Leg Extension Muscles Worked

Without a doubt, the extension machine is one of the most popular accessory movements when it comes to leg day and it is easy to see why.

It allows you to effectively isolate your quadriceps to maximise the gains in your quads. However, there are a few tips and tricks to increase muscle activation which we will cover later.


What is a Leg Extension?

For those of you who don't know, the leg extension has you sitting down in an elevated position so that your feet cannot touch the ground.

You sit with your knees bent at a 90° angle and a pad in front of your shins and ankles. This pad is attached to a cable or pulley system, which is attached to a weight stack.

The point of the exercise is to push upwards and fully extend your legs so that they are facing out and away from you. Today, we want to figure out what muscles the leg extension machine works, how to do a leg extension, and so much more.

Related: Alternative Leg Extension Exercises


Muscles Worked by a Leg Extension

leg extension muscle activation

Unlike some of the other exercises you might see out there, known as compound exercises that work out a variety of muscles at once, the leg extension is an isolation exercise.

Therefore, it is designed to work out one specific muscle or muscle group, and that is the quadriceps or quadriceps femoris.

Technically speaking, the quadriceps femoris is a major muscle group which includes four distinct muscles that are located on the front of the thigh.

  • First, we have the Vastus Lateralis, which is the muscle found on the outer part of the upper thigh, and it's very important for extending the knee.
  • Next is the Vastus Medialis, which is found on the inner part of the upper thigh, and it's very important for stabilizing the kneecap and for allowing for knee extension.
  • Third is the Vastus Intermedius, which can be found directly underneath the rectus femoris, and it also works to extend your knee joint.
  • Finally, we have the rectus femoris, which is the only part of the quadriceps muscle that crosses over the hip joint, and it helps with both knee extension and hip flexion.

Although each of the four separate muscles that comprise the quadriceps plays its own specific role, they all work together with one key goal, which is to straighten the knee.

When you perform a leg extension, the goal is of course to straighten your knee by pushing against a certain amount of weight that is holding you back, and then returning to the starting position.

The resistance that a leg extension machine provides you with puts your quadriceps under constant tension, which is of course what is required to exercise them.


Benefits of the Leg Extension

The benefits of doing leg extensions regularly are very widespread, and it really all comes down to the benefits that you reap from having stronger quadriceps muscles.

So, why do leg extensions and why should you try to train your quadriceps muscles more?


More Strength and Size

Benefits of the Leg Extension

One of the most obvious benefits of doing the leg extension is that it helps to build both strength and size in your upper legs, which as you'll see below, comes with many different benefits.

At the very least, your legs will look more aesthetically appealing when you wear those short shorts.


More Explosive Power

Something that the leg extension most certainly helps with is building more explosive power.

For those who do leg extensions with extreme amounts of weight, all of that resistance builds massive power.

To lift several hundred pounds during the leg extension takes a lot of split-second force, and that translates to explosive power throughout the rest of your day.


Stabilizing the Knee Joint

Benefits of the Leg Extension

Perhaps one of the biggest benefits that you get from doing leg extensions is that it helps to stabilize your knee.

Your quadriceps muscles are very important for keeping your knees stable and aligned.

So whether you do a lot of high-impact activities, or you just have a weak knee joint, keeping your quadriceps strong is crucial. It helps prevent injuries from occurring.


Overall Improvement in Performance

Seeing as the quadriceps are important for extending the knee, it serves to reason that they're required for you to jump.

If you're an athlete, running and jumping are two things that you'll likely always have to do no matter the sport you're in.

When it comes down to it, doing regular leg extensions helps to improve your overall athletic performance.

Related: Alternative Hack Squat Exercises


Aids in Injury Recovery and Daily Activities

Benefits of the Leg Extension

As mentioned above, having strong quadriceps is important for stabilizing the knee joint, and This is why leg extension is often used as a rehabilitation exercise for people who have suffered knee injuries.

It helps to strengthen the quadriceps muscles without putting too much stress on any other part of the body.

On that note, having strong quadriceps and doing leg extensions is just ideal for supporting you in your daily activities, whether you want to stand up, walk, or climb up a set of stairs.


Fall and Injury Prevention

The other benefit that you get from doing a leg extension is that because it helps to stabilize your knees, it also helps you balance better.

On that note, having strong upper legs also allows for faster reaction time and better balance in general.

This is important as you get older because it might allow you to stabilize yourself faster in the event of a slip.


How to Do a Leg Extension

How to Do a Leg Extension

The great thing about the leg extension is that it's an exercise that virtually everybody can do, whether the newest beginners or the most seasoned weightlifting experts.

Of course, you do need to have a leg extension machine, because it's really the only way you'll be able to put that much pressure on your quadriceps. Follow the instructions below to do a leg extension.

  1. The leg extension machine features a seat with an adjustable backrest. For the best position, adjust the backrest so that the back of your knees are resting against the edge of the seat when you sit down, not unlike you would actually sit on a chair.
  2. Adjust the pad of the leg extension machine so that it is sitting comfortably in front of your shins and ankles. You don't want the pad to be on your feet, but you also don't want it to be too high up on your lower legs. If it's anywhere close to your knees, it's too high up.
  3. You now need to set the weight on the weight stack of the leg extension machine. Although you should be able to lift a good deal of weight with your quadriceps, it's always a good idea to start slow, so start with something like 30 or 40 pounds.
  4. With the weight set and your knees bent at a 90° angle, with your lower legs being perpendicular to the ground, it's time to push. Extend your leg by straightening your knee until your toes and feet are in line with your hips, or in other words until your legs are perfectly horizontal.
  5. Hold this position for one or two seconds and then return to the starting position in a very slow and controlled manner. The slower and more controlled this movement is, the more pressure you'll put on your quadriceps, and therefore the more benefit you will gain.

Related: Tips to Smith Machine Squats


Hitting Different Angles on Leg Extension

Although the leg extension machine works all parts of your quadriceps, depending on the way you point your toes, you can shift the focus from the interior, to the metal, and the exterior.

For instance, if you have your toes pointed straight upwards and perfectly in line with your knees, you'll work out the middle of your thighs the most.

However, if you want to work the outer side of your quadriceps, angle your foot inwards while pushing the pad upwards. If you point your toes inwards, you'll notice the outer portion of your thighs being worked out much more.

On the other hand, if you're looking to work out the inner thighs the most, turn your foot outwards as much as you can.


Rounding Up

What it really comes down to is that the leg extension is by far one of the best exercises to work out your quadriceps.

All four of the distinct muscles in the quadriceps femoris get worked out by the leg extension, and exactly which portion of your quadriceps get worked out the most depends on the angle at which you have your feet.

Author's Photo

William Parrett

Will, co-founder of Home Gym Supply, launched the company in 2019 after 15-years in the fitness industry. His expertise stems not only from his professional background but also from his athletic pursuits. A former competitor in the World Beauty Fitness & Fashion (WBFF) and a competitive rugby player, Will has always been dedicated to health and fitness.