Open kinetic chain quadriceps after ACL reconstruction: history, myth, and current reality

Why are we still talking about this?

Thirty years ago, after anterior cruciate ligament reconstruction, open kinetic chain exercise for the quadriceps was considered dangerous. The fear was clear: the anterior tibial shear force generated by open-chain knee extension would stress the graft and jeopardize healing.

From that principle, applied without nuance, came protocols that for years excluded any isolated quadriceps work in the first 4–6 months. The price of that caution was a quadriceps strength deficit still often present two years after surgery — an independent predictor of re-injury and poor functional outcome.

Today we have data we did not have in 1995. It is time to reframe the original question: not whether open kinetic chain is safe, but when, at what load and in which sequence to integrate it into rehabilitation.

A weak quadriceps is not the inevitable consequence of surgery: it is the consequence of how we rehabilitated it.

Snyder-Mackler L., FisioScience editorial

The history of a clinical dogma

The fear of open kinetic chain exercise stems from in vitro biomechanical models of the 1980s and from an overly conservative reading of cadaveric work. In those studies, resisted extension with the knee near full extension generated an anterior load on the native ACL in a range between 0° and 30°.

Translating that data to the living graft during ligamentization, and to the loads actually applied in the clinic, is a logical leap that has been taken for granted over the years. Beynnon and Fleming, as early as the late 1990s, showed in vivo loads comparable between low-load open-chain extensions and a single-leg squat.

The dogma persisted because it was convenient: prescribing leg extension was perceived as risky, while a squat was ‘functional' by definition. In truth, either exercise can be safe or wrong, depending on how it is dosed.

What the evidence really says

Trials over the last fifteen years converge on one point: early introduction (from 4 to 12 weeks) of open kinetic chain quadriceps work does not worsen graft laxity measured with an arthrometer, does not increase the re-rupture rate, and produces greater strength gains compared with protocols that delay it beyond six months.

The effect on strength is particularly evident on the distal portions of the quadriceps and on terminal-extension control, where the closed-chain recruitment pattern is not enough to close the deficit.

How I translate it into the clinic

In practice, I use three criteria to decide when to introduce open kinetic chain: absence of significant effusion, full passive extension, and the ability to generate an isometric quadriceps contraction at 60° with at least 70% of the contralateral limb.

The progression starts with low-intensity multi-angle isometrics, moves to concentric extensions in a reduced range (90°–45°) from the sixth week, and progressively opens the range towards full extension no earlier than twelve weeks, with load guided by RPE and pain.

The machine is not the problem. The problem is the wrong dose at the wrong time.

Knee extension, when properly dosed, is a tool. When demonised, it becomes a hole in the program.

Snyder-Mackler L., 2024

Take-home for the clinician

Open kinetic chain exercise for the quadriceps is neither dangerous nor miraculous. It is a tool that, integrated into a progressive strength program, helps close a deficit that otherwise remains chronic.

If the patient has residual effusion, incomplete extension or still-inhibited volitional contraction, this is not the time. If those criteria are met, avoiding it ‘just to be safe' is no longer prudence: it is a dosing error.