Are Leg Extensions Bad for Your Knees? Good for Building Muscle?

Some say that the leg extension, or knee extension, is bad for your knees and should be avoided at all costs. Any time you do the leg extension, you’re increasing your chances of sustaining a crippling knee injury that will put you out of action for months.

Not only that, the leg extension is no good for building muscle, and has no carry over into real life, giving it zero benefit from a functional training point of view. 

An exercise with no redeeming features, the leg extension is nothing more than a complete waste of time that would be much better spent doing compound exercises like squats or step-ups.

Or at least, that’s what some people will tell you.

Personally, I like using the leg extension machine. I do it most weeks and my knees feel fine.

The leg extension does have the potential to be bad for your knees, depending on what sort of shape your knees are in, how you do the exercise, and what your overall training program looks like.

However, it also happens to be a very effective way to build certain muscles in your quads, areas that wouldn’t grow to the same extent if all you did was squat.

What’s more, the leg extension has been used in some studies to actually reduce knee pain. 

That’s not to say it’s going to help everyone. There are many potential sources of painful knees, and what works in one context isn’t necessarily going to work in another.

However, it does argue against the popular narrative that the leg extension is a universally “bad” exercise for everyone.

Let’s take a closer look at the pros and cons of the leg extension, so you can make your own mind up about whether or not it merits a place in your training program.

What Muscles Do Leg Extensions Work?

The leg extension is a single-joint isolation exercise designed to focus almost exclusively on your quadriceps, a group of four different muscles in the front of your thigh:

• Vastus Medialis
• Vastus Lateralis
• Vastus Intermedius
• Rectus Femoris

Vastus medialis is the “teardrop” muscle on the inside of your thigh, sitting just above the knee. Vastus lateralis is the largest of the quadriceps, found on the outside of the thigh.

Rectus femoris runs straight down the middle of your thigh, while vastus intermedius sits directly underneath rectus femoris

The leg extension doesn’t work all the muscles equally, and tends to stimulate faster growth in rectus femoris compared to the other muscles that make up the quadriceps [5].

How to Do the Leg Extension

• While sitting on the knee extension machine, adjust the back rest so the back of your knee rests against the seat pad.
• Adjust the shin pad so it sits just above your ankles.
• Take hold of the side bars.
• With your toes pointing up, and your feet hip-width apart, straighten your legs.
• Return to the start position.

Drawbacks of the Leg Extension Machine

One of the main arguments against the leg extension is that it puts stress on the anterior cruciate ligament (ACL), a strong band of connective tissue that attaches your thigh bone to your shin bone.

However, just because an exercise puts “joint stress” on a particular tissue isn’t necessarily a bad thing.

Tissues like muscles, tendons, ligaments and even bone, adapt to various stresses in such a way that they’re better able to cope with that stress in future.

That’s how your muscles grow. You expose them to a level of stress they’re not accustomed to, which in turn stimulates a positive adaptation, namely muscle growth.

No stress means no growth.

The problem with stress is when it’s applied in such a way that whatever tissue is being stressed can’t adapt.

If you go to the gym and crank out multiple sets of heavy leg extensions the day after ACL reconstruction surgery, you’re applying a level of stress the tissue isn’t strong enough to handle.

But that doesn’t necessarily make the leg extension bad, just a bad idea for some people in certain situations.

Do Knee Extensions Mess Up Your Knees?

Although some people find that leg extensions make their knees hurt, this isn’t going to be true for everyone.

Every exercise has a benefit-to-risk ratio that will vary depending on the exercise in question, the person doing it, and how it’s being done.

One person might find that leg extensions cause their knees to flare up, while someone else might find doing the exact same exercise actually helps to get rid of the pain.

The leg extension isn’t inherently “bad” for your knee joint. In fact, there are studies where it’s been used to improve function and reduce knee pain.

In a study of volleyball and basketball players suffering from patellar tendinopathy (also known as Jumper’s Knee), doing the leg extension three times a week (4 sets of 8 reps, with a 3-second concentric and 4-second eccentric) for a month led to a significant reduction in knee pain and improved function [1].

Here’s what Mark Surdyka, a Doctor of Physical Therapy at E3 Rehab, has to say about leg extensions and knee pain:

“Many authors suggest that individuals with patellofemoral pain should limit themselves to mini squats or leg extensions through half of the range of motion to minimize stress to the patellofemoral joint. I don’t necessarily agree.

“Limiting the range of motion is one option, but it’s not the only option. There’s a simple solution: test it! If it’s tolerable, then there’s absolutely no reason to limit the range of motion. 

“And if someone did have pain with leg extensions, that still doesn’t make the exercise bad. It just means that person can’t tolerate them at this exact moment.”

It’s a subject Dr Surdyka talks more about in the video below.

Are Leg Extensions Good for Building Muscle?

If you want to maximize muscle growth, the leg extension serves as an effective supplement to the squat, split squat or leg press, leading to more complete development of the quads.

That’s because lower body exercises like squats and lunges don’t work all quadricep muscles to the same extent.

One study, published in the journal Medicine and Science in Sports and Exercise, used ultrasound scans to look at muscle growth in the quadriceps after eight weeks of parallel squat training [2].

Three of the four muscles that make up the quads, vastus lateralis, vastus intermedius and vastus medialis, grew by an average of around 15%. In contrast, rectus femoris didn’t grow at all.

Rectus femoris, the muscle running down the front of your thigh, differs from the other three muscles in the quadriceps in that it’s a bi-articular muscle. It crosses two joints, the knee and the hip, rather than just one, functioning as both a knee extensor and hip flexor.

The leg extension has been shown to preferentially recruit rectus femoris [3]. In fact, the leg extension, performed three times a week for three months, leads to faster growth in rectus femoris compared to the other three vastus muscles that make up the quads [4].

Muscle is just contractile tissue that adapts and grows in response to tension. As long as a particular exercise imposes a sufficient level of tension on a given muscle, that exercise can be used to make that muscle grow.

Are Leg Extensions Bad for Athletes?

Those in the functional training camp will often dismiss certain exercises on the basis they have no functional carry over to daily life or sporting activity.

The leg extension is one such exercise.

“The leg extension has no real-world application for athletes,” say the critics. “All it does is load your quadriceps in a non-functional way. There’s no sport or daily activity that requires you to sit down and straighten your legs against resistance. It doesn’t transfer to anything you do outside the gym, and is therefore completely pointless.”

However, functionality isn’t determined by a small number of inputs, such as a specific exercise or piece of equipment, but by output.

And by output, I’m talking about a positive change in whatever physical quality you’re trying to improve.

This will come, not from the exclusion or inclusion of a single exercise or training modality, but from a strength and conditioning program that incorporates many different exercises.

If you had to choose one or the other, you’ll get far more bang for your training buck doing squats rather than leg extensions.

Compound lifts that work multiple muscles at the same time are a far more efficient use of your training time, and do a far better job of increasing lower body strength.

However, if you’ve got the time to do it, and it fits with your goals, there’s no reason to exclude the leg extension from your training program.

Leg Extension vs Squat: Hamstring Activation

The leg extension is also criticized on the basis that it does nothing for the hamstrings, a group of muscles in the back of your thigh responsible for flexing the knee and extending the hip.

You run the very real risk of building your quads at the expense of your hamstrings. Overdeveloped quads combined with weak hamstrings is just setting you up for an injury.

It’s fair to say that the leg extension doesn’t train the hamstrings, but nor is it meant to. And it’s not like you’re limited to a single lower body exercise. Any effective training program will include exercises that target both the quadriceps and hamstrings.

And even with an exercise like the squat, you’re not getting much hamstring activity anyway.

In a Japanese study that assessed muscle growth after ten weeks of squatting, MRI scans revealed that unlike the quads, which grew by around 5 percent, the hamstrings hadn’t changed in size at all [5].

In other words, even if you’re doing plenty of squats, you’ll still need to include some direct hamstring work in your training program.

Final Thoughts

Leg extensions are often slammed as being “bad for your knees” or “useless for building muscle.”

While they certainly have the potential to be bad for your knees if they’re not done properly, they’re also a very useful way to develop your quads.

If your knees are healthy, the leg extension can be a useful addition to your lower body workouts if you’re trying to make your quads grow.

There will be times when the leg extension isn’t appropriate, such as in the early stages of a program designed to rehab an injured ACL, or if it just bothers your knees.

But if you want to build bigger quads, there’s no good reason why the leg extension can’t form part of a training program designed to do exactly that.