What muscle is weak when the patella moves laterally with knee bending?

Vastus Medialis Oblique Weakness Causes Lateral Patellar Tracking During Knee Bending

When the patella moves laterally during knee bending, the vastus medialis oblique (VMO) muscle is weak. This muscle imbalance is a key contributor to patellofemoral pain syndrome and abnormal patellar tracking 1.

Anatomical and Biomechanical Basis

The patellofemoral joint relies on both passive and active stabilizers to maintain proper alignment during knee movement:

• Active stabilizers: Primarily the quadriceps muscles, with the VMO serving as the main medial stabilizer of the patella 2
• Passive stabilizers: Include the patellofemoral ligaments, joint geometry, and menisci 2

When the VMO is weak or has decreased activation compared to the vastus lateralis (VL), the balance of forces acting on the patella is disrupted, causing lateral displacement during knee flexion activities.

Evidence Supporting VMO Weakness

Recent research confirms that patients with patellofemoral pain syndrome demonstrate:

• Significant atrophy of the VMO compared to healthy controls 3
• A smaller VMO/VL area ratio at the upper pole of the patella compared to individuals without patellofemoral pain 3
• Reduced VMO contraction ratio on shear wave elastography 2

This muscle imbalance leads to:

1. Abnormal patellar tracking
2. Increased lateral pressure on the patellofemoral joint
3. Pain during activities that load the patellofemoral joint (stairs, squatting, prolonged sitting) 1

Clinical Assessment

To determine if lateral patellar tracking is due to VMO weakness:

1. Observe patellar movement during active knee flexion and extension
2. Palpate the VMO to assess muscle bulk and activation during quadriceps contraction
3. Perform functional tests that load the patellofemoral joint:
   • Decline squat test (reproduces pain in patients with patellar tendinopathy) 2
   • Single leg squat (observe for medial knee collapse or lateral patellar displacement)

Treatment Approach

The primary treatment for lateral patellar tracking due to VMO weakness should focus on:

1. Targeted VMO strengthening exercises:

   • Knee extension combined with medial tibial rotation has been shown to produce higher VMO:VL activation ratios 4
   • General quadriceps strengthening combined with specific VMO exercises 3

2. Biofeedback techniques:

   • VMO biofeedback is recommended in the latest clinical guidelines for patellofemoral pain 2

3. Supportive bracing or taping:

   • Patellar taping can help reduce symptoms and improve patellar tracking 2
   • Corrective bracing may be beneficial for moderate to severe cases 2

Important Considerations

• While VMO weakness is commonly associated with lateral patellar tracking, some studies have questioned whether isolated VMO strengthening is effective 5, 6
• A comprehensive approach addressing both VMO strengthening and other biomechanical factors (hip strength, foot biomechanics) is often necessary 2
• The VMO contributes to knee extension torque throughout the range of motion, not just at specific angles 7

Pitfalls to Avoid

1. Focusing only on VMO strengthening without addressing other potential contributors to patellofemoral pain
2. Neglecting proper assessment of patellar alignment and tracking patterns
3. Overaggressive rehabilitation that may increase patellofemoral joint stress and worsen symptoms
4. Ignoring hip and foot biomechanics that may contribute to abnormal patellar tracking

By addressing VMO weakness through targeted strengthening exercises and appropriate supportive measures, lateral patellar tracking during knee bending can be effectively managed in most patients.