HIP FLEXOR STRAINS
Iliopsoas | Rectus Femoris | Athletic Population
Muscle Strain Grading
Critical Must-Knows
- Rectus femoris injuries MORE COMMON than iliopsoas in athletes
- ASIS avulsion = sartorius; AIIS avulsion = rectus femoris
- Iliopsoas bursa largest in body - can communicate with hip joint
- Hip flexor weakness = antalgic gait with increased lordosis
- Return to sport based on STRENGTH not just pain resolution
Examiner's Pearls
- "Thomas test positive = hip flexor contracture
- "Resisted hip flexion at 90 degrees isolates iliopsoas
- "AIIS avulsions common in adolescents (apophysis open)
- "Ultrasound can assess dynamic function in real-time
Clinical Imaging
Imaging Gallery
Critical Hip Flexor Exam Points
Two Key Muscles
Iliopsoas (iliacus + psoas major) and rectus femoris are primary hip flexors. RF crosses two joints (hip + knee) making it vulnerable during high-velocity activities like kicking and sprinting.
Avulsion Fractures
In adolescents, apophyseal avulsions are common. AIIS = rectus femoris, ASIS = sartorius, ischial tuberosity = hamstrings. Always get X-rays in young athletes with acute onset.
Examination Keys
Thomas test for contracture. Resisted hip flexion at 90 degrees (patient supine) isolates iliopsoas. Resisted straight leg raise tests rectus femoris more.
Return to Sport
Strength-based criteria - not just pain-free. Aim for greater than 90% strength compared to uninjured side. Premature return leads to high recurrence rates (30% if inadequate rehab).
Quick Decision Guide
| Presentation | Grade | Treatment | Return Timeline |
|---|---|---|---|
| Mild pain, full ROM, minimal weakness | Grade I | RICE, relative rest, early mobilization | 1-2 weeks |
| Moderate pain, some weakness, antalgic gait | Grade II | Protected activity, progressive rehab | 4-6 weeks |
| Severe pain, significant weakness, palpable defect | Grade III | Immobilization, consider surgery | 3-6 months |
| Adolescent, acute pop, bony tenderness | Avulsion | X-ray, possible surgical fixation | 6-12 weeks |
PRISMHip Flexor Anatomy
Memory Hook:PRISM of muscles bend the hip - ilioPSoas and Rectus lead the way!
AIIS ASIS ITAvulsion Fracture Sites
Memory Hook:AIIS = INFERIOR = RECTUS (below); ASIS = SUPERIOR = SARTORIUS (above)
SPORTReturn to Sport Criteria
Memory Hook:SPORT readiness = STRENGTH first, pain-free second!
Overview and Epidemiology
Hip flexor strains are common in sports requiring explosive hip flexion, including soccer, running, martial arts, and dance. The rectus femoris is affected more frequently than iliopsoas due to its biarticular nature, which increases eccentric loading during activities like kicking.
Mechanism of Injury
Rectus femoris: Eccentric loading during kicking, sprinting (leg deceleration phase). Iliopsoas: Forceful hip flexion against resistance. The dual innervation (femoral nerve for RF, lumbar plexus for iliopsoas) has implications for recovery.
Risk Factors
- Previous strain (strongest predictor)
- Inadequate warm-up
- Muscle fatigue
- Inflexibility/contracture
- Strength imbalance
Common Sports
- Soccer (kicking sports)
- Running/sprinting
- Martial arts
- Dance/gymnastics
- Australian Rules Football
Pathophysiology and Mechanisms
Critical Anatomy
The iliopsoas is the most powerful hip flexor, composed of psoas major (T12-L5 vertebral bodies) and iliacus (iliac fossa), uniting to insert on the lesser trochanter. The rectus femoris has two heads (straight from AIIS, reflected from acetabular rim) and is the only quadriceps component that crosses the hip.
Hip Flexor Anatomy Comparison
| Muscle | Origin | Insertion | Innervation | Clinical Note |
|---|---|---|---|---|
| Iliopsoas | T12-L5 + Iliac fossa | Lesser trochanter | Femoral nerve + L1-3 | Strongest flexor, deep location |
| Rectus femoris | AIIS (straight) + Acetabulum (reflected) | Tibial tuberosity via patella | Femoral nerve (L2-4) | Biarticular = high strain risk |
| Sartorius | ASIS | Pes anserinus (tibia) | Femoral nerve (L2-3) | Longest muscle, weak flexor |
| TFL | ASIS and iliac crest | ITB to Gerdy tubercle | Superior gluteal nerve | Flexion, abduction, IR |
Biarticular Risk
Rectus femoris crosses two joints - it is stretched maximally when the hip is extended AND knee is flexed (e.g., late swing phase of sprinting, follow-through of kick). This is when most RF strains occur.
Iliopsoas Bursa
- Largest bursa in body
- Between iliopsoas tendon and hip capsule
- Communicates with hip joint (15-20%)
- Bursitis can mimic hip flexor strain
Muscle Architecture
- RF: Long fascicles, high velocity capacity
- Iliopsoas: Shorter fascicles, high force
- Pennation angle affects strain risk
- Type II fibres predominate (fast twitch)
Classification Systems
Standard Muscle Strain Classification
| Grade | Pathology | Clinical Features | Imaging | Recovery |
|---|---|---|---|---|
| I (Mild) | Less than 5% fibres disrupted | Minimal pain, no weakness, full ROM | Normal or minor oedema | 1-2 weeks |
| II (Moderate) | 5-50% fibres disrupted | Moderate pain, weakness, antalgic gait | Partial tear visible, haematoma | 4-6 weeks |
| III (Severe) | Greater than 50% or complete rupture | Severe pain, marked weakness, defect | Complete disruption, retraction | 3-6 months |
Clinical vs Imaging
Grade I-II differentiation is often clinical. MRI/ultrasound helps quantify tear extent and predict recovery time. Cross-sectional area of tear on MRI correlates with return-to-play time.
This grading system guides treatment intensity and expected recovery timeline.
Clinical Assessment
History
- Mechanism: Explosive kick, sprint start, sudden change of direction
- Onset: Acute pop/snap vs gradual overuse
- Location: Anterior hip/groin (iliopsoas) vs anterior thigh (RF)
- Functional deficit: Stairs, running, kicking difficulty
Examination
- Gait: Antalgic, shortened stride, increased lordosis
- Inspection: Bruising (delayed), swelling
- Palpation: AIIS, lesser trochanter region, muscle belly
- ROM: Limited hip extension if contracture
Differential Diagnosis
Rule out: Femoral neck stress fracture (night pain, metabolic risk), Hip labral tear (clicking, FADIR positive), Lumbar radiculopathy (dermatomal symptoms), Inguinal hernia (Valsalva worsens), FAI (reduced ROM, impingement tests positive).
Clinical Tests for Hip Flexors
| Test | Technique | Positive Finding | Muscle Tested |
|---|---|---|---|
| Thomas test | Supine, flex opposite hip fully | Tested hip rises off bed | Hip flexor contracture (general) |
| Resisted hip flexion (90 degrees) | Supine, hip at 90, resist further flexion | Pain or weakness | Iliopsoas (primary) |
| Resisted SLR | Supine, straight leg, resist elevation | Pain or weakness | Rectus femoris (primary) |
| Ely test | Prone, passive knee flexion | Hip flexes off bed | Rectus femoris contracture |
Localization
Iliopsoas pain is deep, anterior hip/groin, worse with resisted hip flexion at 90 degrees. Rectus femoris pain is more anterior thigh, often at AIIS or muscle belly, worse with resisted SLR or knee extension.
Investigations
Imaging Protocol
AP pelvis + frog lateral. Essential in adolescents to exclude avulsion fracture. Look for AIIS, ASIS, lesser trochanter avulsions. Often normal in pure muscle strains.
Gold standard for soft tissue assessment. Quantifies tear extent (cross-sectional area predicts recovery). Identifies location (musculotendinous junction vs muscle belly). Assesses oedema, haematoma, retraction.
Dynamic assessment possible. Real-time visualization of muscle contraction. Good for superficial muscles (RF). Operator-dependent but accessible.
MRI Prognostic Factors
Longer recovery associated with: Greater cross-sectional area of tear, involvement of central tendon (RF), proximal location (MTJ), significant retraction. MRI within 5 days is optimal for grading.
MRI Findings
- Grade I: Feathery oedema, intact fibres
- Grade II: Partial disruption, haematoma
- Grade III: Complete disruption, gap, retraction
- Central tendon involvement = poor prognosis
Ultrasound Benefits
- Dynamic assessment during contraction
- Guide injections (bursa, haematoma)
- Monitor healing progress
- Cost-effective, no radiation
Management Algorithm
Standard Management Protocol
Goal: Pain control, restore ROM, progressive strengthening, sport-specific return.
Rehabilitation Phases
Protection and pain control: RICE principles. NSAIDs for 3-5 days. Crutches if antalgic gait. Gentle ROM as tolerated. Avoid stretching in acute phase.
ROM and isometric loading: Pain-free ROM exercises. Isometric hip flexion progressing through range. Pool therapy for unloading. Address any compensatory patterns.
Progressive resistance: Isotonic exercises. Hip flexion against bands/weights. Eccentric loading introduction. Core stability work.
Return to play preparation: Sport-specific drills. Sprinting, kicking progression. Agility and plyometrics. Meet return-to-sport criteria.
Return-to-Sport Criteria
Must achieve: Greater than 90% strength vs uninjured side, pain-free sport-specific activity, full ROM without contracture, passed functional testing (hop tests, agility). Premature return = 30% recurrence rate.
Rehabilitation timeline varies with injury grade - Grade I may be 1-2 weeks, Grade II 4-6 weeks.
Surgical Technique
Rectus Femoris Avulsion Repair
Surgical Steps
Supine on radiolucent table. Bump under ipsilateral hip if needed. Prep from costal margin to mid-thigh.
Anterior approach to AIIS. Bikini incision or longitudinal over AIIS. Develop interval between sartorius (medial) and TFL (lateral).
Identify retracted tendon stump. May need to mobilize from scar. Protect lateral femoral cutaneous nerve. Identify both heads if possible.
Decorticate AIIS footprint. Create bleeding bone bed. Place suture anchors (2-3 typically).
Pass sutures through tendon. Reduce to footprint with hip in flexion to reduce tension. Secure repair. Assess stability through ROM.
Layered closure over drain if haematoma risk. Standard wound care.
Nerve at Risk
Lateral femoral cutaneous nerve runs medial to ASIS, can be injured during approach. Identify and protect. Numbness is common complication if not careful.
This technique restores anatomic footprint and allows early rehabilitation.
Complications
| Complication | Incidence | Prevention | Management |
|---|---|---|---|
| Recurrent strain | 15-30% | Complete rehabilitation, strength-based RTS | Extended conservative management |
| Chronic weakness | 10-20% (Grade III) | Early surgery if complete rupture | Surgical repair if functional deficit |
| Myositis ossificans | Rare | Avoid aggressive massage early, no heat | Observation, excision if symptomatic |
| Hip flexor contracture | 5-10% | Maintain ROM during healing | Stretching program, rarely surgical release |
Re-Injury Risk
30% recurrence rate if return to sport before adequate rehabilitation. Key risk factors: Previous strain, premature return, inadequate strength restoration, poor flexibility. Ensure greater than 90% strength before clearance.
Postoperative Care and Rehabilitation
Rehabilitation After Surgical Repair
Partial weight bearing with crutches. Hip flexion limited to 90 degrees. Brace if needed. Gentle passive ROM. No active hip flexion against resistance.
Progress to full weight bearing. Begin active-assisted hip flexion. Isometric strengthening. Pool therapy. Cycling.
Progressive resistance exercises. Isotonic hip flexion. Eccentric loading. Core stability. Gait normalization.
Return to play preparation. Sport-specific drills. Sprinting progression. Meet RTS criteria before clearance.
Key Principle
Protect the repair for 6 weeks - avoid resisted hip flexion. Rehabilitation is similar to tendon repairs elsewhere - progressive loading after initial protection phase.
Expect 3-6 months before return to competitive sport after surgical repair.
Outcomes and Prognosis
| Grade | Conservative Success | Surgical Success | Return Timeline |
|---|---|---|---|
| Grade I | Greater than 95% | N/A | 1-2 weeks |
| Grade II | 90-95% | N/A | 4-6 weeks |
| Grade III | 50-70% | 85-90% | 3-6 months |
Prognostic Factors
Poor prognosis: Central tendon involvement, proximal location, significant retraction, delayed treatment. Good prognosis: Muscle belly injury, minimal retraction, early appropriate management, good compliance.
Evidence Base and Key Trials
MRI Prognostic Factors in Muscle Injury
- UEFA injury study: MRI findings predict return to play
- Cross-sectional area of injury correlates with recovery time
- Central tendon involvement delays recovery
- Proximal injuries take longer than distal
Rectus Femoris Injury Patterns
- Two distinct injury patterns: acute (biarticular stretch) and chronic (overuse)
- Acute injuries at MTJ during eccentric loading
- Chronic injuries often at reflected head
- Location affects prognosis
Apophyseal Avulsion Fractures in Athletes
- Most apophyseal avulsions heal conservatively
- Surgical indications: greater than 2cm displacement, symptomatic nonunion
- AIIS and ischial tuberosity most common sites
- Return to sport 6-12 weeks typical
Return to Sport After Hip Flexor Injury
- Strength-based criteria reduce reinjury
- Greater than 90% strength compared to contralateral side
- Functional testing should include sport-specific movements
- Premature return leads to 30% recurrence
Surgical Repair of Complete RF Avulsion
- Case series of surgical repair in athletes
- 85-90% return to preinjury level
- Direct repair to bone with anchors
- Better outcomes with acute repair
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Acute Hip Flexor Strain in Footballer
"A 24-year-old male soccer player presents with acute anterior hip/groin pain after kicking during a match yesterday. He felt a sudden sharp pain and had to leave the field. Today he has an antalgic gait and pain with resisted hip flexion. What is your assessment and management?"
Scenario 2: Adolescent with AIIS Avulsion
"A 14-year-old male sprinter presents with sudden anterior hip pain that occurred during a sprint start at athletics training. He heard a pop and couldn't continue. X-rays show a displaced AIIS avulsion fracture with 2.5cm displacement. How would you manage this?"
Scenario 3: Chronic Hip Flexor Pain - Not Improving
"A 28-year-old female dancer presents with persistent anterior hip pain for 6 months after an initial strain. She has completed physiotherapy but still has pain with dancing. Examination shows hip flexor weakness and a positive Thomas test. MRI shows chronic changes at the iliopsoas insertion. What is your differential and management?"
MCQ Practice Points
Anatomy Question
Q: Which hip flexor is biarticular, crossing both hip and knee joints? A: Rectus femoris - It originates from AIIS (straight head) and acetabular rim (reflected head), inserting on the tibial tuberosity via the patella. This biarticular nature increases strain risk during activities like kicking.
Avulsion Question
Q: Which muscle attaches to the AIIS and avulses in adolescent athletes during kicking? A: Rectus femoris (straight head) - AIIS avulsion is common in adolescents before apophyseal fusion (16-18 years). ASIS avulsion = sartorius.
Clinical Test Question
Q: What does a positive Thomas test indicate? A: Hip flexor contracture - With the patient supine and opposite hip fully flexed, if the tested hip rises off the bed, it indicates hip flexor tightness (positive Thomas test).
Return to Sport Question
Q: What is the key criterion for return to sport after hip flexor strain? A: Greater than 90% strength compared to uninjured side - Strength-based criteria reduce recurrence. Pain-free activity alone is insufficient - premature return leads to 30% recurrence rate.
Surgical Indication Question
Q: What displacement threshold suggests surgical consideration for apophyseal avulsion? A: Greater than 2cm displacement - While most apophyseal avulsions heal conservatively, significant displacement (over 2cm) may lead to nonunion and functional deficit, particularly in high-demand athletes.
Bursa Question
Q: What is the largest bursa in the body and where is it located? A: Iliopsoas bursa - Located between the iliopsoas tendon and hip joint capsule. Communicates with the hip joint in 15-20% of individuals. Bursitis can mimic hip flexor strain.
Australian Context and Medicolegal Considerations
Australian Practice
- AFL/NRL context: High incidence in kicking sports
- Medicare: Item numbers for diagnostic imaging
- Sports Medicine Australia: RTS guidelines
- PRP: Not PBS-funded, variable private costs
Key Considerations
- Document strength assessment before RTS
- Explain recurrence risk with early return
- Informed consent for any injections
- Adolescent avulsions need parental consent
Medicolegal Considerations
Document:
- Mechanism and grade of injury
- Explanation of recovery timeline
- Return-to-sport criteria (strength-based)
- Risks of premature return (30% recurrence)
- For adolescents: X-ray performed, avulsion excluded
Consent for surgery: Recurrence, weakness, nerve injury (LFCN), need for revision.
HIP FLEXOR STRAINS
High-Yield Exam Summary
Key Anatomy
- •Iliopsoas = psoas major + iliacus → lesser trochanter
- •Rectus femoris = AIIS → tibial tuberosity (biarticular)
- •AIIS avulsion = RF; ASIS avulsion = sartorius
- •Iliopsoas bursa = largest bursa in body
Classification
- •Grade I = under 5% fibres = 1-2 weeks
- •Grade II = 5-50% fibres = 4-6 weeks
- •Grade III = over 50% or complete = 3-6 months
- •Apophyseal avulsion: surgery if over 2cm displacement
Clinical Tests
- •Thomas test = hip flexor contracture
- •Resisted hip flexion at 90 deg = iliopsoas
- •Resisted SLR = rectus femoris
- •Ely test = RF contracture
Return to Sport
- •Greater than 90% strength vs uninjured side
- •Pain-free sport-specific activity
- •Full ROM, no contracture
- •Premature RTS = 30% recurrence
Complications
- •Recurrent strain: 15-30% (early RTS risk)
- •Chronic weakness: 10-20% (Grade III)
- •Myositis ossificans: rare, avoid early massage
- •LFCN injury: surgical risk