Exercise guide · Beginner · Compound

Leg Extension: How to Do It & Muscles Worked

The leg extension straightens the knee against a padded lever while you sit, isolating the quadriceps. It is the only common exercise that trains the rectus femoris well at the knee, and it adds quad volume without loading the back.

Primary muscles
Quads
Secondary muscles
Hamstrings
Equipment
Leverage Machine
Level · type
Beginner · Compound

Written by the FitStack training team. Last reviewed .

Demonstration of the leg extension
Front view: muscles worked by the leg extension. Primary: quads. Secondary: hamstrings.
Back view: muscles worked by the leg extension. Primary: quads. Secondary: hamstrings.
Primary Secondary

How to do the leg extension

  1. Step 1: Adjust the seat height and backrest of the machine to fit your body.
  2. Step 2: Sit on the machine with your back against the backrest and your feet on the footpad.
  3. Step 3: Grasp the handles or sidebars for stability.
  4. Step 4: Extend your legs forward by straightening your knees, lifting the weight.
  5. Step 5: Pause for a moment at the top, then slowly lower the weight back to the starting position.
  6. Step 6: Repeat for the desired number of repetitions.

Form cues

  • Line your knee joint up with the machine pivot and set the pad just above your ankles.
  • Hold the handles and keep your hips down on the seat.
  • Straighten your legs fully, squeeze the quads for a moment, then lower slowly.

Common leg extension mistakes

  • Kicking the weight up with momentum.

    Fix: Use a two-second lowering phase and a brief pause at the top.

  • Seat set so the knee is out of line with the pivot.

    Fix: Adjust the backrest so your knee crease sits at the axis.

Sets, reps and programming

Do 2 or 3 sets of 10 to 20 reps at the end of a leg session. If it bothers your knees, use a slightly shorter range and a slower tempo.

Suggested sets, reps and rest for the leg extension by training goal
Goal Sets Reps Rest Effort
Strength 3–5 3–6 3–5 min 1–3 reps in reserve
Muscle growth 3–4 6–12 2–3 min 1–2 reps in reserve
Muscular endurance 2–3 12–20 1–2 min 1–3 reps in reserve

Ranges follow the ACSM position stand on progression in resistance training. Muscle grows across a wide range of loads when sets end close to failure (Schoenfeld et al., 2017), longer rests between heavy sets support strength and size (Schoenfeld et al., 2016), and stopping a rep or two short of failure works about as well as training to failure (Grgic et al., 2022). Estimate your working weights with the strength calculators.

Variations and progressions

Leg Extension alternatives

More exercises that train the quads as their main target:

Leg Extension FAQs

Are leg extensions bad for your knees?

For healthy knees, no. The load is controlled and moderate. If you have knee pain, a shorter range of motion and lighter weight are usually tolerated well.

More quads exercises

See the best quad exercises, ranked →

Sources

  1. American College of Sports Medicine. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 2009;41(3):687-708.
  2. Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. Strength and hypertrophy adaptations between low- vs. high-load resistance training. J Strength Cond Res. 2017;31(12):3508-3523.
  3. Schoenfeld BJ, Pope ZK, Benik FM, et al. Longer interset rest periods enhance muscle strength and hypertrophy in resistance-trained men. J Strength Cond Res. 2016;30(7):1805-1812.
  4. Grgic J, Schoenfeld BJ, Orazem J, Sabol F. Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: a systematic review and meta-analysis. J Sport Health Sci. 2022;11(2):202-211.

Step-by-step instructions and demonstration media are from the ExerciseDB exercise dataset; cues, mistakes, programming and FAQs are written by FitStack.

This guide is general fitness information, not medical advice. Stop if an exercise causes pain and speak to a doctor or physiotherapist about injuries or health conditions before starting a new programme.