Assessing VMO–Rectus Femoris Synergy in ACL Injury Patients

28.07.2025 Muscle synergies

By Uruguay Clinic

When it comes to rehabilitation after an anterior cruciate ligament (ACL) injury, one of the most important aspects for restoring proper knee function is the recovery of muscle synergy between the vastus medialis (VMO) and the rectus femoris.

In this guide, you’ll learn how to identify neuromuscular imbalances after injury, which tests to use with surface electromyography (EMG), and how to personalize treatment based on the data you collect.

By the way, want to start using EMG with your ACL patients and improve your outcomes today?
👉 Contact us here to find out how.

What’s the Expected Synergy in a Healthy Knee?

During knee extension movements (e.g., squats, lunges, or isometric extensions), the most efficient neuromuscular pattern is:

✅ Vastus medialis > Rectus femoris

Stronger VMO activation is essential because this muscle helps keep the patella aligned during movement.

By pulling it slightly medially, it prevents lateral drift—critical for reducing imbalances in the extensor mechanism and minimizing the risk of femoropatellar pain or injury.

However, due to its biarticular nature, the rectus femoris tends to dominate in many patients unless synergy is actively retrained.

What Happens After an ACL Injury?

Whether post-surgical or non-operative, it’s common to observe disruptions in this muscle synergy:

VMO inhibition (arthrogenic or central)
Rectus femoris inhibition, or…
Rectus femoris dominance in the absence of proper medial control

Consequences of Altered Synergy

❌ Joint instability
❌ Anterior knee pain
❌ Slow or stalled rehab progress
❌ Higher risk of reinjury or functional failure

How to Assess VMO–Rectus Femoris Synergy with EMG

To detect these alterations objectively and personalize rehab, follow this clinical protocol:

STEP 1: Measure Activation of Both Muscles

Use surface EMG to record the activation levels (in microvolts) of the VMO and rectus femoris during functional tasks.

STEP 2: Compare Healthy vs. Affected Side

Look for:

Absolute differences in activation
Altered synergy pattern (e.g., rectus femoris > VMO)
Hidden deficits during real-life or sport-specific tasks

Recommended Tests for Synergy Assessment

1️⃣ Isometric Knee Extension

Goal: Identify whether the VMO is less active than the rectus femoris on the injured side.

Common Results:

– Right rectus femoris > Right VMO (❌)
– Right VMO << Left VMO (❌)

Diagnosis: VMO inhibition on the affected side

2️⃣ Weighted Squat

Goal: Assess motor patterns during a functional movement.

Common Results:

– Right rectus femoris > Right VMO (❌)
– Both muscles inhibited compared to healthy side (❌)

Diagnosis: Protective pattern with bilateral deficits on the injured side

3️⃣ Weighted Lunges

Goal: Analyze a more dynamic and demanding gesture.

Common Results:

Right rectus femoris > Right VMO (❌)
Reduced activation of both muscles vs. healthy side (❌)

Diagnosis: Ongoing inhibition during functional load-bearing tasks

How to Intervene Based on EMG Findings

Once the deficit is identified, build your treatment plan around these pillars:

1. Strength Training

– Targeted quadriceps exercises: isometric, isotonic, partial weight-bearing
– Unilateral and functional work: lunges, step-ups, leg press with feedback