Leg extensions and leg curls are both staple leg day exercises performed on a machine. They are often performed as part of a superset during a resistance training session as they both isolate different leg muscles to help build strength and size.

But, what exactly is the difference between the two? And when should you be including them in your weight training program?

In this article, we go into detail about both movements and what their differences are!

What are Leg Extensions?

Leg extensions are usually performed on a padded leg extension machine, found in most recreational gyms. You begin by sitting against a padded seat with your feet under roller pads, lifting your lefts to raise your chosen weight.

Leg extensions primarily work the quadriceps (quads), the large muscles found in the front of your thighs. The quads are used for any sort of knee extension movement, such as kicking, cycling, and squatting.

The unique factor about leg extensions is the isolation of the quads, which allows you to build muscle and strength in just this area. This movement also helps strengthen the ligaments and tendons surrounding your knee joint.

How to Perform the Leg Extension

  • Begin by sitting in the leg extension machine with your knees at a 90-degree angle and your feet beneath the leg rollers
  • Tightly grip the handles on either side to help stabilize and tighten your midsection while pressing your lower back into the seat
  • Lift your legs until they reach near extension, ensuring you are moving through a safe range of motion (not fully extending the legs)
  • Pause briefly at the top of the movement then lower your legs to the starting position

What are Leg Curls?

Leg curls are performed on a padded machine, similar to the leg extension machine, also with padded rollers. Seated leg curls require you to sit against a padded seat with your feet over the padded rollers. From this point, you will use your legs to lower your chosen weight.

This exercise primarily works the hamstrings, the large muscles located on the backs of your thighs. The hamstrings are used mostly to bend your knee and are used in movements such as running and walking.

Much like the leg extension which isolates the quads, the leg curls isolate the hamstrings. This means that this movement helps to build muscle and strength in this specific area. Leg curls also help to strengthen the knee and hip joints.

How to Perform Leg Curls

  • Start by sitting on the leg curl machine with your knees slightly bent and your feet resting on the rollers
  • Grip the handles for extra stability and tighten your midsection, pressing your lower back into the seat
  • Push your legs down on the pads until your legs reach a below 90-degree angle, which is the optimal range of motion
  • Pause briefly before returning to the starting position

Leg Extension Vs Leg Curls: What’s the Difference?

Muscles Worked

The main difference between these movements is the muscles they work.

Leg extensions primarily work the quadriceps, found in the front of the legs, as well as helping to sculpt and define the “teardrop” muscle at the front of your thigh.

On the other hand, the leg curl works the hamstrings, found in the back of the legs.

These two movements are often performed together as part of a superset because the muscles worked tend to work together quite often. For example, during a squat, your hamstrings flex the legs and the quadriceps extends them.


The other big difference between these exercises is the type of movements.

Leg extensions move a weight while extending the knee, while leg curls flex the knee. Knee extension will help movements such as kicking and cycling, whereas knee flexion helps with movements such as walking and running.

To help avoid injuries and muscle imbalances, it’s recommended to work both the quadriceps and hamstrings together, such as in a superset, or at least within the same training session.

If one muscle group becomes more dominant than the other, your may increase your risk of injury!

Can the Leg Curl and Leg Extension Replace Squats?

Squats are a compound exercise that should be a primary part of any training program. Squats engage a range of muscles and joints, targeting both the quadriceps and hamstrings, but also the glutes, core, shoulders, and back.

While the leg curl and leg extension can help build strength and mass, they are only a small part of the puzzle. As they are isolation movements, they are not as effective for building all-around muscular strength in comparison to squats.

Though, they should be included in any leg program as isolation exercises make for great accessory exercises, helping to accelerate muscle growth and strength.


Leg extensions and leg curls are two opposing movements that are recommended to be worked together during a leg session.

Leg extensions focus on building the quadriceps, whereas leg curls focus on building the hamstrings.

Both of these movements should be included in your leg workout program, especially as there is a need for muscle balance to reduce the risk of injury.