Do Leg Extensions Build Muscle?

Leg extensions can be performed with or without a leg extension machine, and they are one of the most common exercises for building leg muscle. 

However, many new fitness enthusiasts find themselves getting discouraged when their hours of work on the leg extension machine don’t produce the results they were hoping for. Even worse, home leg extension machine users find that their workouts begin to hurt their knees over time. 

Much of this is down to not properly understanding which muscles leg extensions work and how to maximize progress through leg extensions without putting strain on the knee joints. 

In today’s article, we’re going to be discussing which muscles leg curls work, how best to build muscle through leg extension exercises, and whether or not leg extension machines are actually bad for your knees.

Do Leg Extensions Build Muscle

What Muscles do Leg Curls Work? 

Leg curls are one of the most popular exercises to do on a leg extension machine. 

Performing a leg curl, or a prone leg curl, on a leg extension machine involves lying on your front with your legs outstretched underneath the roller pad. Then, you bend your knees, pushing the lifting bar upwards. Hold for a moment, and carefully lower back to your original position, maintaining control throughout. 

There are a few other variations of the leg curl, but they all have one thing in common: they isolate the hamstring muscles in a way that no other exercise does. 

You may have heard that squats are a good form of exercise for building the hamstrings, and that is true, but the difference is that squats work the hamstrings secondarily to the glutes and quads. When you perform a leg curl, your hamstrings are isolated as the primary strengthening target. 

Now, working your hamstrings is very important. We use our hamstrings throughout our daily lives, whether to walk to the store or hit the treadmill at the gym. Weak or tight hamstrings can cause a lot of discomfort in the knees and lower back, which increases the risk of injury.

In fact, tight hamstrings can actually restructure the alignment of your hips and spine, so it’s crucial to keep these muscles strong and relaxed through proper exercise. 

However, the downside to an exercise like the leg curl that isolates one specific muscle is that it can’t be relied upon for a full leg workout. If you want to build muscle using your leg extension machine or other leg extension exercises, you’ll need to mix things up a bit. 

How to Build Muscle Through Leg Extensions and Curls

While leg extensions and leg curls can be a highly beneficial and effective part of your leg day routine, they shouldn’t constitute your entire workout. 

Leg extensions and leg curls should be used exclusively to train your hamstrings and quads. Although the standard leg curl only works the hamstring muscles, you can perform some leg extension variations to target a wider range of muscles.

For example, performing leg extensions (where you sit at the machine with the lift bar resting on the fronts of your ankles and extend your legs to lift the bar) can strengthen your outer quad sweep, your rectus femoris, and your ‘teardrop’ muscles for a more comprehensive upper leg workout. 

It’s very easy to change up your leg extension exercises to work different areas of your quad muscles. All you need to do is vary the directions in which you point your toes while lifting the bar. 

To target the outer sweep, point your toes inwards as you lift. To strengthen the rectus femoris, you’ll need to turn your toes outwards. The direction of your toes doesn’t matter so much with the teardrops - leg extensions will work these muscles, regardless. 

On top of your leg curls and extensions, you should also be working on your squats and lunges. Combining the above exercises with lunges and squats will yield the best results in terms of muscle building. 

Is the Leg Extension Machine Bad for your Knees?

Unfortunately, the downside to using a leg extension machine to build your hamstrings and quads is that you may be unintentionally putting a lot of strain on your knees in the process.

The anterior cruciate ligament (or the ACL for short) is responsible for ensuring rotational stability in the knee joints. This is key to a lot of the knee’s natural range of motion, so if the ACL gets damaged, you could find your mobility significantly impaired. 

In cases of significant damage to the ACL, surgery will usually be required since this ligament lacks the ability to heal on its own. Often, some of your knee’s original functions will remain permanently altered. 

Leg extensions, especially when performed on an extension machine, put a lot of pressure on the ACL. Lateral patellar deviation, where the kneecap becomes misaligned, is one of the most common injuries resulting from leg extensions. 

Now, this doesn’t necessarily mean that you need to throw out your leg extension machine and swear off leg extensions altogether. It just means that you will need to pay extra attention to your form and listen carefully to your body in order to avoid injury. 

In order to stop your knees from bearing the brunt of your leg extensions, it’s crucial to maintain the correct form and posture at all times when using the machine. 

Make sure you’re sitting up straight against the backrest and use controlled movements rather than jerking your legs up for more momentum. When you reach your extension, avoid locking your knees. Also, remember not to overdo it.

Pair your leg extensions with other forms of quad and hamstring exercises to avoid overworking your knees with the same movements. 

Final Thoughts

While leg extensions and leg curls do build muscle, they should not be your only priority on leg day. 

Leg extensions should be varied with different toe positions to work more areas of your quads, while leg curls exclusively work the hamstrings. 

To effectively build muscle, you should also be performing squats and lunges alongside extensions and curls. 

Remember that leg extension machines have been linked to knee injuries, so if you’re going to use this machine, make sure to take it slow and steady while observing the correct form.