GUIDE · 7 MIN READ · ACL RECOVERY

Knee Range of Motion After ACL Surgery

Range of motion — flexion and extension — is often the first measurable thing that moves after ACL reconstruction. Here's how clinicians think about it week by week, how to measure it at home, and the warning signs that need a call to your physio.

Two numbers that matter: extension and flexion

Range of motion after ACL reconstruction is measured as two angles. Extension is how straight the knee can go — a fully straight knee is 0°. Flexion is how far it bends — a knee bent heel-to-butt is roughly 135-140°, though normal full flexion varies between individuals. TODO: verify population flexion range

Progress is usually tracked as degrees away from each endpoint. "Minus 5 degrees extension" means the knee can't fully straighten and sits 5° short. "120 degrees flexion" means the knee bends to 120°.

Why full extension matters more than people realise

Both directions matter, but losing extension is the harder problem. The reason is biomechanical: without full extension, the quad can't lock the knee, which disrupts normal walking, delays strength gains, and compounds into a flexion contracture if left untreated.

Most ACL rehabilitation protocols prioritise full passive extension within the first one to two weeks. Physios will have you lie down with a towel under the ankle (heel propped, knee hanging), do prone hangs, and push extension work aggressively long before they worry about flexion. This is deliberate: a 5° extension deficit at week 2 is a much bigger clinical problem than being 20° short of full flexion. TODO: cite extension-priority protocol

Typical weekly milestones

These are common targets from standard ACL rehab protocols. Your actual numbers depend on your surgeon, your graft type, any meniscus work done at the same time, and your pre-surgery conditioning. Use these as a reference, not a scorecard. TODO: verify against named protocols (Delaware-Oslo, Melbourne, Cincinnati)

Week 0–2

• Extension: aim for full passive 0°. Prone hangs, heel props, quad sets with the knee locked.
• Flexion: aim for 90° by end of week 2. Heel slides, wall slides, stationary bike with high seat (as cleared).
• Walking: heel-strike gait with crutches, aiming for a straight knee at heel strike.

Week 2–6

• Extension: hold full 0°. Any loss here is urgent.
• Flexion: progress toward 120° by week 4-6.
• Bike loops, light leg press (if cleared), ongoing quad activation.

Week 6–12

• Flexion: progress toward full flexion (approximately 135°+). Many people are near-full by 10-12 weeks.
• Range of motion stops being the active target; the focus shifts to strength and symmetry.

Beyond week 12

Range of motion plateaus for most people around three months post-op. From here, the game is strength (quad lag, hamstring symmetry), neuromuscular control (hop quality, landing mechanics), and sport-specific loading. Range of motion still gets checked, but it's no longer the headline number.

How to measure range of motion at home

Three methods, each with trade-offs:

1. Goniometer. The clinical gold standard for home use. A plastic goniometer costs under $10. Line it up over the knee's rotation axis with the arms along the thigh and shin. Takes a few tries to feel natural.
2. Side-view photo. Phone-camera a side profile of the knee at end-range. Trace or eyeball the angle. Works best if you shoot from the same position every time.
3. Phone motion sensor. Apps (including Palio) can use your phone's accelerometer/gyro to estimate flexion by strapping it to your shin and raising the leg. Fast, repeatable, but requires the same body position for consistent numbers.

Whatever you use, consistency beats precision. A 2° measurement error is fine if it's the same error every time — the trend still tells the truth. The worst thing you can do is use a different method each session.

Common range-of-motion problems

Extension loss (flexion contracture)

The most common post-ACL complication. The knee refuses to fully straighten, often because the graft or scar tissue in the notch (cyclops lesion) is physically blocking extension. Early detection is huge — the longer it sits, the harder it is to recover without a secondary procedure.

Flexion plateau around 90-100°

Normal mid-recovery. Usually caused by swelling, soft-tissue stiffness, or scar tissue around the surgical sites. Progress is slower here but it keeps moving with consistent work.

Pain with end-range

Some discomfort at end-range is expected. Sharp pain, catching, or a locking sensation is not — those are physio-call-today signs, not "push through it" signs.