leg extensions

Leg Extensions: Muscles Worked, Proper Form, Variations

The leg extension has gotten a bad rap in the gym industry. For decades it’s been considered an isolation exercise for the quadriceps, something that you might want to throw in for a couple of sets after you’ve repped out on squats. Then there are those who advocate avoiding leg extensions altogether in the belief that they cause something called ‘knee shearing’, which is supposed to damage the knee joint.  The reality is that the leg extension is the single best resistance exercise that you can do for your quadriceps. In this article, you’re about to learn why. I’ll also show you how to optimize your leg extension form and show you how to get the most out of them as part of your leg training routine. 

Why Do Leg Extensions?

There are four parts to the quadriceps muscles:

quadricep muscle anatomy
  •  The rectus femoris
  • The vastus intermedius
  • The vastus lateralis
  • The vastus medialis

These four muscles have different origin points but they all insert on the upper end of the femur, or thigh bone. As a result, you cannot do an exercise to isolate an individual muscle; they all work together.

Your quadriceps do one thing and one thing only – knee extension. That means that they allow you to move your lower leg from a fully contracted to a fully extended position. In that sense, it is identical to the triceps, which is the elbow extender. To work the quadriceps muscles you must do an exercise that performs the same movement as the function of the muscle. So, what exercise involves knee extension?

That’s right – the leg extension. This exercise allows you to move your quad muscle fibers through their complete range of motion and, therefore, achieve total muscle engagement. In fact, it is the only exercise that allows you to do that. That’s why the leg extension is the best exercise for building muscle in your quads. 

Leg Extension Proper Form

Leg Extension Set Up

When you perform the leg extension, it is imperative that you set the machine up properly. Adjust the foot pad just above your ankle joint. Align your knees with the pivot. To fully engage your quads, the seat should not be in the upright position. In the upright position, your spine is arched and your hip joint is at 90 degrees. This hip extension stretches the hamstrings and adductors, which compromises the ability of the quads to contract with maximum force. The closer your torso is to parallel the more your leg muscles will be engaged. As a result, you should set the seat position as far back as possible. 

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Leg Extension Performance

You should only perform the leg extension with the middle 80 percent of its range of motion. That means that you cut out the first and last 10 percent. When you are going heavy on the exercise there is some potential for knee joint strain at the very start and the very end of the movement. Cut out the top and bottom portions to remove injury risk.

You will not lose any benefit in terms of muscle or strength development by only moving through 80 percent of the range of motion, so it makes to cut out the potential problems areas that may cause knee issues. 

To perform the exercise, simply move smoothly through the middle 80 percent of the leg extension movement. Perform both the positive and the negative parts of the rep under full control, taking about a second and half in each direction. Some people believe that holding the top contracted position will help with muscle definition. That is not true. The only way to define your leg muscles is to get rid of body fat through diet and cardio.

Leg Extension Alternatives

The seated leg extension is the single best exercise that you can do to build quad strength. However, if you are working out at home you may not have access to a leg extension machine. Most gyms will have a leg extension machine but there will be times when it is being used. Having alternative exercises up your sleeve will allow you to continue your leg workout. 

The best leg extension alternative exercises will simulate knee extension as closely as possible. What the means, in practical turns, is that the tibia (lower leg bone) needs to extend from a standing position down to the floor. In other words, the knee must go over the toes and down towards the floor. 

The best exercise to achieve this maximum knee extension is the sissy squat. This is a similar movement to the leg extension, except that it is your upper body and quads that are moving rather than your lower legs.

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The next best exercise to use as an alternative for the leg extension is the cable squat. This version of the squat allows you to lean back so as to achieve a greater knee extension. You can also load your quads without placing compressive stress on your lower back as you would with the standard barbell back squat.

The third best leg extension alternative is forward lateral lunges. This exercise allows you to extend the forward knee over the toe and down toward the floor in order to achieve knee flexion. You are also able to train each thigh muscle unilaterally. 

Leg Extension Variations

Lunge Switch

Step-by-step how-to

  1. Assume a staggered stance and drop down into the lunge position with your front foot planted on the floor and the rear heel lifted.
  2. Explode into the air as you switch your leg position.
  3. Return to a low lunge stance position with opposite legs forward and back.
  4. Do 3 sets of 12 reps.

Knee Get Ups


Step-by-step how-to

  1. Get down on the floor on your knees, with your butt resting on your heels.
  2. Explode up to a standing position to plant your feet on the floor. In this position your knees should be bent and torso slightly angled forward.
  3. Do 3 sets of 8 reps.

Forward Lunges

Step-by-step how-to

  1.  Stand with your feet hip width apart and kettlebells led at your sides.
  2. Lunge forward with your left foot and drop your rear knee until it almost touches the floor. Maintain an upright torso throughout the movement.
  3. Reverse the action by pushing through the front heel.

Leg Extension Common Mistakes to Avoid

1. Short Range of Motion

I have already pointed out that you should cut out the first and last 10 percent of the movement. However, I see a lot of guys who overload the weight and use a ridiculously short range of motion. They are only moving through something like 40 percent of the middle of the movement. That is robbing you of most of the benefits of the exercise. So, be sure to move through that full 80 percent of the movement.

2. Moving Too Fast

If you move the weight up too fast, as when doing explosive leg extensions, momentum will take over and rob you of the benefit of the exercise. If you don’t move with control during the negative part of the exercise, you will also rob yourself of the eccentric training benefits. You should be taking about a second a half on the way up and another second and a half on the way down. There is no reason that you should ever do explosive leg extensions.

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3. Going Too Heavy

If you use a weight that is too heavy you will not be able to perform the exercise with proper form. This will force you to make one or both of the previous mistakes. Use a weight that is challenging but controllable. 

Leg Extension Set & Reps

The leg extension should be your main quad exercise. I recommend doing a total of 10 sets where you pyramid down in reps as you increase the weight. In order to activate nth your fast twitch and slow-twitch muscle fibers, you should start with very high reps of 50 and work down to low reps of 6 with your heaviest weight.

Here is how your set and rep scheme should look:

Set# of Reps
Set One50 reps
Set Two30 reps
Set Three20 reps
Set Four15 reps
Set Five10 reps
Set Six10 reps
Set Seven8 reps
Set Eight8 reps
Set Nine6 reps
Set Ten6 reps

Your rest between sets should be 30-45 seconds.


The leg extension deserves to be at the top of your quadriceps workout exercise list. Use it as your go-to thigh developer, performing 10 sets over a wide range of repetitions, from very high to very low.  Do this every 5 days and your quads will get bigger and stronger. 

Frequently Asked Questions

Are leg extensions better than squats?

If your goal is to develop quadriceps muscle mass then, yes, leg extensions are better than squats. Squats do not effectively produce knee extension, which is the only biomechanical action of the quadriceps. Leg extensions do, making them the smarter choice for your quadriceps workout.

Should you go heavy on leg extensions?

Yes, but be sure to maintain proper form. Use the rep plan outline above to gradually progress from a lighter weight with high reps down to sets of 6 reps with heavy weights over 10 sets. There is a muscle-building advantage to go lower than 6 reps.

How many leg extensions should I do?

The leg extension should be your main quad exercise. I recommend doing them first in your quad workout with 10 sets every 5 days, varying your from a high of 50 down to allow of 6.

Steve Theunissen

Steve Theunissen is a former gym owner and personal trainer based in Tauranga, New Zealand. He is the author of six hardcopy books and more than a hundred ebooks on the topics of bodybuilding, fitness, and fat loss.

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