Leg Extension vs Squat: Which Builds More Quad Muscle?

Neither exercise wins outright for quad size. In an 8-week randomised trial of 63 untrained young women, leg extensions grew the rectus femoris more at all three measured sites, while squats grew the vastus lateralis more at its lower end. For squat strength, the squat was far ahead.

This article covers what that trial measured, how far its results stretch, and a practical way to program both exercises and log them so your own training history tells you what works for you.

Which builds more quad muscle, the leg extension or the squat?

It depends on which part of the quadriceps you mean. The best direct comparison found the leg extension grew more rectus femoris and the squat grew more of the lower vastus lateralis. Neither exercise grew everything more, so the data does not support dropping either one on growth grounds alone.

The quadriceps is four muscles. The rectus femoris crosses both the hip and the knee, while the vastus lateralis (outer quad) crosses only the knee. Kassiano and colleagues (2026, Journal of Strength and Conditioning Research) measured two of them, the rectus femoris and vastus lateralis, with ultrasound. They did not measure the vastus medialis or vastus intermedius, so the trial says nothing about those.

The authors' conclusion was:

"The leg extension induce greater rectus femoris hypertrophy, while the back squat promotes greater vastus lateralis hypertrophy, particularly at the distal site."

Kassiano et al., Journal of Strength and Conditioning Research, 2026

What did the leg extension vs squat trial find?

Sixty-three untrained young women were randomly assigned to the Smith machine back squat (30 women) or the leg extension (33 women). Both groups did 3 sets of 8 to 12 repetitions to repetition maximum, twice a week, for 8 weeks. Muscle thickness was measured at proximal, middle and distal sites of the rectus femoris and vastus lateralis.

The rectus femoris differences were all significant (all p below 0.001), as was the distal vastus lateralis difference. The study reported the other vastus lateralis sites without highlighting a between-group difference, so we are not claiming one either way.

Strength told a different story:

The squat group gained much more on the squat, which is expected because they practised it. On the leg extension test the groups did not differ (p = 0.824), so squatting built leg extension strength about as well as leg extensions did, at least in beginners.

Why might the two exercises grow different parts of the quad?

The honest answer is that nobody has proven it. The authors' results are clear but the mechanism is open. We can say the exercises load the muscles differently, and offer a cautious reading of the wider literature.

The rectus femoris is the only quad muscle that crosses the hip, so exercises that change hip position change its job. A squat moves the hip and knee together, while a leg extension isolates the knee. That is general anatomy, and it is a plausible reason the two exercises treated this muscle differently, but the trial did not test it.

A 2026 review by Pedrosa, Pereira and Kassiano looked at muscle length and exercise selection. It concluded that training at longer muscle lengths tends to favour growth when there is meaningful external torque in the lengthened position, while admitting that muscle length is hard to separate from other factors. Our full range of motion vs partial reps guide covers the related evidence, including the 2020 systematic review by Schoenfeld and Grgic that favoured full range of motion for lower-body hypertrophy.

Treat all of this as hypothesis. The trial shows the result, not the reason.

Does the squat build more strength than the leg extension?

For squat strength, yes, by a wide margin. The squat group's 3RM squat rose 46.7% against 21.3% for the leg extension group. That is simple specificity: you get better at what you practise.

For leg extension strength the two groups looked alike. That does not mean the exercises are interchangeable for performance. A lifter who wants a bigger squat should squat. A lifter who wants a bigger leg extension number mostly needs to do leg extensions, though the squat group did gain on that test too.

Because strength carries over only partly, our leg press vs squat and Smith machine vs barbell squat guides are worth reading if you are choosing your main lift.

How far can you trust one trial?

Not all the way. The result is clear but narrow, and a few features limit how far you can stretch it.

• Untrained participants. Beginners grow quickly from almost anything. Experienced lifters may respond differently.
• Young women only. The authors studied one group. We would not assume the same size of effect in everyone.
• Eight weeks. Short trials can miss slower differences.
• Smith machine squat. The squat group used a guided bar path, not a free barbell, so the result may not match a free barbell squat.
• Two muscles measured. The vastus medialis and vastus intermedius were not measured, so the full picture of quad growth is incomplete.
• A single trial. One study is a data point. Wait for replication before treating it as a rule.

None of that makes the finding useless. It is the sort of evidence that justifies trying both exercises, not one that justifies a strong claim.

How should you program leg extensions and squats together?

Use a compound quad lift as your main exercise and add leg extensions as an accessory. This is our reasoning from the trial, not a protocol any study tested.

1. Start with the squat, hack squat or leg press. These train multiple joints and let you load heavily. The trial's squat group made the biggest strength gains. Our compound vs isolation exercises guide explains where each type fits.
2. Add leg extensions after the heavy lift. They are easy to take close to failure with little technical demand. See exercise order in a workout for how order affects performance.
3. Work in the 8 to 12 rep range for most sets. That is what the trial used. Our hypertrophy rep range guide compares other loading ranges.
4. Take hard sets close to failure. The trial trained to repetition maximum. Our training to failure vs RIR guide shows how to judge effort without failing every set.
5. Check your weekly set total. Three sets of each exercise twice a week was enough to grow in beginners, but your needs will rise as you advance. How many sets per muscle group covers volume.
6. Use a full, controlled range. Do not shorten the range to hit a bigger number.
7. Progress with double progression. Add reps until every set reaches the top of your range, then add load. See double progression.

Which should you choose?

Choose by situation. These are practical judgements built on one trial, not findings.

How do you track leg extensions and squats?

Log them as separate exercises. A 50 kg leg extension and a 50 kg squat are not comparable, and one shared history would blur both trends.

IronLedger records weight, reps, RPE or RIR and a note on every set. A practical set-up:

• Weight and reps: the load you used and the reps you completed, aiming for the top of your range.
• RPE or RIR: how close each set came to failure, so 12 reps at RIR 3 is not mistaken for 12 reps at RIR 0. Our RPE vs RIR guide shows how to rate sets.
• Notes: the machine, the seat setting or "paused at the top". Free text stays with the set in your history.

For a lift like the squat, the per-lift trend shows estimated 1RM and best set across every session you have logged. Leg extension estimated 1RMs are less meaningful, so rely on best set and reps at a given load. Run a block with and without the accessory, compare your trends, and keep what moves.

What mistakes should you avoid?

• Treating one trial as settled. The finding comes from untrained young women over 8 weeks.
• Skipping the main lift. Leg extensions are an accessory. The squat group gained far more squat strength.
• Dropping leg extensions because they look easy. The trial found they grew the rectus femoris more.
• Comparing numbers across exercises. Squat and leg extension loads are not interchangeable.
• Cutting range of motion to lift heavier. Use a full range you control.
• Changing everything at once. Change one exercise at a time so your history stays readable.

Keep learning

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