Updated May 2026. Written by the Upwell Health Collective clinical team. Clinically reviewed May 2026. Next review due November 2026. For educational purposes only.
Ask any athlete who has had ACL reconstruction what they were told about knee extensions and the answer is almost always the same: don't do them. Or at least: not for a long time. The seated leg extension machine — the one that isolates the quadriceps with the foot free, pushing a pad from a bent knee toward full straightening — has been treated as contraband in ACL rehabilitation clinics for the better part of three decades.
The reasoning sounds logical on the surface: open kinetic chain (OKC) knee extension produces anterior shear force on the tibia. Anterior shear force stresses the ACL. Therefore, OKC knee extension after ACL reconstruction must be dangerous. Don’t do it. Do squats instead. Do leg press. Keep the foot on the ground. Stay in the closed chain.
Here is the problem: the evidence does not support this. And the evidence against avoiding OKC knee extensions has been building for over 20 years — accelerating significantly since 2020 — to the point where continuing to categorically ban knee extensions after ACL reconstruction is not conservative clinical practice. It is outdated clinical practice, and it is costing patients quadriceps strength, delaying return to sport, and potentially increasing long-term re-injury risk.
This article explains the biomechanics, the evidence, the risks that are real versus the risks that aren’t, and the criteria-based framework for safely integrating OKC knee extensions into ACL rehabilitation from as early as two to four weeks post-surgery.
Before going further, clarity on terms matters.
Open kinetic chain (OKC) exercises are those in which the distal segment — the foot — is free to move. The seated knee extension machine is the canonical example. Single-leg hamstring curls, straight-leg raises, and terminal knee extensions with a resistance band at the ankle are also OKC movements. The defining feature is that the foot is not fixed against a surface.
Closed kinetic chain (CKC) exercises are those in which the distal segment is fixed. Squats, leg press, lunges, step-ups, and single-leg squats are all CKC. The foot contacts a surface and the load is distributed across multiple joints simultaneously.
The historical argument was that CKC exercises produce hamstring co-contraction, which reduces anterior tibial translation, which reduces ACL strain. OKC exercises — particularly terminal knee extension approaching 0 degrees — produce anterior shear forces without adequate hamstring counterforce, therefore stressing the ACL graft. The argument was compelling. The problem is that it was based predominantly on biomechanical modelling, extrapolated to clinical practice without adequate clinical outcome data, and has since been substantially contradicted by direct ACL strain measurement and clinical trials.
The critical biomechanical finding is from Beynnon et al., who placed strain gauges directly into the ACLs of normal subjects and measured in vivo ACL strain during various activities. The key results:
ACL strain during OKC leg extension in the safe range is less than one-third of the strain experienced during normal walking (Criteria-Based Decision Making for Introducing OKC Exercise after ACLR, PMC11993518, 2025). This single finding fundamentally undermines the argument that OKC knee extensions are categorically dangerous to the ACL graft.
The important nuance is range of motion. The strain profile is not uniform across the arc of knee extension:
At terminal extension (the last 40 degrees of the movement), quadriceps force required to extend the knee is 3–4 times greater, and there is minimal hamstring co-contraction to counterbalance anterior tibial translation. As resistance increases in this range, ACL strain increases proportionally (Beynnon et al.; Considerations with OKC Knee Extension Exercise Following ACLR, PMC8341750).
This is not an argument against OKC knee extension. It is an argument for range-of-motion restriction in early and mid-phase rehabilitation. The safe range is 90 to 40 degrees (some protocols specify 90 to 35 degrees). In this range, OKC knee extension is biomechanically safer than walking.
The clinical literature on this topic has been building for two decades and has reached a reasonably clear consensus that does not match the clinical practice of most rehabilitation providers.
The foundational clinical fear is that OKC knee extensions will stretch or loosen the graft — producing increased anterior tibial translation and knee instability. Multiple trials and systematic reviews now directly contradict this:
The fear of graft laxity from OKC exercise is not supported by the best available clinical evidence when OKC is introduced progressively and in an appropriate range of motion.
This is the finding that should be driving a complete reassessment of ACL rehabilitation practice. OKC knee extension is superior to CKC for restoring quadriceps strength in the early and mid phases of ACL rehabilitation.
A comprehensive systematic review and meta-analysis (Frontiers in Sports and Active Living, 2024) found that OKC exercises are superior to CKC for improving quadriceps strength at 3 to 4 months post-surgery, whether as part of conservative or post-surgical rehabilitation. Studies comparing CKC and OKC exercises early after ACLR consistently report superior isokinetic strength and endurance of the knee extensor muscles with OKC at 3, 6, or 12 weeks post-operatively (PMC11993518, 2025).
Why? Because CKC exercises, for all their functional advantages, do not specifically isolate the quadriceps. In a leg press or squat, the hamstrings, glutes, and calf contribute to the movement. The quadriceps do not receive the same isolated stimulus. After ACL reconstruction — where the quadriceps is specifically, severely, and persistently inhibited by arthrogenic muscle inhibition — an isolated stimulus matters enormously.
A 2025 scoping review (PMC11993518) found that OKC strengthening, introduced appropriately, reduces the risk of anterior knee pain — the donor-site morbidity complaint that is most associated with bone-patellar tendon-bone (BTB) grafts in particular. This is a clinically important finding for BTB patients who already face the highest risk of anterior knee pain and who most need quadriceps strengthening to offload the patellofemoral joint.
The stakes of this debate are not abstract. They are directly connected to re-injury rates, return-to-sport failure, and long-term osteoarthritis risk.
After ACL reconstruction, the quadriceps does not simply weaken from disuse. It is actively inhibited. Arthrogenic muscle inhibition (AMI) is a neurally-mediated reflex inhibition of the quadriceps following joint injury or surgery. It is triggered by effusion, inflammation, and altered afferent signalling from the joint — and it produces quadriceps activation failure that is independent of, and in addition to, simple muscle atrophy.
A 2026 cohort study from the SANTI Study Group (Sonnery-Cottet et al.) found that AMI occurs in over half of patients with acute ACL injuries and persists into the early post-operative period. AMI produces knee extension deficit, quadriceps activation failure, and gait abnormality. A scoping review of AMI interventions confirmed that it is associated with gait abnormality, long-term quadriceps atrophy, poor function, dynamic instability, persistent knee pain, and early osteoarthritis (PMC6579490).
Most critically: AMI does not resolve spontaneously. It requires specific, targeted intervention. And the primary intervention that directly addresses AMI-related quadriceps inhibition is progressive, loaded, isolated quadriceps strengthening — which is precisely what OKC knee extension provides.
Even at the point of clinical return to sport at 9 to 12 months, quadriceps strength deficits of 15 to 30% on the operated limb are common. Patients who return to sport with these deficits have altered biomechanics, asymmetric loading, and meaningfully elevated re-injury risk. A 2025 narrative review on AMI and return-to-sport decision-making confirmed that deficits in voluntary quadriceps activation have been reported months after ACLR, that AMI does not necessarily resolve naturally, and that it must be specifically addressed through targeted rehabilitation (PMC12027548).
Withholding OKC knee extension from these patients — in the name of graft protection that the evidence does not support — means withholding the most effective tool for resolving a deficit that directly drives re-injury risk.
This bears repeating because it is the single most important datum in this debate: ACL strain during OKC leg extension in the 90–40 degree range is 3.8%. ACL strain during normal walking is 13%. Every patient who walks to the clinic is already placing more strain on their graft than they would during a properly performed OKC knee extension. The argument that OKC is too dangerous to use is therefore not an argument about graft protection during exercise. It is an argument that patients should not be allowed to walk.
This reductio is not facetious. It exposes how dramatically the fear of OKC has been disconnected from the actual biomechanical reality of what is happening during rehabilitation and daily life.
Understanding why the OKC prohibition became so entrenched is useful context for understanding why it has been so hard to shift, even as the evidence accumulates.
The movement away from OKC and toward CKC exercise in ACL rehabilitation began in the mid-1980s and accelerated through the 1990s. The primary drivers were:
The CKC preference became encoded in clinical guidelines, rehabilitation textbooks, post-operative surgeon protocols, and the cultural knowledge base of sports physiotherapy globally. It was reinforced generation to generation through training, supervision, and clinical habit. When the contradicting evidence began to emerge — first from Beynnon’s direct strain measurement work, then from clinical trials showing no laxity increase and superior quadriceps recovery — it faced the substantial inertia of an established clinical paradigm.
As of 2026, the evidence has reached the point where the paradigm deserves to change. The 2025 scoping review (PMC11993518) found that 80.7% of studies in the literature now use time-based criteria for introducing OKC exercises, with a median time of just 15 postoperative days. Not weeks. Not months. Days.
The question is no longer whether to use OKC knee extensions after ACLR. The current evidence-based question is: when, how, at what range, and under what clinical criteria should OKC be introduced?
The 2025 scoping review (PMC11993518) synthesised the literature on this question and identified the key criteria employed across studies. Here is the framework that emerges:
The majority of well-designed studies introduce OKC at 2 to 4 weeks post-operatively. Some protocols introduce unloaded or minimally loaded OKC from week 2. Loaded OKC in the restricted 90–40 degree range is well-supported from week 4 onward. The old standard of waiting 12 or 16 weeks is not supported by the current evidence base.
The 90–40 degree range (some studies cite 90–35 degrees) is the evidence-based safe zone for loaded OKC in the early and mid phases of rehabilitation. This restricts the movement to the portion of the arc where ACL strain is lowest. Terminal extension (40–0 degrees) under load should be deferred until later rehabilitation phases — typically Phase 3 onward — when graft maturation and quadriceps strength recovery support it.
The following clinical criteria are broadly supported as appropriate gating criteria before OKC introduction:
OKC should follow the same progressive overload principles as any strength training:
The biomechanical argument about graft vulnerability to OKC is strongest for hamstring autograft in the very early post-operative period, because hamstring grafts undergo soft-tissue-to-bone tunnel healing (slower than bone-to-bone) and are more susceptible to graft creep under sustained load. This is not an argument against OKC for HT patients — it is an argument for careful range restriction and load management in the first 6 weeks. BTB patients, whose bone-to-bone fixation is more rapid, have somewhat less vulnerability at this specific point, though the 90–40 degree restriction applies to all graft types in early rehabilitation. Allograft patients — whose grafts remodel more slowly and are more susceptible to creep — warrant the most conservative OKC introduction timeline.
Blood flow restriction (BFR) training combined with OKC knee extension is arguably the most evidence-backed early rehabilitation tool for addressing the quadriceps deficit after ACLR. BFR applies a cuff at the proximal thigh to partially restrict venous outflow during exercise, producing a metabolic environment that drives hypertrophy and strength gains at loads of 20–30% of 1RM — loads that are entirely safe for the healing graft in the restricted 90–40 degree range.
The combination solves both problems simultaneously: the low absolute load protects the graft from excessive strain, while the BFR-mediated metabolic stimulus delivers the muscle activation and hypertrophic signal needed to overcome AMI and build quadriceps volume. At Upwell, BFR is applied as standard from Phase 1 onward — not as an optional add-on, but as a core quadriceps recovery tool for all ACLR patients.
If you are currently in ACL rehabilitation and have been told not to do knee extensions, or to wait until month four or five before introducing them, the following questions are worth raising with your clinician:
What is the specific clinical rationale for avoiding OKC in my programme? The evidence-based answer should reference graft type, time from surgery, clinical criteria (swelling, extension, pain), and range-of-motion considerations — not a blanket prohibition.
How is my quadriceps strength and AMI being assessed and managed? If the answer is “we’re doing squats and leg press,” that is a partial answer. It does not specifically address the isolated quadriceps deficit and the neural inhibition driving it.
What is my current LSI for knee extension strength, and what is the plan to get it to 90%+? This number matters for return-to-sport clearance. If it is not being tracked, ask why.
Is BFR being used in my programme? If not, it should be discussed. The combination of BFR and early OKC knee extension is the most evidence-supported approach to early quadriceps rehabilitation after ACLR.
The evidence-based position in 2026 is clear. OKC knee extension after ACLR is safe, effective, and superior to CKC-only approaches for quadriceps recovery. The blanket prohibition on OKC knee extensions that has been standard practice for 30 years is not supported by direct strain measurement, clinical laxity outcomes, or strength outcomes data.
The clinician’s job is not to avoid OKC. It is to introduce OKC appropriately — at the right time, in the right range, with the right load, in response to the right clinical criteria. That is criteria-based practice. It requires clinical skill and ongoing assessment. It is harder than a blanket prohibition. It also produces better outcomes.
The key criteria for safe OKC introduction are not complex:
This is not a controversial clinical position. It is what the current evidence says. The controversy is between the evidence and the practice — and the job of evidence-based clinicians is to close that gap.
At Upwell, OKC knee extensions are a standard component of ACL rehabilitation from Phase 1 onward, introduced according to clinical criteria and progressed according to tissue response and strength data. Our position is based directly on the evidence summarised in this article:
If you are a patient wondering why your previous rehabilitation felt like it stalled at the quad, or why your leg still feels weak six months post-surgery, this is the most likely explanation. And it is addressable.
Contact our team or book an assessment if you want to discuss your current programme or get an objective assessment of where your quadriceps recovery is at. VALD force plate testing gives us bilateral strength data to the decimal point — so we know exactly what we’re dealing with and can build the programme around the actual deficit, not the estimated one.
This article is for educational purposes only and does not substitute for individual clinical assessment. If you are currently in ACL rehabilitation, please work with a qualified physiotherapist for personalised guidance. Information last reviewed May 2026.