🏃 Patellofemoral Pain Syndrome (Runner's Knee) Simulator

Normal Patellar Tracking

A healthy kneecap glides straight down a centered groove.

• 5-7 mm: Trochlear groove depth (guides patella centrally)
• 10-14°: Normal Q-angle (measured hip to shin)
• ~2-6 cm²: Patellofemoral contact area (grows with flexion)
• ~25%: Lifetime PFPS prevalence (most common knee complaint)

The patellofemoral joint

The patella rides in a bony groove on the front of the femur.

A centered patella spreads load evenly across smooth cartilage.

What keeps it centered

Groove depth, ligaments, and balanced quad pull share the job.

Why runners care

Repetitive knee bending magnifies any small tracking error.

Q-Angle and Lower-Limb Biomechanics

A wider Q-angle bends the quad pull line laterally.

• ASIS-patella-tibia: Q-angle formula (measured in degrees)
• ~17°: Female average Q-angle (wider pelvis, higher risk)
• >20°: Elevated risk threshold (linked to maltracking)
• +3-5°: Foot pronation link (effective Q-angle increase)

Defining the Q-angle

It is the angle between the quad line and patellar tendon.

A steeper angle adds a constant lateral tug on the kneecap.

Pelvis and femur factors

Wider hips and femoral internal rotation both raise the angle.

Foot pronation adds up

Flat, overpronating feet rotate the tibia and worsen tracking.

Quadriceps Imbalance and VMO Weakness

A weak VMO leaves the lateral quad pull unopposed.

• ~55°: VMO fiber angle (pulls patella medially)
• ~12-15°: VL fiber angle (pulls patella laterally)
• ~50 ms: VMO activation lag (in symptomatic knees)
• 6-8 wks: Strengthening timeline (typical rehab window)

A tug-of-war of muscles

VMO pulls medially while vastus lateralis pulls laterally.

Even small VMO weakness lets lateral forces dominate.

Why VMO weakens first

It is the last muscle recruited and first to atrophy.

Rehab implication

Targeted VMO strengthening restores medial pull balance.

Lateral Patellar Maltracking

The kneecap drifts and grinds against the lateral condyle.

• 10-30°: Peak deviation window (early knee flexion)
• +40-60%: Lateral contact pressure (vs centered tracking)
• "J-sign": Classic sign (lateral jump near extension)
• ~4-5 mm: Cartilage thickness (patella) (thickest joint cartilage)

The lateral shift

Combined Q-angle and weak VMO push the patella outward.

Friction concentrates on a small lateral facet zone.

The J-sign

Near full extension the patella visibly jumps laterally.

Repetitive load damage

Every stride re-grinds the same irritated cartilage patch.

Anterior Knee Pain (Runner's Knee)

Irritated cartilage causes pain with stairs and squatting.

• Stairs, squats, sitting: Pain trigger activities ("theater sign")
• ~1 in 4: Runners affected annually (common overuse injury)
• ~80-90%: Conservative recovery rate (with rehab, no surgery)
• 6-12 wks: Typical recovery time (with corrected mechanics)

Where the pain comes from

Subchondral bone and synovium sense the cartilage irritation.

Cartilage itself has no nerves; surrounding bone does.

Everyday pain triggers

Stairs, squats, and prolonged sitting load the joint most.

Breaking the cycle

Correcting Q-angle load and VMO strength resolves most cases.