Leg Extensions is a MUST For ACL Rehab
Your best friend after ACL Surgery - and getting back to sport stronger.
Here's a scenario that has been played out in ACL rehab.
In the past - as an athlete approaches 4 weeks post-reconstruction. They’re walking fine. No swelling. Moving well. And then someone — a surgeon, a physio, even a well-meaning gym instructor — tells them to avoid the leg extension machine. “It’ll stress the graft.”
The fear makes intuitive sense. The graft is healing.
“Why would you isolate the quad and pull on it directly?”
But as time goes on and as evidence builds - the story begins to point in the other direction. And a 2023 cohort study by Forelli et al. makes it harder than ever to justify that restriction.
What the study actually did.
103 patients post-ACLR (hamstring tendon graft).
with patients being active individuals and wanting to return to sport.
Two groups.
One did the usual closed kinetic chain work — squats, leg press, step-ups.
The other added open kinetic chain exercises — leg extensions and seated leg curls on an isokinetic machine — starting at 4 weeks post-op.
Frequency: 3x per week.
Prescription:
Isokinetics @ 60°/sec: 10 sets of 8 reps
Traditional Knee Extension Machine (0-90deg ROM): 8 sets of 8 reps
Tempo: 3s concentric, 1s hold, 3s eccentric
Load: 60% max, re-tested weekly with a dynamometer.
Both groups were tested at 3 and 6 months using isokinetic dynamometry.
The researchers looked at two things:
Quad and Hamstring strength (via LSI and peak torque to bodyweight ratio)
Graft laxity — measured with an automated GNRB arthrometer.
What did they find?
At 3 months Post Op:
Quad LSI: 76% (OKC + CKC) vs 47% (CKC Only)
At 6 months Post Op:
Quad LSI: 91% (OKC + CKC) vs 62% (CKC Only)
No significant difference in graft laxity between groups at either 3 or 6 months. The graft was fine.
Hamstring strength showed a similar pattern — the OKC group had significantly higher hamstring symmetry at both timepoints too. That matters because the hamstring graft is literally the tissue being used to rebuild the ACL. And yet loading it early, progressively, didn't loosen the knee.
But why does the quad gap matter so much?
Quadriceps weakness after ACLR isn’t just a performance issue. It’s a safety issue. Lower quad strength at return to sport is one of the best predictors of re-injury risk — some data puts it as high as a 4x increased risk when LSI is below 90% at clearance.
The quads are extremely important in producing braking forces (alongside the soleus - which also needs to be strengthened) during deceleration and play a large role in vertical jump.
The previous rehab model of “CKC only, avoid leg extensions” leaves athletes at 6 months with a quad LSI around 62%. That’s not close to 90%. That’s not a safe return to sport number.
This is why I keep saying: pain-free isn’t the finish line. Getting from 62% to 90% quad symmetry takes months of targeted loading — and the earlier you start building that base, the better positioned you are when the clock runs down.
"but the hamstring graft is at risk" argument?
Well this study used hamstring grafts. And found no increased laxity. A 2024 follow-up study (also by the same French group) replicated the finding. The concern about hamstring graft vulnerability with early OKC may be more theoretical than real.
The criteria that mattered
The OKC group in this study didn’t just start leg extensions at 4 weeks automatically. There was a clinical criteria that had to be cleared — and it could be the difference between safe early loading and recklessness.
The criteria:
Stroke test <1+ — minimal or no knee effusion
Range of motion (0-110 deg)
Single leg raise without lag — basic quad function/activation present
Graft laxity at 134N < 1.55mm — graft is not already loosening
This is criteria-based progression, not time-based permission. There's a meaningful difference. A patient who hits 4 weeks but still has a swollen, stiff knee with no active quad control doesn't start leg extensions. A patient who clears all four criteria does.
What this means in practice…
The takeaway isn’t “give everyone leg extensions at 4 weeks.” The takeaway is that the restriction against early OKC after ACLR — especially with a hamstring graft — is not well-supported by the evidence, and the cost of waiting is a significant strength deficit at a critical window.
The goal is always the same: get you back to what you love, ready for it — not just structurally healed, but strong enough to trust your knee on the field again. That gap between “the graft is healed” and “I’m actually ready” is where most ACL re-injuries live.
Early OKC, done right, helps close it.
Concluding Thoughts
Hopefully, you enjoyed and/or learned something this post. It has been fun personally to learn more and share what I have learned about ACL rehab — as it has been a clinical interest of mine for a long time. And something that I’ve been thinking about doing a PhD in, in the future.
Until next time,
Raymond Lau - (@ray.therehabchiro)
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