How to Do Leg Extension: Muscles Worked & Proper Form

Leg Extension exercise technique

Muscles Worked in Leg Extensions

Muscles worked in leg extensions

Primary muscles worked:

How to Do Leg Extensions

  1. Adjust the machine so that you are correctly positioned. Your knees should be in line with the machine’s joint.
  2. Extend your knees with control, until they are completely straight.
  3. Slowly lower the weight again.

Text and graphics from the StrengthLog app.

Introduction to the Leg Extension

The seated leg curl is an isolation exercise for the quadriceps and is one of the most popular leg exercises in our free workout log app StrengthLog.

Whether your goal is to get big quads or to get stronger legs, the leg extension is a great exercise to incorporate into your workout routine.

Which Muscles Do the Leg Extension Work?

The leg extension is an isolation exercise for the quadriceps, especially for the rectus femoris.1

Muscles worked in leg extensions

The vastus muscles pass over only the knee joint, while the rectus femoris passes over both the hip and knee.

This means that if you’re doing squats or a similar movement, you will extend your knee while simultaneously extending your hip, leading to your vastus muscles shortening, but your rectus femoris muscle length doesn’t really change, and it can’t contribute much to the work, because it is shortening in one end (the knee) but lengthening in the other (the hip).

For the rectus femoris to be worked, you need to either fixate your hip during the exercise, or perform some kind of kicking exercise where your hip is being flexed during knee extension, not extended. Enter; the leg extension!

Benefits of the Leg Extension

  • Easy to learn. The leg extension is a relatively simple exercise to perform, which makes it suitable for lifters of all levels.
  • Isolates the quads effectively. The leg extension lets you train the quadriceps effectively without involving other muscle groups. This helps you focus 100% on the working muscles without other aspects, such as grip strength or balance, interfering.
  • Targets the rectus femoris. As mentioned earlier, the leg extension keeps your hip fixated during the movement, which leads to better activation of the rectus femoris compared to movements where both the knee and hip joints move simultaneously.

Leg Extension: Proper Form & Technique

Leg extension start position
Starting position
Leg extension top position
Top position


Make sure to adjust the machine so that it fits your body properly. Make sure that the joint of the machine aligns with your knee joint, and that the back pad is adjusted so that you can press your back against it.


When you’ve ensured you have a good position in the machine, brace your core and start the movement by extending your legs as far as possible. Keep the motion slow and controlled to maximize muscle engagement. Make sure not to overextend your knees in the top position.

Common Mistakes in the Leg Extension

  • Adding too much weight. Loading more weight than you can handle can increase the risk of injury and probably lead to using excessive momentum to move the weight. It’s not only a way of making the exercise less effective, but it can increase your injury risk. Keep the movement slow and controlled.
  • Not activating your core. Not activating your core, and therefore arching your lower back during the exercise can lead to discomfort and also move the focus from your quads. Let them do the work, and keep your back pressed firmly against the backrest during the entire movement.
  • Too short range of motion. Not fully extending your legs reduce the effectiveness of the exercise. Aim for a long and consistent movement, and go as far as possible without compromising form.

Leg Extensions Alternatives

It’s hard to find a good substitution for leg extensions since it’s an exercise where you extend your knee without simultaneously extending your hip. The closest we can think of is the sissy squat or the reverse Nordic curl.

However, there are other good quadriceps exercises, and we’ll list some of the most classic ones below. We still think that you should include the leg extension in your routine if you want optimal quadriceps growth, though, since it’s quite unique in how it targets the quads.

1. Squat

Do we even need to say something about the barbell squat? The king of all exercises! It can initially feel scary, but it’s a great exercise once you master it.

Apart from your quads, the barbell squat also targets your glutes, adductors, and lower back.

2. Leg Press

The leg press requires less balance, control, and mobility than the barbell squat. This means that this exercise can be easier to start with for a beginner and also that you can train closer to muscular failure without thinking about balance. The leg press targets your quads, but also your glutes, adductors, and hamstrings.

3. Hack Squat

The hack squat is like a mix of the barbell squat and the leg press. The upright body position gives you a movement reminding of the barbell squat (or even the front squat), but the stability of the machine is more similar to the leg press. It targets your quads, glutes, and adductors.

How Many Sets and Reps Should You Do in the Leg Extension

The number of reps that you should do varies depending on your goals, and this goes for all exercises.

  • For muscle growth, around 6–15 reps per set is good to aim for.
  • For strength, around 3–8 reps per set is good.

There are no clear-cut lines between these two goals, however. Training in the “muscle growth range” will still increase your strength, and training in the “strength range” will still cause your muscles to grow. It’s just a matter of what you are emphasizing.

Read More: How Many Reps to Build Muscle vs. Strength?

Regarding how many sets you should do, that depends more on your training background and your capabilities. How much work are you used to doing, and how much does it take for you to stimulate growth?

It also depends on how many workouts you do per week. You can tolerate (and grow from) a higher training volume if you distribute it over more workouts.

Training Programs & Workouts that Include the Leg Extension

These programs and workouts, and many more, are available in our workout log app StrengthLog.

Download StrengthLog for free with the buttons below:

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  1. J Sports Sci. 2021 Oct;39(20):2298-2304. The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial.