If you’ve had ACL surgery or you’re dealing with patellar tendon pain, you may have been told to focus on “functional” exercises like squats, lunges, and step-ups. These are hugely important particularly post ACL surgery— but they’re not the whole story.

One exercise that often gets overlooked (or avoided) is knee extension with weight. When done properly and progressed gradually, it plays a huge role in recovery of the quadriceps which are significantly weakened in both these conditions.

The Real Issue: Your Quad Muscle Isn’t Firing Properly

After ACL injury or surgery — and often with patellar tendon pain — the quadriceps muscle (front of the thigh) can become “switched off” as a result of pain.

This doesn’t mean you’re weak or lazy. It’s a normal response to:
• Pain
• Swelling
• Injury or surgery
• Your brain trying to protect the knee

Even months after injury, the quad may still not be working the way it should.

Why “Functional” or closed kinetic chain Exercises Aren’t Always Enough

Squats and lunges use lots of muscles at once. That’s great — but it also means your body can cheat.

You might:
• Shift more weight onto your good leg
• Use your hips more than your knee
• Avoid fully straightening the knee
• Move in a way that avoids loading the painful area

So even though you’re exercising, the quad on the injured side may still not be getting strong.

How Knee Extension Helps

Directly strengthens the quad without other muscles being able to “take over”.
Knee extension targets the quad muscle better than almost any other exercise. There’s no hiding and no compensating — the muscle has to work. This helps wake up the quad and restore normal strength.

Builds strength in a safe, controlled way
Because the movement is simple and supported, we can start with light weight, progress slowly and safely, control the range and speed, and adjust based on pain or confidence.

Improves confidence in your knee
Many people feel nervous loading their knee after surgery or when they’ve had pain for a long time. Knee extension helps you trust the knee again, see clear progress, and feel stronger week to week.

Especially helpful for patellar tendon pain
For patellar tendon issues, controlled loading is key. Knee extension allows steady loading of the tendon, less stress on other joints, and better pain control. When progressed properly, it can actually reduce pain over time, not increase it.

“I Heard Knee Extension Is Bad for the Knee…”

This is a very common myth.

When knee extension is prescribed at the right time, done with good technique, and loaded progressively, it is safe and beneficial for ACL rehab and tendon health.

In fact, avoiding it often leads to ongoing weakness, imbalance, and slower return to normal activity or sport.

It’s Not One or the Other

This doesn’t mean you should only do knee extension.

The best rehab programs include:
• Knee extension to rebuild quad strength
• Squats, lunges, step-ups and other functional movements for everyday movement
• Sport-specific exercises later on

Each type of exercise has a purpose.

The Take-Home Message

If your quad muscle isn’t strong on its own, it won’t work properly during more complex movements.

Knee extension helps rebuild strength, control, and confidence in the injured knee. When combined with functional exercises, it leads to better long-term outcomes and a stronger, more resilient knee.

References:

Pamboris et al. (2024) – Effect of open vs. closed kinetic chain exercises in ACL rehabilitation. Front Sports Act Living.

Jalili Bafrouei et al. (2025) – Strengthening exercises improve knee muscle strength after ACLR: systematic review & meta-analysis.

A Comprehensive RCT on OKC vs CKC post-ACL injury (OKC > CKC for quadriceps strength).

Jean LM et al. (2022) – Squat training effects on knee extensor strength in ACL-deficient individuals.

Restoring Knee Extensor Strength Clinical Commentary, outlining rationale for isolated knee extension in rehab.