The psoas muscle is a low back muscle located deep in the body, very close to the spine and inside the hip and thigh bones. This depth, combined with the fact that the psoas originates from the sides of the five lumbar vertebrae, means it plays an important role in back health. The psoas is a hip flexor muscle, as is the quadriceps muscle.
Related: Stretch Your Quadriceps
You use your psoas when you walk, run, climb stairs, sit, play sports or do most anything that requires flexion at the hip.
The psoas starts on the side the lumbar vertebrae and, making a triangular shape, tapers down into a tendon that inserts on a little bump on the inside of the top of the thigh bone called the lesser trochanter. On its way to the lesser trochanter, the psoas meets up with another muscle called the iliacus. The iliacus is a big, flat muscle that resides on the inside of the front of your hip bone. Together the illiacus and the psoas muscles (called the iliopsoas muscle) works to flex the hip joint.
Flexing the hip joint can happen one of two ways. You can lift your leg, or you can bend your trunk forward. We’ve already talked about lifting the leg. This is a common movement that occurs in pretty much every activity you do. From sitting to running a marathon to making the kids' beds, hip flexion is a very basic body action.
Bending over -- the second type of hip flexion -- involves keeping your legs stationary and folding your trunk down. A word of caution: During the action of bending, many people make the mistake of rounding their back. These people often erroneously believe that they are flexing at the hip joint. They are not – they’re rounding their backs. This back-rounding habit may be born out of lack of awareness of body mechanics. To use the psoas muscle for bending over, you can't be rounding your back. The action comes from the hip.
Related: Lift Heavy Objects The Back Safe Way
The Psoas and Sit Up Exercises
Another common misconception about the work of the psoas muscle relates to sit up exercises. Many people, in the quest for gorgeous abdominal muscles, believe they're doing sit ups incorrectly if they use hip flexors (the psoas and the quadriceps) during the last phase of the movement - when your trunk comes fully off the floor. They believe that a sit up is supposed to work your abs, so if the abs aren’t doing the work, you’re cheating.
But this is not the case.
What really happens during a sit up is the abdominal muscles shorten to bring your head, neck, shoulders and thoracic spine up off the floor. Once the abdominals have shortened to their fullest extent, they can do no more to bring your trunk off the floor. In other words, the abs can only bring you up part of the way.
In fact, the abdominal muscles don't even cross the hip joint. This means they cannot affect the hip flexion action. So once the abs have taken you as far as they can, the hip flexors kick in to finish the job.
The psoas muscle also plays an important role in posture. In most positions, it helps maintain a slight anterior tilt of the pelvis, which in turn helps maintain your normal lumbar curve. When only one psoas muscle is contracting, or one is contracting more than the other, psoas action tilts your trunk to the side. This action is called lateral flexion.
Consequences of a Tight or Weak Psoas
When your psoas muscle gets too tight or shortened, you may tend toward an arched back, especially when you challenge your abdominal muscles. If your psoas is weak, it will likely be harder to flex your hip joint. This may negatively affect your ability to climb stairs, walk uphill, get up from a position in which you are lying on your back (supine) or preparing to stand up from a sitting position.
Kendall, Florence Peterson, McCreary, Elizabeth Kendall, and Provance, Patricia Geise. Muscles Testing and Function with Posture and Pain. 3rd. Baltimore, Maryland: Williams & Wilkins, 1983.