How much weight should I use compared to the two-leg leg extension?

Start with roughly half of your two-leg working weight, or a little less, and adjust from there. Some people handle a bit more than half per leg and some a bit less, so let clean, controlled reps with your hips in the seat decide the load.

Should I start with my weaker leg?

Many coaches have clients start with the weaker leg and then do the same number of reps with the stronger one. That keeps the stronger side from setting the pace and makes the gap between legs easier to track over time.

Is the single-leg leg extension bad for your knees?

Not when the machine is set up correctly and the weight is controlled. The knee is the only joint that moves, so pain around the kneecap is a cue to lighten the load or shorten the range, and anyone with a knee injury or recent surgery should get loading guidance from their clinician.

Does the backrest angle matter?

It can. In a 10-week study where untrained men trained each leg at a different hip angle, the leg trained leaning back at 40 degrees of hip flexion grew more rectus femoris than the leg trained sitting upright at 90 degrees, while vastus lateralis growth was similar. If the backrest on your machine adjusts, a more reclined setting is a reasonable choice as long as your knee still lines up with the pivot.

How many reps should I do?

Most people use it for moderate to higher reps, around 8 to 15 per leg, since it is an accessory movement and very heavy sets tend to pull the hips off the seat. Pick a load that lets you finish every rep with a controlled lowering.

Machine Single-Leg Leg Extension

What Is the Machine Single-Leg Leg Extension?

Machine Single-Leg Leg Extension is an isolation exercise that primarily targets the quads. It is a beginner-level movement performed on a plate-loaded leg extension machine: you sit back against a reclined pad, hook one lower shin behind a roller pad and straighten that knee against the load while the other leg rests.

It is the one-leg version of the Machine Leg Extension. Working each side on its own keeps the stronger leg from carrying the weaker one, and it makes any difference in load or reps between the two sides easy to see and track from week to week. Most programs use it as an accessory after the main lower-body lifts, for quad size, or to even out a side that lags behind.

Because the hip stays still and only the knee moves, the work falls on the quadriceps: the vastus lateralis, vastus medialis and vastus intermedius, plus the rectus femoris, which also crosses the hip. The glutes and hamstrings do very little. In a 12-week study where untrained adults trained one leg on a single-leg knee extension machine and the other on a single-leg leg press, the rectus femoris grew 13.2% in the knee extension leg against 1.1% in the leg press leg, while growth of the vasti and the quadriceps as a whole was similar, and only the leg press leg grew the gluteus maximus and adductor magnus. That makes this exercise a complement to presses and squats, such as the Machine Single-Leg Leg Press, rather than a replacement for them.

The machine sets the path and the seat supports the back, so no balance is needed, which makes it a good fit for beginners and for anyone whose single-leg squats or lunges are limited by balance rather than leg strength. If you are new to the machine, learn the seat and pad settings on the two-leg version first. Since one muscle group does all the work, focusing on the front of the thigh and squeezing it at the top of each rep is a useful cue for keeping the load where it belongs.

Muscles Worked

MuscleActivation

Rectus Femoris: Primary
Vastus Lateralis: Primary
Vastus Medialis: Primary
Biceps Femoris: Minimal
Gluteus Maximus: Minimal
Semitendinosus: Minimal

Instructions

Load plates onto the weight horn on the machine's movement arm, starting lighter than you would use for both legs.
Adjust the seat so that when you sit back against the reclined pad, the side of your knee lines up with the machine's pivot point.
Sit with your hips at the back of the seat and hook your working leg behind the roller pad so it rests on the front of your lower shin, just above the ankle, with the knee bent to about a right angle.
Rest your other foot on the footrest in front of the seat, with that knee bent and relaxed.
Grip the handles at the sides of the seat and press your back and hips into the pads.
Exhale and straighten the working knee, lifting the pad until your leg is straight out in front of you and about level with the seat.
Squeeze your quads for a moment at the top without snapping the knee straight.
Lower the pad under control over 2 to 3 seconds until your knee is back at about a right angle, without letting the plates drop.
Finish your reps, then switch legs and do the same number of reps on the other side.

Common Mistakes

Knee out of line with the pivot
Sitting too far forward or back from the machine's pivot point makes the pad slide along the shin and shifts strain toward the knee instead of the quads.
Pad too high on the shin
A pad halfway up the shin shortens the lever and a pad over the foot lets the ankle help, so set it on the lower shin just above the ankle.
Hips lifting off the seat
A load that is too heavy makes the hips rise or the torso twist to help the lift, which takes work off the working quad and strains the lower back.
Kicking the weight up
Swinging the leg up and letting the plates drop uses momentum instead of the quads and jars the knee at both ends of the rep.
Letting the stronger leg set the pace
Doing extra reps or more weight on the stronger side keeps the gap between legs from closing, so match both sides to what the weaker leg can do well.

Injuries

Machine Single-Leg Leg Extension is a low risk exercise when performed with proper technique. The machine guides the path and the seat supports your back, so the main strain point is the knee itself: the area around and under the kneecap and the patellar tendon just below it. These tend to get irritated when the load is too heavy, the reps are kicked up fast, or the knee does not line up with the machine's pivot.

To keep the work in the quads, line the side of your knee up with the pivot, set the roller pad low on the shin just above the ankle, and keep your hips in the seat. Warm up with a light set on each leg, raise and lower the weight under control, and add load in small steps. Use the same load and reps on both legs so the stronger side does not drift ahead.

If one leg at a time feels unstable or too heavy even with the lightest load, go back to the Machine Leg Extension with both legs. People who feel discomfort around the kneecap near the straight-leg position often work in a shorter range that stays pain-free. To progress, add load gradually, slow down the lowering, or pause at the top, and for single-leg strength that also trains the hips and balance, add the Dumbbell Bulgarian Split Squat.

Stop if you feel sharp pain in or around the kneecap, pain that lingers after the set, swelling, or a sense of the knee catching or giving way. Anyone recovering from a knee injury or surgery should follow their clinician's guidance on whether, when and how to load this movement.

Overview

Primary Focus: Quads
Equipment: Machine
Modality: Strength
Difficulty: Beginner

Alternative Exercises

Frequently Asked Questions

Q: How much weight should I use compared to the two-leg leg extension?: Start with roughly half of your two-leg working weight, or a little less, and adjust from there. Some people handle a bit more than half per leg and some a bit less, so let clean, controlled reps with your hips in the seat decide the load.
Q: Should I start with my weaker leg?: Many coaches have clients start with the weaker leg and then do the same number of reps with the stronger one. That keeps the stronger side from setting the pace and makes the gap between legs easier to track over time.
Q: Is the single-leg leg extension bad for your knees?: Not when the machine is set up correctly and the weight is controlled. The knee is the only joint that moves, so pain around the kneecap is a cue to lighten the load or shorten the range, and anyone with a knee injury or recent surgery should get loading guidance from their clinician.
Q: Does the backrest angle matter?: It can. In a 10-week study where untrained men trained each leg at a different hip angle, the leg trained leaning back at 40 degrees of hip flexion grew more rectus femoris than the leg trained sitting upright at 90 degrees, while vastus lateralis growth was similar. If the backrest on your machine adjusts, a more reclined setting is a reasonable choice as long as your knee still lines up with the pivot.
Q: How many reps should I do?: Most people use it for moderate to higher reps, around 8 to 15 per leg, since it is an accessory movement and very heavy sets tend to pull the hips off the seat. Pick a load that lets you finish every rep with a controlled lowering.