenaline rushes into the bloodstream, the psoas will jump into action and will either help you run faster (hip flexion), tighten up around your organs (preparing for a fight, like armor), or immobilize you (stuck on the spot- freeze!). Since the psoas also attaches to the diaphragm their actions will affect your breathing. When the psoas is activated it will help you breathe deeper if you are running or fighting (pulls the diaphragm down with more force), or slow your breath way down if you are going into freeze mode, like a opossum playing dead (diaphragm movements frozen). The psoas is also known to hold emotional energy. People have been caught off guard when they are in a psoas stretch and burst into tears. It is a fairly regular occurance in a yoga class.
A tight psoas, especially if it is just on one side, can cause lumbar back pain, sacroiliac joint instability, nausea, weakness in the core, instability, hip pain, knee pain and shoulder misalignment. Tight psoas muscles can also cause an excess of guarding of ones emotions as well as poor sleep, poor digestion and low sexual drive.
Many people will think they have back problems when their pain is actually coming from the psoas. It is not always obvious if the psoas is weak, tight, or in a hyper-contracted state.
Stretching is not always the best thing for the psoas, especially if it needs to be released.
Here is a good way to both strengthen and release the psoas-
Sit on the ground with your feet about 3-4 feet apart.
Lean back slightly and support yourself with your hands behind you.
Turn your toes out at a 45 degree angle.
Keep your knees soft. Then lift the right right leg up about 3 inches from the ground on an inhale. Exhale lower it back down. Imagine you are lifting the leg from the inner ankle, or inner knee. Try not to compensate by using your abdominals, shoulders or the opposite leg. It is also important not to lift the leg too high. Repeat on the other side. If one side is more difficult to lift, that is your weaker side. If they feel the same, and the movement is not too hard to do, walk your hands closer to your body, arch your lower back forward and try to lift the leg again. See if one is easier than the other. If the movement is too hard and you can not lift your leg off the ground, lay down on your back. Have your legs the same distance apart as above and try to lift each leg. Up on the inhale, down on the exhale. Lift up only about 3 inches.
This movement is both a test to see if there is asymmetry in your psoas strength and also a way to correct it. If one leg is hard to lift, start with 3-5 times lifting. Even if your leg does not leave the ground, you are still toning the muscle. You might want to work the weak side twice as much as the stronger side.
You can stretch the psoas like this:
Get a stable chair (kitchen or dining room chair is best) and place it against a wall. Stand alongside the wall and use it for support as you put your right foot on the chair. Sink your hips down. You can lift your back heel off the ground and have a little bend in the left knee. Make sure that when you sink your hips down, your right knee does not extend past your right toes. If it does, step your left leg back further or your right foot forward more, or both. You want to give a lot of space between the left leg on the ground and the right foot on the chair. When you sink your hips look for a stretch in the front of your left hip. If your left toes are facing the chair that your right foot is on, you will be stretching your superficial hip flexors. Now, if you turn your left toes out to the side at a 45 degree angle and you sink your hips you should feel the stretch on the left side closer to your groin. That will be your psoas, and that sensation you feel will be your psoas stretching. It is best to pulse the movement here- inhale pull your hips up, exhale sink your hips down. 5-10 pulses on each side is good.
It is important to know that some people will not feel a strong stretch in the psoas, especially if it is weak and hyper contracted. You may also feel it very differently on one side than the other.