ProgreviaSingle-leg leg extension: proper form, muscles worked and common mistakes
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How to do it
Knee extension working one leg at a time to fix imbalances. Extend to near lockout squeezing the quad for a beat and lower under control; no momentum and keep the glute down.
Muscles worked
Main muscle: quads. When it plans weekly volume, every set of this exercise counts in full for quads: the generator does not split it with other muscles.
Single-leg leg extension: when one leg lags behind
The single-leg leg extension does the opposite of the two-leg version: there's no way for one side to rest while the other pushes. Each leg does its full reps, and the weaker one decides the weight. That makes it the option for spotting and narrowing left-right gaps, something the regular machine leg extension tends to hide. The trade-off is that you'll use noticeably less total weight and take longer, because you work one leg and rest the other on every set.
Always start with your weaker leg and match its rep count with the other, without trying to beat it. It fits well at the end of a leg session or as the only quad exercise on a short day. It also works as a tougher last set once the two-leg version feels easy. Progress by matching reps between sides first and adding weight second.
Hold the machine's handles firmly: pushing with one leg makes the pelvis want to twist or lift off the seat, and the quad loses tension when that happens. Keep your hips flat against the seat throughout the set.
Technical sheet
- Movement pattern
- Isolation
- Minimum level
- Beginner
- Technical demand
- Basic
- Fatigue it generates
- Low (0.6 out of 2.2)
- Risk if technique breaks down
- Low
- Worked one side at a time
These are the values the generator uses to decide where the exercise goes within the session.
Recommended sets and reps
As an isolation exercise, it works best in 3–4 sets of 10–15 reps with 0–2 RIR. Use it at the end of the session to add volume without systemic fatigue.
Common mistakes to avoid
- Lifting your hips off the seat and jerking the weight up instead of extending the knee under control all the way.
- Not adjusting the seat and pads to your body: your joint should line up with the machine's pivot.
- Always starting with your good side and doing fewer reps on the other: start with the weaker side and match its reps on the strong one.
What the research says
As an isolation move, it is mostly a way to add volume to one muscle. Gentil and colleagues (2013) found that, in beginners, adding it to a multi-joint exercise gave no extra arm growth; it pays off later on, or for muscles the compounds barely reach.
It works one side at a time. Speirs and colleagues (2016) compared the Bulgarian split squat with the back squat in rugby players over five weeks and strength improved by a similar amount, so a single-leg version does not mean giving up progress.
On how hard to push: the meta-analysis by Refalo and colleagues (2023) found no clear advantage in going to failure over stopping just short of it. That is why the generator programs it with reps in reserve (RIR) instead of asking for failure on every set.
- Gentil P, Soares SR, Pereira MC, et al. Effect of adding single-joint exercises to a multi-joint exercise resistance-training program on strength and hypertrophy in untrained subjects. Applied Physiology, Nutrition, and Metabolism. 2013;38(3):341-344.
- Speirs DE, Bennett MA, Finn CV, Turner AP. Unilateral vs. bilateral squat training for strength, sprints, and agility in academy rugby players. Journal of Strength and Conditioning Research. 2016;30(2):386-392.
- Refalo MC, Helms ER, Trexler ET, Hamilton DL, Fyfe JJ. Influence of resistance training proximity-to-failure on skeletal muscle hypertrophy: a systematic review with meta-analysis. Sports Medicine. 2023;53(3):649-665.
How Single-leg leg extension compares with other quads exercises
Of the 16 quads exercises in the catalogue, ranked from least to most fatiguing by the values the generator uses, Single-leg leg extension is number 2.
That makes it a good candidate for adding quads sets at the end of a session or in heavy weeks, when you want volume without extra fatigue.
| Exercise | Equipment | Fatigue | Technique |
|---|---|---|---|
| Leg extension | machine | Low (0.5 out of 2.2) | Basic |
| Single-leg leg extension | machine | Low (0.6 out of 2.2) | Basic |
| Goblet squat | dumbbells | Moderate (0.8 out of 2.2) | Basic |
| Dumbbell step-up | dumbbells | Moderate (0.9 out of 2.2) | Basic |
| Static lunge | bodyweight | Moderate (0.9 out of 2.2) | Basic |
| Banded squat | resistance band | Moderate (1 out of 2.2) | Basic |
| Leg press | machine | High (1.2 out of 2.2) | Basic |
| Dumbbell lunges | dumbbells | High (1.2 out of 2.2) | Basic |
One side at a time. The meta-analysis by Kassiano et al. (2025), across nine studies, found no significant difference in hypertrophy between unilateral and bilateral training; for strength, each format improved most in its own type of test.
On a machine. In the meta-analysis by Haugen et al. (2023), with thirteen studies and 1,016 participants, there were no significant differences in muscle growth between training on machines and with free weights.
An isolation exercise. In the trial by Gentil et al. (2015), with untrained young men over ten weeks, the biceps curl and the lat pulldown were equally effective for gaining strength and size in the elbow flexors: isolation is not filler.
- Kassiano W, Nunes JP, Costa B, Ribeiro AS, Loenneke JP, Cyrino ES. Comparison of muscle growth and dynamic strength adaptations induced by unilateral and bilateral resistance training: a systematic review and meta-analysis. Sports Medicine. 2025;55(4):923-936.
- Haugen ME, Vårvik FT, Larsen S, Haugen AS, van den Tillaar R, Bjørnsen T. Effect of free-weight vs. machine-based strength training on maximal strength, hypertrophy and jump performance: a systematic review and meta-analysis. BMC Sports Science, Medicine and Rehabilitation. 2023;15(1):103.
- Gentil P, Soares S, Bottaro M. Single vs. multi-joint resistance exercises: effects on muscle strength and hypertrophy. Asian Journal of Sports Medicine. 2015;6(2):e24057.
Related guide
Best leg exercises for the gym and at home
Alternative exercises
More isolation exercises
- Overhead cable triceps extension
- Triceps pushdown
- Seated overhead dumbbell triceps extension
- Overhead dumbbell extension
- Face pull
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Frequently asked questions
What muscles does Single-leg leg extension work?
Single-leg leg extension mainly works the quads muscle group, plus synergist and stabilizer muscles depending on the variation.
How many sets of Single-leg leg extension should I do?
Between 3 and 5 sets per session, adjusting the total weekly volume for quads to your level (about 10–20 sets per muscle per week).
Should I do extra sets for my weaker leg?
Not at first. Start with the weak leg and match its reps with the other, and the gap will narrow gradually. If the difference is still large after several weeks, you can add one extra set on the weaker side, while still matching reps. If you feel pain in the knee, get it checked.
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