The psoas major is the primary hip flexor and the only muscle that connects the lumbar spine directly to the lower extremity. It originates at the T12 to L5 vertebral bodies and discs and inserts at the lesser trochanter of the femur. When it contracts, it flexes the hip and, in its eccentric role, stabilises the lumbar spine during movement. When it is chronically shortened, as it becomes in adults who spend large portions of the day seated, it exerts a persistent pull on the lumbar vertebrae and anterior pelvis, producing anterior pelvic tilt, increased lumbar lordosis, and compressive loading on the posterior spinal elements including facet joints and intervertebral disc posterior margins.
The Sitting-Back Pain Pathway
The hip flexors, particularly the psoas and iliacus, are at their shortest when the hip is in flexion, as it is during sitting. Hours of seated posture each day progressively adapt the muscle-tendon unit to a shortened resting length. This adaptive shortening reduces the available hip extension range when standing, causing compensatory anterior pelvic tilting and lumbar hyperextension with each step during walking and running. The cumulative impact on posterior spinal structures over hours, days, and years produces the degenerative changes and pain sensitisation seen in chronic low back pain.
Assessment and Treatment
The Thomas test is the standard clinical assessment for hip flexor tightness. The subject lies supine and holds one knee to the chest while allowing the other leg to hang. If the hanging leg rises above the level of the table or the knee does not fully extend, hip flexor tightness is present. Research by Winters et al. found hip flexor tightness in a high proportion of adults with chronic low back pain, at rates significantly higher than pain-free controls. Addressing hip flexor extensibility through stretching, strengthening antagonists including the gluteus maximus, and reducing prolonged sitting time all have evidence for reducing back pain.
The Most Effective Interventions
The kneeling hip flexor stretch, performing a lunge and posteriorly tilting the pelvis, stretches the psoas and iliacus effectively when held for 60 to 90 seconds per side. Strengthening the gluteal muscles, which are the primary antagonists of the hip flexors and the muscles that most directly control pelvic tilt, is equally important and often more durable than stretching alone. Breaking up prolonged sitting every 30 to 60 minutes with standing or walking produces a meaningful reduction in hip flexor adaptive shortening over time.
- Perform the kneeling hip flexor stretch for 60 to 90 seconds per side, twice daily if seated for most of the day
- Strengthen the glutes with hip thrusts, glute bridges, and Romanian deadlifts: they control pelvic tilt more effectively than stretching alone
- Break up sitting every 30 to 60 minutes with standing or walking: this limits the degree of adaptive shortening that accumulates
- Avoid sleeping in a foetal position: it maintains the hip in flexion overnight and compounds daytime shortening
- Include prone lying for 10 minutes in the evening to passively extend the hip and stretch the anterior chain
Sources
- Winters, M.V., et al. (2004). Passive versus active stretching of hip flexor muscles in subjects with limited hip extension. Physical Therapy, 84(9), 800-807.
- Liebenson, C. (2014). Rehabilitation of the Spine: A Practitioner's Manual. Lippincott Williams and Wilkins.