The psoas (pronounced “SO-as”) is a deep core muscle that runs from your lower spine down through your pelvis to attach at the top of your thigh bone. It’s one of the most important muscles for walking, standing up, and stabilizing your lower back, yet most people have never heard of it until something goes wrong. Because it connects your upper and lower body, the psoas plays a role in posture, hip mobility, back pain, and even how deeply you breathe.
Where the Psoas Sits in Your Body
The psoas major is a long, thick muscle that originates along the sides of your lumbar vertebrae, the five bones that make up your lower spine. From there it travels downward through the pelvis and attaches to a small bony bump near the top of your femur called the lesser trochanter. This path makes it one of the few muscles that directly links your spine to your leg.
Near its lower end, the psoas merges with another muscle called the iliacus, which lines the inside of your pelvic bowl. Together they form what’s known as the iliopsoas. In anatomical specimens, the two muscles are clearly separate near the spine, but their tendons fuse together by the time they reach the thigh bone. When doctors or physical therapists talk about “the iliopsoas,” they’re referring to this combined unit. When they say “the psoas,” they usually mean the psoas major specifically.
Some people also have a smaller companion muscle called the psoas minor, which sits in front of the psoas major. Not everyone has one; it’s absent in roughly 40 to 60 percent of the population and plays a minor role in trunk flexion when it is present.
What the Psoas Does
The psoas is your body’s primary hip flexor. Every time you lift your knee, whether climbing stairs, walking, or getting out of a car, the psoas is doing much of that work. Its full list of jobs includes flexing the hip, pulling the thigh slightly inward, and rotating the hip outward. But its stabilizing role is just as important as its movement role.
When you’re sitting, the psoas holds your lower back steady. When you’re standing, it helps keep your torso upright over your pelvis. It activates when you do a sit-up, pulling your trunk up from a lying position. It stabilizes the head of your thigh bone inside the hip socket during movement. Essentially, any activity that involves your hips, lower back, or the transition between them relies on the psoas to some degree.
How It Affects Your Posture and Lower Back
Because the psoas attaches directly to the lumbar spine, its length and tension have a direct effect on spinal curvature. When the psoas becomes chronically shortened or tight, it pulls the lumbar spine forward, increasing the natural inward curve of the lower back. This exaggerated curve is called hyperlordosis, and it often comes with an anterior pelvic tilt, where the front of the pelvis drops downward and the tailbone tips up.
A study of desk job workers published in the Indian Journal of Occupational and Environmental Medicine found a moderate positive correlation between iliopsoas tightness and the degree of lumbar lordosis. Workers with the tightest hip flexors had the most pronounced spinal curves. This chain reaction, tight psoas pulling the pelvis forward and deepening the lumbar curve, is recognized as one of the common mechanical causes of low back pain. People who sit for long stretches each day are especially vulnerable because the psoas stays in a shortened position for hours at a time.
The Psoas and Breathing
The psoas and the diaphragm, your main breathing muscle, are physically connected through shared fascial tissue and spinal attachments. When the psoas is chronically tight, it can restrict how far the diaphragm moves during a breath, leading to shallower breathing patterns. This isn’t just a mechanical inconvenience. Shallow breathing reduces stimulation of the vagus nerve, which is responsible for shifting your body into a calm, restorative state.
The psoas is sometimes called the “muscle of fear” because of how directly it responds to the body’s stress response. When your brain perceives a threat, the sympathetic nervous system activates, and the psoas contracts to prepare you to run or curl into a protective position. In people dealing with chronic stress or unresolved trauma, this contraction can become a persistent holding pattern: a tight psoas sends tension signals that the brain interprets as ongoing danger, which triggers more stress hormones, which tightens the psoas further. Breaking that cycle often requires addressing both the muscle tension and the nervous system state behind it.
Psoas Syndrome: When the Muscle Becomes a Problem
Psoas syndrome refers to pain and dysfunction caused by irritation, inflammation, or injury to the psoas or the broader iliopsoas unit. The hallmark symptom is deep pain in the front of the hip or groin that gets worse when you extend your hip backward or resist flexion. The pain can also radiate into the thigh, the lower back, the buttock, or the sacrum, though it typically doesn’t travel below the knee.
Certain patterns make psoas syndrome recognizable. Pain often flares during transitions, like standing up from a chair or trying to maintain an upright posture. Some people feel a catching or snapping sensation in the groin when the knee bends to 90 degrees. Walking and stair climbing commonly trigger discomfort because the muscle fires with every step. In chronic cases, the tight psoas can pull the lower back into a visibly exaggerated curve. Symptoms generally worsen with activity and improve with rest.
Groin pain from psoas irritation is particularly common in sports that involve kicking and in adolescents going through growth spurts. Diagnosis is primarily clinical. A physical therapist or doctor may use the Thomas test, where you lie on your back and pull one knee to your chest while the other leg hangs off the table. If the hanging leg lifts or won’t rest flat, that suggests a tight or irritated psoas. Pressing on the lesser trochanter, where the muscle inserts, often reveals tenderness. Reduced range of motion in hip extension is another consistent finding.
Stretches and Exercises for the Psoas
Keeping the psoas both flexible and strong matters more than focusing on one or the other. A tight psoas needs lengthening, but a weak one can’t do its stabilizing job properly.
Strengthening
Because the psoas lifts your knee toward your chest, any exercise that involves raising the thigh against gravity will work it. Simple options include marching in place (holding a chair for balance if needed), marching while seated, or standing on one leg and lifting the opposite knee with toes pointed down, holding for a few seconds per rep. These exercises are low-impact enough for most fitness levels and can be done throughout the day.
A quadruped donkey kick, where you balance on your hands and one knee while lifting the opposite leg toward the ceiling with the knee bent, is another effective choice. This movement strengthens the glutes on the working side, which allows the psoas on the opposite side to relax and reset its resting tension. That reciprocal relationship between the glutes and the psoas is one reason weak glutes often accompany a tight psoas.
Stretching
The classic psoas stretch is a half-kneeling lunge. Drop one knee to the floor, plant the opposite foot in front of you, and gently shift your hips forward until you feel a stretch deep in the front of the hip on the kneeling side. Keeping your torso upright prevents the lower back from compensating. Hold for 20 to 30 seconds and repeat on both sides. For people who sit most of the day, doing this stretch two or three times is a practical way to counteract the hours spent with the psoas in a shortened position.
Because of the psoas’s connection to the diaphragm, pairing stretches with slow, deep belly breathing can amplify the release. As the diaphragm expands fully, it gently lengthens the fascial tissue shared with the psoas, creating a two-for-one effect that addresses both muscle tension and the nervous system patterns that maintain it.