Leg Extensions
Also called knee extension, leg extension machine
Quick answer
The leg extension is a seated machine exercise where you straighten your knees to lift a padded lever resting on your lower shins. It isolates the quadriceps, the four muscles at the front of your thighs, with very little help from other muscles.
Written by the GymStreak coaching team · Updated
Muscles worked
Primary
How to do Leg Extensions
- 1
Adjust the back pad so your knees line up with the machine's pivot point, and set the shin pad so it rests just above your ankles.
- 2
Sit back with your hips against the seat, hold the side handles and let your knees bend to a comfortable starting angle. This is your starting position.
- 3
Straighten your knees to lift the pad until your legs are fully or nearly straight.
- 4
Squeeze your quads for a moment at the top, keeping your hips down on the seat.
- 5
Lower the pad slowly, stopping just before the weight stack touches down, then start the next rep.
Form cues
- Line up your knee joint with the machine's pivot before you start.
- Hold the handles and keep your hips pinned to the seat.
- Squeeze your quads hard at the top of every rep.
- Take two to three seconds to lower; don't let the stack slam.
- Set a start angle that feels comfortable for your knees.
Breathing: Breathe out as you straighten your legs; breathe in as you lower the pad back down.
Common Leg Extensions mistakes
Knees not lined up with the pivot
Move the back pad until the middle of your knee sits level with the machine's axis so the pad doesn't slide along your shins.
Hips lifting off the seat
Hold the handles, sit back and reduce the weight if your hips rise to finish the rep.
Kicking the weight up
Start each rep smoothly and pause at the top; if you can't pause, the load is too heavy.
Letting the stack drop
Control the lowering phase, since it is half the work for your quads.
Sets and reps by goal
| Goal | Sets | Reps | Rest |
|---|---|---|---|
| Muscle growth | 3–4 | 8–15 | 60–90 s |
| Muscular endurance | 2–3 | 12–20 | 30–60 s |
Starting ranges in line with the ACSM guidance on resistance training. Pick a load you can lift with good form for every rep, and add weight or reps gradually.
Leg Extensions variations and alternatives
Variations
- Single-leg extension
Working one leg at a time shows up and evens out strength differences between sides.
- Paused leg extension
Holding the top for 1–2 s makes a lighter weight much more challenging.
Alternatives
- Barbell back squat
The main compound lift for the quads, glutes and whole lower body.
- Leg press
A supported compound press that loads the quads without balancing a bar.
- Machine hack squat
A quad-focused machine squat with a fixed path.
- Bulgarian split squat
A single-leg option that trains the quads and glutes with minimal equipment.
Leg Extensions FAQ
What muscles do leg extensions work?
Leg extensions work the quadriceps: the rectus femoris, vastus lateralis, vastus medialis and vastus intermedius. Because your hips stay still on the seat, almost all the work comes from the quads straightening your knees. Your hamstrings, glutes and calves do very little, which is why leg extensions are usually paired with squats, leg presses or lunges.
Are leg extensions bad for your knees?
For most healthy people, no. Leg extensions load the knee, which is how they train the quads, and the tissues around the knee adapt to gradual loading like any other. Line your knee up with the pivot, control the weight and choose a start angle that feels comfortable. If they cause knee pain rather than muscle effort, stop and get it checked by a qualified professional.
Should I do leg extensions or squats?
Both have a place. Squats train the quads, glutes and adductors together and build overall lower-body strength, while leg extensions isolate the quads with little fatigue elsewhere and are easy to learn. Many programs put squats or leg presses first, then add leg extensions for extra quad work. If you can do only one, a squat pattern covers more muscles.
How far should I lower the weight on leg extensions?
Lower until your knees are bent to around 90 degrees, or as far as the machine's start position allows while it still feels comfortable, and stop just before the stack touches down to keep tension on your quads. Using most of the available range is a good default, but it should feel like hard muscle work, never joint pain.
Sources
Related calculators
Read more
- Surprise, Surprise: Most “Worst Gym Exercises” Are Perfectly Safe and Growth-StimulatingLeg extensions. Smith machine squats. And deadlifts. Are these truly the "worst gym exercises" you could do? Well, that depends. Learn more here.6 min read
- The Only 3 Quad Exercises You Need for Massive LegsThere's nothing complicated about getting massive quads. TBH, all you need are these 3 quad exercises — and done correctly. Learn more here.5 min read
- How to Gain Muscle and Lose Fat Like a Minimalist in the GymWant to spend less time in the gym, but still want to achieve your physique goals? Mm. Here's how to gain muscle and lose fat like a minimalist.6 min read
Perfect Your Leg Extensions Form
Get detailed 3D demonstrations, track your progress, and receive personalized workout plans featuring this exercise.
