Leg Extensions: Rectus Femoris Length-Tension, Knee Joint Forces, and Knee Safety

Are leg extensions bad for knees? For healthy knees, the research does not say so. The exercise loads the knee differently from a squat, and it is one of the few movements that reliably grows the rectus femoris, the quad muscle that squats and leg presses tend to miss.

This article sits inside our complete lower-body and core training guide. Here we stay on one machine: what the knee actually experiences, why the rectus femoris loves this exercise, how to set the seat, and when to be careful.

Are leg extensions bad for knees?

The “leg extensions ruin your knees” line comes from a simple observation. In a leg extension the shin swings freely and the resistance sits at your ankle. That is called an open kinetic chain exercise, and it loads the knee joint differently from a squat, where your foot is fixed to the floor. We cover that distinction in depth in open vs closed kinetic chain for hypertrophy.

What does the biomechanics research say? In a classic lab study, ten men did squats, leg presses and knee extensions at their 12-rep max while researchers estimated joint forces. Patellofemoral compressive force (the squeeze between kneecap and thigh bone) peaked near full knee bend in the squat, and in the mid-range of the extending phase in the knee extension (Escamilla et al., 1998). Tension in the ACL showed up only in the open chain exercise, and only near full extension.

Read that carefully. It says the forces are different and sit at different angles. It does not say they are harmful. This was ten men in a single session of three reps, with modelled (not directly measured) internal forces. Nobody followed lifters for years to see whose knees broke down.

What you can control is execution. Knees generally adapt to load that is built up gradually. Jumping to the whole stack, kicking the weight up with momentum and slamming the lockout is a poor bet for any joint, and it is the part of this exercise worth worrying about.

Why the rectus femoris needs leg extensions

Your quadriceps has four heads. Three (the vasti) only cross the knee. The fourth, the rectus femoris, also crosses the hip. That detail changes everything about how to train it.

In a squat or leg press, your hips and knees extend together. The rectus femoris is stretched across the hip while it shortens at the knee, so it contributes less. In a leg extension, the hip stays put and only the knee moves, so the muscle gets a direct, loaded contraction. The 1998 lab study found exactly that pattern: open chain knee extension produced more rectus femoris activity, closed chain exercises more vasti activity (Escamilla et al., 1998).

Do the growth trials agree? Yes, and the gaps are large.

• Versus leg press (12 weeks, MRI). Seventeen untrained adults trained one leg with knee extensions and the other with the leg press. Rectus femoris volume rose 13.2% with knee extensions versus 1.1% with the press. Vasti growth (about 5.0-7.2% vs 4.4-6.2%) and whole-quad growth (7.1% vs 4.9%) were comparable (Kinoshita et al., 2026).
• Versus back squat (8 weeks, ultrasound). Sixty-three untrained young women did either a Smith machine squat or leg extensions, 3 sets of 8-12, twice a week. Leg extensions grew the rectus femoris more at every site measured (for example +17.5% vs +7.9% distally). The squat grew the distal vastus lateralis more (+18.2% vs +11.2%) (Kassiano et al., 2026).

The authors of the MRI trial put it plainly: the leg press is time efficient for the vasti and glutes, but knee extension is essential for effectively targeting the rectus femoris (Kinoshita et al., 2026).

One caveat applies to both trials. Participants were untrained, and the interventions lasted 8-12 weeks. Trained lifters may respond differently, so treat the sizes as indicative rather than guaranteed. If you want the other end of the lengthened-quad spectrum, see sissy squats and reverse Nordic curls.

Do you need leg extensions at all?

Not always. A 2017 review of single- versus multi-joint exercises concluded that lifters may not need single-joint work to get equivalent muscle activation and hypertrophy, and that single-joint exercises may mainly be needed for muscles like the lumbar extensors or for fixing imbalances (Gentil et al., 2017).

How does that square with the rectus femoris data? The review looked at prime movers and overall size, and its long-term hypertrophy comparisons came mainly from upper-limb studies. The newer trials look at individual quad heads with imaging. Both can be true: a squat or press pattern can grow the quads well overall while the rectus femoris lags.

So here is a cautious takeaway. If you are short on time, a leg press or hack squat covers most of the quad. If you want complete development, or you notice your upper thigh looks underdeveloped compared with your outer quad, add leg extensions. For a squat-focused comparison, see hack squats and quad stimulus-to-fatigue.

Seat angle: why leaning back may help

Most machines have an adjustable backrest, and most people never touch it. Because the rectus femoris crosses the hip, the hip angle changes how stretched it is. A more reclined backrest opens the hip and puts the muscle on a longer length before you start.

A 2025 trial tested this directly. Twenty-two untrained men trained each leg with a different hip angle, 90 degrees versus 40 degrees of hip flexion, for ten weeks, two sessions a week. The 40-degree (more reclined) side showed more rectus femoris hypertrophy, while the vastus lateralis grew the same either way (Larsen et al., 2025). The authors suggest reduced hip flexion if you want to maximise rectus femoris growth, and note that both options are viable for the quads overall.

Two honest limits. It was untrained men over ten weeks, and the machine settings in your gym will not map exactly onto those angles. So treat it as a direction, not a prescription: recline the backrest a few notches, keep your hips planted and see how it feels.

Leg extension form cues

Good form here is mostly about removing momentum and respecting the knee. The steps below are practical coaching guidance built around the seat-angle finding, not a protocol from one study.

• Line up the pivot. Your knee joint should sit level with the machine’s axis. A pad that is misaligned drags the knee around a path it does not want to take.
• Pad on the lower shin. Just above the ankle bone, not on the top of the foot.
• Stay glued to the seat. Hips down, hands on the handles. If your hips lift, the load is too heavy or the seat is too far forward.
• Lift, pause, lower. Squeeze for a beat at the top without slamming into lockout. Take roughly 2-3 seconds on the way down.
• Use a range you control. A systematic review found that full-range training favours lower-body muscle growth compared with partial range (Schoenfeld & Grgic, 2020). That review did not test leg extensions, so apply it sensibly: go as deep as you can without the weight bouncing.
• Toes neutral or slightly up. There is little good evidence that toe position changes quad growth, so pick what is comfortable.

A common mistake here is loading the stack and kicking it up. That turns a quad exercise into a momentum exercise and dumps load on a knee that is still extending at speed.

How many sets, and where it fits

The trials above used about 3 sets of 8-12 reps, 2 sessions a week (Kassiano et al., 2026), or 5 sets of 10 reps at 70% of one-rep max (Kinoshita et al., 2026). Both used untrained participants over short periods, so they are a starting range, not a ceiling.

For most lifters, 2-4 hard sets, once or twice a week, after a compound lift is plenty. Stop a rep or two shy of failure on early sets and push the last set closer. Add weight when you can hit the top of your rep range with clean form. To check your weekly total across all quad work, use the training volume calculator, and see our leg day plan for where this sits in a session.

Leg extensions with knee pain or after ACL surgery

This is where blanket advice stops working, so let’s be careful.

Patellofemoral pain (pain around or behind the kneecap). A randomized trial of 112 people with long-standing patellofemoral pain compared knee-focused exercise, hip-focused exercise and free activity, each with education. After three months, there were no differences in pain scores between groups (Hott et al., 2019). Everyone improved. The takeaway is not that leg extensions fix knee pain. It is that the choice of exercise mattered less than staying active and understanding the problem. If you have this kind of pain, work with a physio, and see our guide to knee pain and leg workout modifications.

After ACL reconstruction. A 2025 scoping review of 26 studies notes that open chain strengthening appears safe and improves quadriceps strength, but many clinicians remain reluctant because of worries about graft laxity. Median time to allow it was 15 postoperative days, with a huge range (1 to 270 days), and fewer than one in three studies added clinical criteria on top of the calendar (Forelli et al., 2025). In other words, there is no consensus timeline.

A small lab study of 15 patients after double-bundle ACL reconstruction found that a loaded (10 kg) seated knee extension produced more anterior tibial translation and anteromedial graft elongation at low flexion angles than a single-leg lunge. The authors note that some translation is physiological and does not by itself mean danger (Zhang et al., 2025).

See a doctor if you have swelling, locking, giving way, pain that wakes you at night, or pain that does not settle within a few days of backing off. Those are not “push through it” signs.

How to keep the leg extension knee-friendly

• Build load over weeks. Do not start at the top of the stack.
• Keep the lowering slow and the lockout soft.
• If front-of-knee pain appears at one angle, shorten the range slightly or lighten the load before dropping the exercise.
• Pair it with a squat or press pattern rather than relying on it alone, as in the back squat versus front squat comparison. For the wider squat-and-knees question, see are squats bad for your knees.
• Training straight after a long commute or a desk day? Do a light set or two first so the knee is warmed up before the working sets.

The practical takeaway

Healthy lifters can use leg extensions without fear if they progress load sensibly and control the rep. They are a well-supported way to grow the rectus femoris, particularly with the backrest reclined a little. They are not mandatory for every program, and they are not the right call for a knee that is hurting or recently operated on until a professional clears it.