leg extension vs leg curls

Leg Extension vs. Leg Curls – What’s the Difference?

Leg extensions and leg curls are typically performed together as part of a superset during resistance exercise. They both isolate the leg muscles to build muscle and strength. But what’s the difference between the two? When should you include it in your resistance training program?

In the following article, we’re going to go into detail about each exercise and what their differences are.

Leg Extension

Leg extensions are commonly performed on a padded leg extension machine. You’ll begin sitting against a padded seat with your feet under roller pads. You’ll then use your legs to raise a weight.

Leg extensions primarily work the muscles in the front of your legs known as the quadriceps. The quadriceps are used for any sort of knee extension movement such as kicking and cycling. Football players and MMA fighters report improved performance with stronger kicking abilities.

What’s unique about the leg extension is it isolates the quadriceps. This allows you to focus on building muscle and strength in just this area. Leg extensions also help strengthen the ligaments and tendons around your knee cap. 

How to Perform the Leg Extension

  1. Begin in the seated position on the leg extension machine with your knees at a 90-degree angle and your feet anchored beneath the leg rollers. 
  2. Grip the handles, tighten your midsection, and press your lower back into the seat. 
  3. Lift your legs until they reach near extension. Ensure you’re moving through a safe range of motion by not fully extending the legs.
  4. Pause briefly, then slowly lower your legs to the starting position. 

Leg Curl

Leg curls are performed on a padded leg curl machine. Seated leg curls involve sitting against a padded seat with your feet over a padded bar. From here, you’ll use your legs to lower a weight.

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Leg curls primarily work the muscles in the back of your legs known as the hamstrings. The hamstrings are used to bend your knee. They’re used in movements such as running and walking.

Just like the leg extension, leg curls isolate the hamstrings. Isolating a muscle is a great way to increase muscle and strength in that particular area. Leg curls also help strengthen the knee and hip joint. 

How to Perform the Leg Curl

  1. Sit on the leg curl machine with your knees slightly bent and your feet resting on top of the rollers. 
  2. Grip the handles, tighten your midsection, and press your lower back into the seat. 
  3. Push down on the bar until your legs reach a below 90-degree angle. Pushing past the 90-degree angle is an optimal range of motion for the leg curl. 
  4. Pause briefly, then slowly raise your legs to the starting position. 

Differences Between Leg Extensions and Leg Curls

Muscles Worked 

Leg extensions primarily work the quadriceps muscles. This is the muscle found at the front of your thigh. If you’re looking to increase size in your quads, or want to define that “teardrop” section, leg extensions are your go-to exercise.

Conversely, leg curls work the hamstring muscles. This is the muscle found at the back of your leg.

Leg Extensions vs Leg Curls MW copy

Leg extensions and curls are typically performed together as part of a superset because the quadriceps and hamstrings typically work in pairs. For example, during a squat, you’ll use your hamstrings to flex the legs and quadriceps to extend them. 

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The other primary difference between a leg extension and a leg curl is the type of movement.

Leg extensions move a weight while extending the knee. Leg curls move a weight while flexing the knee. Knee extension will help in kicking movements and cycling, whereas knee flexion helps walking and running.

But, each of these movements still works its opposite muscle group, just to a lesser extent.

To help avoid injury and muscle imbalances, both the quadriceps and hamstrings should be worked together, like in a superset. If one muscle group becomes more dominant than the other there is a higher risk of injury. 


Leg extensions and leg curls are two opposing movements that should be worked together. Leg extensions focus on working the quadriceps and knee extension. Whereas, leg curls focus on working the hamstrings and knee flexion.

Both the quadriceps and hamstrings need to be balanced to reduce the risk of knee injuries. Not only is symmetry important for aesthetics, but also for injury prevention. 

Frequently Asked Questions

Are Leg Extensions and Leg Curls Necessary?

Leg extension and leg curl exercises are some of the best ways to isolate the quadriceps and hamstrings muscle. But, this doesn’t mean they’re the only exercise to do this. If you’re limited on equipment or don’t want to wait around to use the machine, there are some alternatives you could try.

To focus on the quadriceps you can try Bulgarian split squats, cyclist squats, or forward lunges. If you want to work your hamstrings, try reverse lunges, good mornings, or Romanian deadlifts. 

Are Leg Extensions as Good as Squats?

Put simply, it depends on your goal for the exercise. Squats are a compound exercise that engages a range of muscles from movement at multiple joints. Squats will target your quadriceps, glutes, hamstrings, core, shoulders, and back.

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They’re great for building overall strength, but don’t focus on one area. Leg extensions allow you to focus fully on your quadriceps but don’t engage much else. If your goal is isolation, leg extensions are better than squats. If your goal is to hit all the major muscle groups in your legs, squats are better than leg extensions.

Note: if you have poor squatting techniques, it’s best to work your strength on the machines while practicing your squatting form. 

Can You Build Big Legs With Leg Extensions?

Yes and no. Leg extensions focus on building muscle in the front of your legs, or your quadriceps. If you performed just leg extensions with progressive overload you would build big quadriceps.

But, if you didn’t perform exercises to hit other parts of your legs, your legs wouldn’t be “big” overall. For aesthetics and injury prevention, it’s important you work all areas of your legs.

Muscle imbalances significantly increase your risk of injury. For overall leg development, I recommend starting your exercise routine with a compound movement such as the squat, then adding isolation exercises at the end.

For your legs to grow you will also need to ensure you’re practicing progressive overload and eating for muscle growth. Progressive overload involves progressing most weeks throughout your exercise programming.  

Michaela Summers

Michaela Summers is a health and fitness content creator. She holds a BSc in Exercise and Sport Sciences and a Master of Research in Health and Wellbeing from the University of Exeter. She is on a mission to help people live a fulfilling, impactful life through fitness and lifestyle. When she's not writing, she can be found in the gym, playing tennis, or exploring the great outdoors.

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