Hip Flexor Strains

Hip flexor strains are common in sports that demand explosive hip flexion: soccer and the other kicking sports, running and sprinting, martial arts, dance and gymnastics, and Australian Rules Football. The primary hip flexors are the iliopsoas and the rectus femoris, and the rectus femoris is affected more often, because it crosses two joints and that increases its eccentric loading during kicking.

Mechanism. The two muscles fail under different loads. The rectus femoris fails under eccentric load while kicking or sprinting, in the leg deceleration phase. The iliopsoas fails under forceful hip flexion against resistance.

Risk factors. A previous strain is the strongest predictor. The others are inadequate warm-up, muscle fatigue, inflexibility or contracture, and strength imbalance.

The adolescent. In adolescents, apophyseal avulsions are common: a different injury wearing the same clinical presentation. They are covered under Classification and Investigations.

Two primary flexors. The iliopsoas is the most powerful hip flexor, and it lies deep: psoas major, from the T12-L5 vertebral bodies, and iliacus, from the iliac fossa, unite at the pelvis and insert together on the lesser trochanter. The rectus femoris is the only part of the quadriceps that crosses the hip, arising by a straight head from the AIIS and a reflected head from the acetabular rim. Sartorius, the longest muscle in the body, is a weak flexor in its own right; pectineus and tensor fasciae latae are the secondary flexors.

The dual innervation, the femoral nerve to the rectus femoris and the lumbar plexus to the iliopsoas, has implications for recovery.

Architecture. The rectus femoris has long fascicles and a high capacity for velocity; the iliopsoas has shorter fascicles and generates high force. Type II (fast-twitch) fibres predominate, and the pennation angle affects strain risk.

The iliopsoas bursa. The largest bursa in the body lies between the iliopsoas tendon and the anterior hip capsule, over the iliopectineal eminence. It communicates with the hip joint in 15-20%, through the interval between the iliofemoral and pubofemoral ligaments. Bursitis can mimic a hip flexor strain.

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History. Establish the mechanism (an explosive kick, a sprint start, a sudden change of direction), whether the onset was an acute pop or snap or a gradual overuse, and what the patient can no longer do: stairs, running or kicking.

Where it hurts localises the muscle. Iliopsoas pain is deep, in the anterior hip and groin, and worse with resisted hip flexion at 90 degrees. Rectus femoris pain sits more in the anterior thigh, often at the AIIS or the muscle belly, and is worse with a resisted straight leg raise or knee extension.

Examination. Hip flexor weakness gives an antalgic gait with a shortened stride and increased lordosis. Look for swelling and for bruising, which is delayed; palpate the AIIS, the lesser trochanter region and the muscle belly; and check hip extension, which is limited if there is a contracture. Then test each muscle in turn.

Radiographs first. An AP pelvis and frog lateral. In adolescents they are essential, and every young athlete with an acute hip flexor injury should have them, to exclude an avulsion of the AIIS, ASIS or lesser trochanter. In a pure muscle strain they are often normal.

MRI is the gold standard for the soft tissue. It quantifies the extent of the tear, places it at the musculotendinous junction or in the muscle belly, and shows oedema, haematoma and retraction. MRI within 5 days is optimal for grading.

What predicts a longer recovery. On MRI, a longer recovery is associated with:

• greater cross-sectional area of tear, which correlates with return-to-play time
• involvement of the central tendon of rectus femoris, a poor prognostic sign
• proximal location, at the musculotendinous junction
• significant retraction

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Ultrasound allows dynamic assessment, watching the muscle contract in real time, and suits the superficial rectus femoris. It is operator-dependent but accessible, cost-effective and free of radiation. It can also monitor healing and guide injections into the bursa or a haematoma.

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The decision. The grade sets the treatment, and in the adolescent the radiograph decides whether it is a strain at all.

Endoscopic AIIS fixation. A displaced AIIS avulsion can also be fixed endoscopically, as in the sequence below.

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Re-injury. The key risk factors are a previous strain, premature return, inadequate strength restoration and poor flexibility. The return-to-sport criteria under Management are the defence against it.

Iliopsoas (iliopectineal) bursitis. Where the bursa communicates with the joint, a hip effusion or synovitis can distend it, and an enlarged bursa can present as a groin mass or even compress the femoral vessels. Secondary bursitis is common: in hip OA, in rheumatoid or other inflammatory arthritis, and with overuse in dancers and runners.

The pain is deep and anterior in the groin, worse on resisted flexion and on passive extension, and it often coexists with internal snapping, which is covered in the snapping-hip topic.

Iliopsoas impingement after THA. A recognised cause of persistent groin pain after total hip arthroplasty, in a small but important minority of painful THAs. The tendon is irritated by:

• an anteriorly prominent or oversized acetabular cup
• an uncovered anterior component
• retained cement
• a protruding screw or long neck

Pain comes on active straight-leg raise, stair climbing and rising from a chair, and resisted hip flexion reproduces it. Cross-sectional imaging (CT) quantifies anterior cup overhang relative to the native acetabular rim.

Management. Native-hip bursitis is managed conservatively with activity modification, NSAIDs, stretching and eccentric loading, and a bursal injection for refractory cases. Post-THA impingement escalates stepwise, with activity modification and a diagnostic or therapeutic injection first. If symptoms persist, iliopsoas tenotomy (arthroscopic or open) is appropriate when the acetabular component is well positioned, whereas a malpositioned or grossly oversized or retroverted cup causing the impingement should be revised rather than the tendon simply released.

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The clinical picture. Acute groin or iliac-fossa pain, with the hip held flexed and externally rotated because that shortens the compartment and eases the pain (a positive psoas sign). Consider it in any anticoagulated patient with sudden groin pain and a flexed hip. The femoral nerve signs follow:

• quadriceps weakness, with loss of knee extension
• a diminished or absent patellar reflex
• sensory loss over the anterior thigh and medial leg (saphenous distribution)

Investigation and management. CT or ultrasound confirms and sizes the iliacus or retroperitoneal haematoma. Check coagulation status (INR, platelets, factor levels) and reverse any coagulopathy; active extravasation on contrast CT supports urgent correction of coagulopathy and consideration of embolisation. Most are managed conservatively, with reversal of anticoagulation, analgesia and observation of the neurology. Surgical or image-guided decompression or evacuation is reserved for a progressive or dense femoral nerve deficit.

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Protect the repair for 6 weeks, with no resisted hip flexion. After that, rehabilitation follows the pattern of tendon repairs elsewhere, progressive loading after the initial protection phase, and return to competitive sport takes 3-6 months.

In grade III, conservative management often leads to prolonged weakness.

Prognosis. The poor prognostic factors are the MRI features that predict a longer recovery (see Investigations) and delayed treatment. A muscle belly injury with minimal retraction, early appropriate management and good compliance carry a good prognosis.

Global Epidemiology

Hip flexor and iliopsoas-related injuries cluster in kicking and change-of-direction sports (soccer, Australian Rules, Gaelic football, rugby, sprinting, dance). In the UEFA Elite Club Injury Study, thigh muscle injuries (quadriceps and hamstring) are among the most common time-loss injuries in professional football. Using the Doha framework, iliopsoas-related groin pain is a recognised distinct entity, though adductor-related pain predominates and multiple causes coexist in roughly 40-45% of athletes presenting with groin pain. Apophyseal avulsions concentrate in adolescents aged 13-17 years before apophyseal fusion, with a strong male predominance.

Side-by-Side Guidance

PRP and orthobiologics. Platelet-rich plasma for muscle strains shows inconsistent results across trials. There is no robust evidence that it accelerates return to sport in acute hip flexor strains, and it remains adjunctive at best.

Classification choice. The traditional grade I-III scheme is simple but crude. The Munich and BAMIC systems add prognostic detail, separating functional from structural injury and recognising intratendinous involvement, but their inter-rater reliability, and their superiority for the hip flexors specifically, are unproven.