The Psoas Major: More Than a Hip Flexor
Why this deep muscle may influence the lower back, pelvis, nerves, and the ability to rest in a deep squat.
The psoas major is often described as a tight hip flexor that simply needs to be stretched. That explanation is easy to understand, but it is incomplete.
The psoas is a deep muscle that connects the lumbar spine directly to the femur. It helps coordinate the lumbar spine, pelvis, and hip, while much of the lumbar plexus lies within the substance of the muscle.
Because of this unique position, the psoas may influence much more than hip flexion. It may affect:
Lumbar and pelvic control
Hip movement
Load transfer between the trunk and lower limbs
Neural sensitivity
Groin and anterior hip symptoms
Movement symmetry
Recovery from recurrent low-back and lower-limb pain
At YOON CLINIC, we do not view the psoas as an isolated muscle. We view it as part of a larger clinical system.
Where Is the Psoas Major?
The psoas major begins along the lower thoracic and lumbar spine. Its more superficial fibers attach to the sides of the vertebral bodies and adjacent intervertebral discs. Its deeper fibers attach to the lumbar transverse processes.
The muscle then travels downward through the pelvis, joins the iliacus to form the iliopsoas, and inserts onto the lesser trochanter of the femur.
This creates a direct anatomical connection:
Lumbar spine $\rightarrow$ Pelvis and hip $\rightarrow$ Femur
Because of this position, the psoas can participate in hip flexion, lumbar stabilization, pelvic control, and the transfer of force between the trunk and lower limbs. It is not simply a muscle that lifts the leg; it is one of the key structures through which the spine and lower body interact.
The Psoas and the Lumbar Plexus
One of the most clinically important features of the psoas is its relationship with the lumbar plexus.
The lumbar plexus is not simply located behind the muscle. Anatomical studies show that much of it lies within the substance of the psoas, with its exact layout varying by spinal level and between individuals.
The lumbar plexus contributes to several major nerves, including:
Femoral nerve
Obturator nerve
Lateral femoral cutaneous nerve
Genitofemoral nerve
Ilioinguinal nerve
Iliohypogastric nerve
This does not mean that every tense psoas compresses these nerves. However, it does mean that the psoas forms an unusually close mechanical interface between muscle and nerve tissue. Changes in muscle tension, tissue pressure, movement, or local sensitivity may alter the environment surrounding these neural structures in some patients.
Why Can the Psoas Matter in Low-Back and Pelvic Pain?
The psoas should not be blamed for every form of low-back pain. However, it may become an important contributor through several distinct patterns:
1. Persistent Tension
Following injury, prolonged sitting, repetitive hip flexion, or altered movement, the psoas may remain in a protective state of increased tension. This can be associated with:
Deep anterior hip or groin pressure
Discomfort during active hip flexion
Reduced tolerance to hip extension
Compensatory lumbar movement
Recurrent low-back or pelvic discomfort
Increased sensitivity around the lumbar plexus
Note: Persistent tension does not necessarily mean the muscle is physically shortened, it often represents an ongoing protective motor response.
2. Reduced Length or Adaptability
If the iliopsoas cannot lengthen adequately as the hip moves into extension, the body compensates through the lumbar spine, pelvis, or lower limb. This compensation is often visible during walking, running, squatting, split-stance movements, standing after prolonged sitting, or prone hip extension.
3. Weakness or Poor Coordination
Not every painful psoas is too tight. In some patients, the muscle may be weak, poorly coordinated, or unable to work effectively with the trunk, pelvis, and hip. This is why simply stretching or "releasing" the psoas is not always the correct solution.
MRI research examining psoas size and quality in people with low-back pain shows mixed findings. While some studies show an association between greater psoas muscle area and lower pain intensity, systematic reviews show no single imaging measurement can definitively determine whether it is the primary pain generator.
4. Altered Hip–Spine Coordination
Patients with low-back pain commonly demonstrate differences in hip range of motion, strength, muscle activation, and movement performance. The psoas is often a central structure within this altered system, particularly when a patient shows reduced hip extension, pelvic rotation, or poor squat mechanics.
What Symptoms May Be Associated With the Psoas Region?
A psoas-related clinical pattern may include:
Symptom Category | Associated Presentations |
Spine & Pelvis | Low-back pain, recurrent pelvic discomfort, difficulty standing upright after prolonged sitting |
Hip & Groin | Deep groin pain, anterior hip discomfort, reduced hip extension |
Nerve & Radiation | Anterior/medial/lateral thigh pain or altered sensation |
Movement | Asymmetric squat mechanics, compensatory side shifting |
Because these symptoms overlap with other conditions (such as lumbar radiculopathy, hip-joint pathology, meralgia paresthetica, or inguinal hernia), the psoas should never be selected as a treatment target based solely on pain location.
Groin or testicular pain can have non-musculoskeletal causes. Sudden, severe, unexplained, or worsening symptoms require medical assessment before assuming a muscular cause.
The Deep Squat as a Human Resting Position
The deep squat is often viewed as an exercise, but it is fundamentally a resting position that the human body evolved to use daily.
Research involving Hadza adults in Tanzania found that squatting and kneeling were regularly used during non-walking rest, maintaining higher lower-limb muscle activity than sitting in a chair. Removing chairs from daily life makes the ability to settle close to the ground practically essential.
At YOON CLINIC, we use the ability to remain comfortable in a deep-squat resting position for 1–2 minutes as a practical functional benchmark.
When the deep squat is restricted, the body compensates through:
Side shifting or pelvic rotation
Knee collapse or heel elevation
Excessive forward trunk lean
Lumbar spine compensation
While the inability to deep squat can stem from the ankles, knees, hip joints, or balance, clinical experience shows the psoas major is one of the most frequent modifiable barriers within this pattern.
How We Evaluate and Treat the Psoas at YOON CLINIC
Rather than asking "Is the psoas tight?", our clinical inquiry focuses on: "Is the psoas acting as a meaningful barrier to recovery?"
Evaluation includes:
Functional movement (Deep squat, gait, split squat mechanics)
Hip range & strength (Modified Thomas test, active/resisted hip flexion, prone extension)
Spine & Neural testing (Lumbar mobility, reflexes, sensation, neural tension)
Treatment as an Assessment Tool
When the psoas appears to be a barrier, targeted treatment is followed by immediate reassessment of the deep squat. We look for real-time changes in pain, depth, knee alignment, and weight distribution.
If pain decreases while squat depth and control improve immediately, it confirms the psoas was contributing to the dysfunction.
From Restoring the Squat to Maintaining Recovery
Improving the deep squat is not the end of treatment, it is the beginning of movement retraining.
Treatment restores access to the movement by removing the structural or neural barrier.
Practice makes the movement familiar, repeatable, and durable.
A common pattern we observe is that patients improve, stop practicing the deep squat once pain disappears, and experience symptom recurrence months later. Pain relief often occurs before movement recovery becomes truly durable. Continuing to practice the deep squat as a daily resting position helps protect against future flare-ups.
Summary: Treatment Should Match the Barrier
The psoas major is neither a universal villain nor an unimportant hip flexor. It is a deep muscular and neurological interface connecting the spine, pelvis, and lower limbs.
If tension/irritability is dominant: Focus on reducing sensitivity and restoring extension tolerance.
If weakness/poor coordination is dominant: Focus on controlled hip flexion and lumbopelvic stability.
If global compensation is dominant: Address secondary barriers like ankle mobility, gluteal function, and pelvic control.
Treat the dominant barrier. Reassess the movement. Maintain the recovery.
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