DO NOT Avoid Leg Extensions After Your ACL Surgery
By: Tanner Bishop — Physiotherapist, CSCS, MSc PT, BSc Kine, NCCP Coach
Someone has probably told you to avoid the leg extension machine after ACL reconstruction. The reasoning sounds sensible: the exercise strains your new graft, so leave it alone.
The evidence does not support that advice. It has been tested repeatedly, and the fear does not hold up.
Meanwhile, avoiding quad work in the first twelve weeks costs you something real. Your quadriceps start shutting down and shrinking almost immediately, and the deficits you accumulate early are the hardest ones to claw back later. So the advice meant to protect your knee ends up weakening it.
Here is what the research actually shows, and how I use leg extensions from the first weeks after surgery.
Where the Fear Came From
The concern is not invented. Loaded knee extension does increase strain on the ACL, and that strain is highest in the last part of the range, as your knee approaches full extension. That is a real mechanical finding.
The mistake was turning a range-specific caution into a blanket ban. The strain concentrates between roughly 40 degrees and full straight. Above that, in the deeper part of the range, the loading is far more forgiving. Banning the entire exercise to avoid one portion of it is like refusing to drive because highways are faster than side streets.
What the Evidence Says About Safety
This question has been studied directly, and the answer keeps coming back the same.
Perriman and colleagues pooled the randomized trials comparing open-chain quad work against closed-chain exercise after ACL reconstruction. They found no meaningful difference in knee laxity between the two — the graft did not loosen in the open-chain groups. What they did find was better quadriceps strength, and less pain, in the people doing the open-chain work.
A more recent systematic review reached the same conclusion, and reported no increased risk of clinical laxity at any time point measured. Forelli and colleagues then tracked graft laxity directly with an automated arthrometer in patients doing early leg extensions, including patients with hamstring grafts, and found no harm.
So the concern has been tested with the tools designed to detect exactly the thing people are worried about, and the loosening does not show up.
The Aspetar expert panel now recommends starting open-chain knee extension at around week four, with roughly eighty-eight percent agreement among the clinicians involved. This is no longer a fringe position.
Why Waiting Costs You
Your quadriceps do not sit patiently while you protect the knee. Two things happen at once.
The first is arthrogenic muscle inhibition — your nervous system reduces the signal reaching the quad because the joint is swollen and painful. It is a reflex, not a choice, and it shows up in as many as half of patients within six weeks. The second is straightforward atrophy, the muscle physically getting smaller.
The combination is stubborn. Quadriceps volume deficits are still measurable six to twelve months after surgery, and activation deficits have been documented as far out as two years. Those first twelve weeks are when the hole gets dug — and the longer you leave it, the longer the climb back.
That is the real trade-off. Not safety versus risk, but strength versus weakness.
The Meniscus Repair Problem
This is where the ban does the most damage.
If you had a meniscus repair alongside your reconstruction, you are likely under weight-bearing restrictions. That rules out most of the closed-chain options — squats, split squats, leg press, step-ups all need you to load through the foot.
Take away leg extensions too, and you have essentially nothing left for the quad during the exact window when it is disappearing fastest. Open-chain extension lets you load the muscle without loading the joint through the foot. In my practice, that makes it the primary quad exercise for these patients, not an optional extra.
How I Actually Program It
Start in the deep range. From ninety degrees of knee flexion up to about forty-five degrees, keeping load moderate. This is the forgiving part of the range, and it lets you build real strength while the graft is at its most vulnerable.
Progress the load before you progress the range. Get strong in that arc first.
Around the twelve-week mark, begin extending into the fuller range as strength and healing allow. This is where clinical judgment matters — graft type, meniscus status, swelling, and how your knee responds all feed into the timing.
One honest caveat: if your graft came from your hamstring tendon, some guidelines still advise more caution with added load. The direct laxity data has not shown harm even in these patients, but the debate is not fully settled. Worth a conversation with your surgeon and physio rather than a rule applied blindly.
The Bottom Line
Avoiding leg extensions after ACL reconstruction is outdated advice built on a partial reading of the mechanics. Done in the right range, with sensible load, the exercise is safe — and it is one of the most effective tools available for the problem that actually threatens your outcome, which is quadriceps weakness.
If you have been told to stay off the machine entirely, ask why. If the answer is "it strains the graft," that is worth a longer conversation.
References
Forelli, Florian, Wassim Barbar, Gwendal Kersante, Amaury Vandebrouck, Pascal Duffiet, Louis Ratte, Timothy E. Hewett, and Alexandre J.M. Rambaud. "Evaluation of Muscle Strength and Graft Laxity With Early Open Kinetic Chain Exercise After ACL Reconstruction: A Cohort Study." Orthopaedic Journal of Sports Medicine 11, no. 6 (2023). https://pmc.ncbi.nlm.nih.gov/articles/PMC10334004/
Perriman, Angela, Edmund Leahy, and Adam I. Semciw. "The Effect of Open- Versus Closed-Kinetic-Chain Exercises on Anterior Tibial Laxity, Strength, and Function Following Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis." Journal of Orthopaedic & Sports Physical Therapy 48, no. 7 (2018): 552–566.
Escamilla, Rafael F., Toran D. MacLeod, Kevin E. Wilk, Lonnie Paulos, and James R. Andrews. "Anterior Cruciate Ligament Strain and Tensile Forces for Weight-Bearing and Non-Weight-Bearing Exercises: A Guide to Exercise Selection." Journal of Orthopaedic & Sports Physical Therapy 42, no. 3 (2012): 208–220.
"Considerations with Open Kinetic Chain Knee Extension Exercise Following ACL Reconstruction." International Journal of Sports Physical Therapy. https://ijspt.scholasticahq.com/article/18983
Effect of Open Kinetic Chain Exercises During the First Weeks of Anterior Cruciate Ligament Reconstruction Rehabilitation: A Systematic Review and Meta-analysis. Physical Therapy in Sport (2025). https://www.sciencedirect.com/science/article/pii/S1466853X25000355
"The Effect of Physiotherapy on Arthrogenic Muscle Inhibition After ACL Injury or Reconstruction: A Systematic Review." https://pmc.ncbi.nlm.nih.gov/articles/PMC11678747/
Sonnery-Cottet, Bertrand, et al. "Arthrogenic Muscle Inhibition After ACL Reconstruction: A Scoping Review of the Efficacy of Interventions." British Journal of Sports Medicine 53, no. 5 (2019): 289–298. https://pmc.ncbi.nlm.nih.gov/articles/PMC6579490/
Kuenze, Christopher, et al. "Muscle Atrophy Contributes to Quadriceps Weakness After Anterior Cruciate Ligament Reconstruction." Journal of Science and Medicine in Sport 18, no. 6 (2015). https://pmc.ncbi.nlm.nih.gov/articles/PMC4500756/
"Persistent Quadriceps Muscle Atrophy After Anterior Cruciate Ligament Reconstruction Is Associated With Alterations in Exercise-Induced Myokine Production." https://pmc.ncbi.nlm.nih.gov/articles/PMC9263390/
van Melick, Nicky, et al. "Evidence-Based Clinical Practice Update: Practice Guidelines for Anterior Cruciate Ligament Rehabilitation Based on a Systematic Review and Multidisciplinary Consensus." British Journal of Sports Medicine 50, no. 24 (2016): 1506–1515.
This article is educational content and not individual medical advice. Rehabilitation after ACL reconstruction depends on your specific surgery, graft type, associated injuries, and healing response. Work with your surgeon and physiotherapist to determine what is appropriate for you.
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