Leg Extension vs. Leg Curl: A Definite Guide from Experts

Friends don’t let friends skip leg day – or so the popular meme goes. Leg training will improve your overall appearance and athletic performance and can even help you lose fat and control your weight.

Bottom line: If you can train your legs, you should – hard and often!

Most experts agree that compound leg exercises like squats, lunges, deadlifts and leg presses are the best way to build muscular, strong legs, but how do leg extensions and leg curls fit into lower body training?

After all, both of these popular exercises appear in many of the best strength training and bodybuilding programs, and most gyms have leg extension and leg curl machines.

Do you need to leg extensions and leg curls? Are they safe and effective, or just a waste of time? Is one exercise better than the other? What can you do instead?

We have the definitive answers to all your leg extension and leg curl-related questions!

Leg Extensions Vs. Leg Curls: A Brief Overview

While you can develop strong, powerful, good-looking legs with compound exercises like squats, leg presses, and deadlifts, these muscles work your quadriceps, hamstrings, and other lower body muscles at the same time.

Leg extensions and leg curls are single-joint or isolation exercises that allow you to focus exclusively on your quadriceps or hamstrings alone.

Leg Extensions 101

Leg extensions are a lower body exercise for the quadriceps, which are the muscles on the fronts of your thighs. There are four quadriceps – vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris.

The main function of the quadriceps is the extension of the knee joint. However, rectus femoris is also involved in hip flexion as it crosses the knee and hip joints.

While exercises like squats, leg presses, lunges, and step-ups all work your quads, leg extensions are arguably the most effective way to isolate these powerful muscles.

Leg Curl 101

Leg curls train your hamstrings which are located on the back of your thighs. There are three hamstring muscles – the semimembranosus, semitendinosus, and biceps femoris.

The hamstrings have two functions – knee flexion and hip extension. However, in leg curls, they are only responsible for the flexion of the knees.

The hamstrings are involved in most compound lower body exercises, especially those that involve hip hinging, such as Romanian deadlifts, good mornings, and deadlifts. However, leg curls are the only exercise that emphasizes knee flexion over hip extension.

How to Do Leg Extensions

To get the most from any exercise, you need to do it correctly. Follow these steps to perform leg extensions correctly.

Leg extensions are best done for sets of 8-20 reps and two to five sets. Use a weight that brings you to within a couple of reps of failure in your chosen rep range. Rest 1-2 minutes between sets.

1. Sit on the leg extension machine and adjust the backrest, so your knees are in line with the machine’s lever-arm pivot point.
2. Adjust the leg pad, so it’s just above your ankle. Flex your ankles and pull your toes up toward your shins.
3. Set the weight by moving the selector pin up or down. Hold the handles next to the seat to stabilize your upper body.
4. Without jerking or kicking, extend your legs to lift the weight. Straighten your knees and pause for 1-2 seconds at the top of each rep.
5. Bend your legs and smoothly lower the weight, but do not allow the weight plates to touch down.
6. Continue for the desired number of reps.

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Note: You can also work one leg at a time, i.e., single-leg leg extensions.

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Related: Best Leg Extension Machines

Tips for mastering the leg extension

Get more from leg extension with these proven pro tips!

Take it slow – the more slowly you extend your legs, the less momentum you’ll generate, and the more tension there will be on your quads. Going faster will allow you to lift more weight but also makes leg extensions less effective and potentially more dangerous.

Pause at the top of each rep – squeeze your quads as hard as possible at the mid-point of each rep to maximize muscle tension and make your workout more effective.

Keep your torso stationary – this is a leg isolation exercise, not a full-body movement. Use the handles to keep your upper body locked in place so all the work falls on your quads.

Pull your toes up – increase quadriceps engagement by pulling your toes up toward your shins. This is called dorsiflexion and will allow you to tense your quads even harder at the top of each rep.

Common leg extension mistakes

Avoid these mistakes to make your leg extension workouts even more effective!

Using too much weight – leg extensions work best when done for moderate to high reps with medium to light weights. Going too heavy means you may not be able to fully extend your legs or could end up kicking the weight up, both of which take tension off your quads.

Going too fast – slow and smooth is the best way to do leg extensions. This keeps your muscles under constant tension for a better muscle-building effect.

Making leg extensions your main quads exercise – in nature, your quads seldom work in isolation. Instead, they almost always work alongside your hamstrings, glutes, abductors, and adductors. So, while leg extensions are a good exercise, they’re best done in conjunction with compound exercises like squats, lunges, leg presses, etc. 

Leg extension alternatives

No leg extension machine? No problem! You can also isolate your quads by doing the following leg extension machine alternatives:

1. Dumbbell leg extensions
2. Ankle weight leg extensions
3. Cable machine leg extensions
4. Resistance band leg extensions
5. Kneeling bodyweight leg extensions

Leg extensions and injury risk

Leg extensions are a reasonably safe exercise for most people. However, there is always a risk of muscle strains if you use too much weight or try to kick the weight up. Leg extensions also put a shearing force through your knee joints, which puts stress on the cruciate ligaments, which are responsible for anterior/posterior knee stability.

Despite this, leg extensions are often used to rehab knee injuries. The movement is guided by the machine, and you can adjust the weight easily to match your current strength.

Done correctly, leg extensions are no more dangerous than most other lower body exercises.

How to Do Leg Curls

To get the most from leg curls, you need to do them correctly. Follow these guidelines to perform leg curls safely and effectively.  

Leg curls are best done for sets of 8-20 reps and two to five sets. Use a weight that brings you to within a couple of reps of failure in your chosen rep range. Rest 1-2 minutes between sets.

1. Select the appropriate weight.
2. Lie on the leg curl machine so your knees are in line with the machine’s lever arm pivot point. Adjust the leg pad, so it’s resting against the lower part of your calf.
3. Grip the handles and brace your abs. Press your abdomen into the bench, and do not allow your lower back to lift or arch.
4. Bend your legs and pull your heels up toward your butt. Try to touch your hamstrings with your calves.
5. Slowly lower the weights but stop just short of the plates touching down.
6. Continue for the desired number of reps.

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Note: You can also work one leg at a time, i.e., single-leg leg curls.

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Tips for mastering the leg curl

Get more from leg curls with these proven pro tips!

Squeeze and don’t kick the weights up – eliminate momentum and maximize muscle tension by doing each rep slowly and deliberately. Start each rep with a smooth contraction and not a kick.

Use a full range of motion – maximize hamstring engagement by bringing your feet as close to your butt as you can at the midpoint of every rep. Cutting your reps short will make this exercise less effective.

Don’t go too heavy – using heavy weights mean you are more likely to need to use momentum and less able to complete a full range of motion. Instead, check your ego and reduce the load to get a better hamstring workout.

Common leg curl mistakes

Avoid these mistakes to make your hamstring training even more effective!

Lifting your hips – bring your feet to your butt and not your butt to your feet by lifting your hips off the bench. Lifting your butt not only makes the exercise less effective but also puts more stress on your lower back.

Hurrying your reps – slow and steady work best for leg curls. Make your reps more effective by lifting and lowering the weight with a controlled, smooth tempo, e.g., 2-3 seconds up and 2-3 seconds down.

Letting the weights touch down between reps – letting the weights touch takes tension off your hamstrings. It could also allow you to bounce out of the bottom of each rep. Both of these errors will make your leg curl workout less effective.

Leg curl variations and alternatives

There are a few different ways to do leg curls, as well as some ways to do leg curls without a specific leg curl machine:

1. Seated leg curl machine
2. Standing leg curl machine
3. Cable machine leg curls
4. Resistance band leg curls
5. Dumbbell leg curls
6. Suspension trainer leg curls
7. Nordic leg curl
8. Valslide leg curl

Leg curls and injury risk

Leg curls are a very safe exercise. Done correctly, the main risk of injury is muscle tears and putting stress on the lower back by lifting the hips off the bench.

However, if you do not position your legs correctly on the leg curl machine bench, there is a danger that you could hyperextend your knees and damage your cruciate ligaments, which are responsible for anterior/posterior knee stability.

Avoid this risk by lowering the weight slowly and never extending your knees beyond their natural range of motion.

Leg Extension and Leg Curls Pros and Cons

Not sure if leg extensions and leg curls deserve a place in your workouts? Consider these pros and cons, and then make your decision!

Leg extension pros

Isolate your quads – no exercise isolates your quads like leg extensions. If you want to work your quads without involving your glutes and hamstrings, leg extensions are the way to go.

No hip or back stress – unlike squats, leg presses, lunges, etc., leg extensions put no strain on your lower back or hips, so they may be helpful for working around hip pain or lower back pain.

Could improve squat performance – if weak quads prevent you from squatting as much weight or doing as many reps as you want, leg extensions can help. They’re a useful way to target your quads and strengthen them in isolation.

Improve terminal joint extension strength – most leg exercises get easier as you approach knee extension. This could mean you are weak at the end of your range of motion. Leg extensions get harder as you straighten your leg and could help improve your strength at terminal knee extension.

Localized hypertrophy – if you want to beef up your quads and pay less attention to your hamstrings and glutes, leg extensions are arguably the best way to go about it. Leg extensions are an excellent quad-building exercise.

Ideal for beginners – leg extensions are easy to learn, and most machines allow you select very light weights. This means they’re the perfect exercise for beginners, the elderly, and the unfit.

Leg extension cons  

You need a leg extension machine to do them – while most gyms have leg extension machines, you probably won’t have space for one if you work out at home. As such, you’ll need to find another way to isolate your quads, such as dumbbell leg extensions.

Knee stress – while leg extensions are mostly safe, if you have preexisting knee problems, they could make matters worse. However, problems with leg extensions are relatively rare, especially if you avoid going too heavy and do them with good technique.

Leg curl pros

Isolate your hamstrings – Romanian deadlifts, good mornings, etc., work your hamstrings but also train your glutes. Leg curls are arguably the only way to work your hammies in isolation. This can be useful if you find your glutes tend to overpower your hamstrings during other leg exercises.

Improve your mind-muscle connection – a lot of lifters are unable to feel their hamstrings working during Romanian and stiff-legged deadlifts. Leg curls are an excellent way to reconnect with your hamstrings and strengthen the mind-muscle connection for better future workouts.

Reduce your risk of hamstring injuries – the hamstrings are one of the most commonly injured muscles, especially for athletes. Leg curls allow you to gradually increase hamstring strength and lower the risk of injuries.

Minimal lower back and hip stress – done correctly, your lower back and hips should remain stationary during leg curls. In contrast, Romanian deadlifts and hyperextensions involve a lot of lower back and hip activity. Leg curls are a very lower back and hip-friendly exercise.

Leg curl cons

You need a leg curl machine – while there are a few different types of leg curl machines to choose from, not all gyms have them. If you train at home, you may not have a leg curl machine at all. In that case, you’ll need to make do with one of the non-leg curl machine alternatives listed elsewhere in this guide.

Knee stress – leg curls can cause knee stress if you let your hamstrings relax at the bottom of a rep and hyperextend your knees. However, some machines have range of motion limiters to prevent this problem.

Not a functional movement – it’s very rare for your hamstrings to work without your glutes. As such, if you want to develop functional strength, e.g., for sports, the leg curl may not be the best exercise for you.

Choosing Between Leg Extensions and Leg Curls

Still unable to choose between leg extensions or leg curls? Consider these final points and then decide!

What is the primary difference between leg extensions and leg curls?

Leg extensions work two different muscles. As such, you should choose the exercise that trains the muscles you want to develop. A few reps of each exercise will soon tell you which muscles you are working!

Leg extensions work the quadriceps, which are the muscles on the front of your thighs. These muscles are responsible for extending or straightening your knees. There are four of them, which is why they’re called the quadriceps – quad means four.

Leg curls train the hamstrings, which are the muscles on the back of your thighs. The hamstrings are responsible for flexing your knee and extending your hip. However, knee flexion is the name of the game during leg curls. One of the hamstrings is called the biceps femoris, which means two-headed leg muscle. It bends your leg in much the same way as your biceps bend your arms.

Which is better – leg extensions or leg curls?

When comparing leg extensions to leg curls, it’s impossible to say that one is better than the other. They’re opposite sides of the same coin and develop opposing muscle groups. The best one depends on whether you want to train your quads or hamstrings.

In most cases, you’ll probably get better results from your workouts if you do both of these exercises. Training only your quads or hamstrings could create muscle imbalances, which would affect the appearance and function of your lower body.

Which one is right for me?

Only you know the answer to that question! If you want to train your quadriceps, leg extensions are the right choice. But, if you want to focus more on your hamstrings, leg curls are the exercise for you.

If in doubt, you should probably do both as they’re equally beneficial and effective exercises. Try supersetting leg extensions with leg curls to train your quads and hamstrings in the most time-efficient way.

Frequently Asked Questions

Do you have any burning questions about leg extensions and leg curls? Unable to decide which of these exercises is the right one for you?

These FAQs will give you the answers you’ve been looking for!

Are leg extensions and leg curls necessary?

Leg extensions and leg curls are best thought of as supplementary exercises. While they’re undoubtedly useful, you can develop muscular, strong legs without them, so they’re not compulsory.

However, if you want to preferentially isolate the muscles on the front of the back of your thighs, leg extensions and curls are one of the best way to do it.

Can you build big legs with leg extensions?

If you want bigger quadriceps, leg extensions can help. They keep your muscles under almost constant tension and produce a deep pump and burn, all of which can help build muscle.

On the downside, leg extensions do not train your hamstrings, and if you want more muscular legs, you need to develop the back as well as the front of your thighs.

So, while leg extensions can make your legs bigger, you’ll get better results if you combine them with exercises for your hamstrings.

What are the alternatives to leg extensions?

Leg extensions are not the only way to isolate or emphasize your quadriceps. Other exercises you can do include:

1. Narrow stance goblet squat
2. Sissy squat
3. Hack squat machine
4. Barbell hack squat
5. Narrow stance leg press
6. Narrow stance Smith machine squat
7. Short-step lunges
8. Dumbbell leg extensions
9. Kneeling bodyweight leg extensions
10. Seated straight leg raises
11. Wall squats
12. Backward sled drags

Read about these exercises and more in our guide to quadriceps isolation exercises.

Are leg extensions as good as squats?

The best exercise is the one that most closely matches your goal. For example, suppose you want to isolate your quadriceps and avoid using your hips and hamstrings. In that case, the leg extension is your best choice.

But squats rule the roost if you want to train all your major leg muscles in one movement.

Think about what you are trying to achieve, and then choose the exercise that most effectively meets your needs. For some, that’ll be leg extensions, while for others, it’ll be squats.

There may even be another exercise that’s best, such as leg presses or hack squats.

Which is better: stiff-legged deadlift or leg curl?

As with the previous Q & A, the best exercise is the one that’s right for you. Stiff-legged deadlifts work your hamstrings, glutes, and lower back, while leg curls isolate your hamstrings. The right one for you depends on what you are training for.

In most cases, you don’t even need to choose between stiff-legged deadlifts and leg curls, or squats and leg extensions, for that matter. You’ll probably get the best results from combining these exercises.

Can these exercises replace squats and deadlifts?

Squats and deadlifts are compound exercises involving multiple muscles and joints working together. This includes the ankle, knee, and hip joints, quadriceps, hamstrings, glutes, and lower back muscles. Both exercises also mirror everyday movements, like getting out of a chair or picking an object off the floor.

While leg extensions and leg curls work some of the same muscles, they are totally knee-centric and do not involve the hips. Nor do they mirror any everyday activities.

It’s probably best to think of leg extensions and leg curls are supplements to and not replacements for squats and deadlifts. This makes a lot of sense given that squats are often called the king of exercises, and deadlifts rank just as highly.

Who should avoid leg extensions and leg curls?

Leg extensions and leg curls are well tolerated and safe for most people, especially when performed correctly and without too much weight. However, both exercises do put a lot of shearing force on the knee joints, so they may not be suitable for everyone.

Avoid leg extensions and leg curls if you have a history of:

• Meniscal tears
• Bursitis
• Sprained or torn knee ligaments
• Severe crepitus
• Osteoarthritis
• Patellar tendonitis
• Patellar tendonosis
• Patellofemoral pain syndrome
• Any knee pain made worse by leg extensions or leg curls

Leg press vs. leg extension, which is better?

When comparing exercises, it’s always best to consider your reason for doing the specified exercises.

The leg extension is the way to go if you want to focus on your quads and ignore your hamstrings and glutes. No other exercise can isolate your quads like leg extensions. But, if you want a general lower body workout that works your quads, glutes, and hamstrings, the leg press is probably best.

So, think about what you want to achieve, and then choose the exercise that most closely matches your goal. Sometimes that’ll be leg extensions, while at other times it could be the leg press.

Are the leg extension and leg curls machines any good?

They sure are! In the same way that most people do biceps curls and triceps pushdowns to train their arms, leg extensions and leg curls are great exercises for isolating your quadriceps and hamstrings, respectively.

While you CAN work these muscles with compound exercises, e.g., leg presses or hip Romanian deadlifts, working them in isolation can be useful, too, especially if your muscles are weak, need extra attention, or you want to build them as big as possible.

When it comes to leg extensions and leg curls, should I go light or heavy?

Going too heavy with leg extensions or leg curls means you’ll probably have to use momentum to lift the weight. That means your muscles won’t be under as much tension, or under tension for as long. This would make either exercise less effective.

Because of this, it’s usually best to use moderate to light weights and do medium to high reps for leg extensions and curls.

Sets of 8 to 20 reps with a weight that you can lift and lower with controlled form will undoubtedly produce superior results to low reps, heavy weights, and kicking the weights up.

Leave heavy weights and low reps for squats, deadlifts, leg presses, etc.

Should I do leg extensions as a runner/sprinter?

Leg extensions can be beneficial for sprinters because they emphasize the vastus medialis, which is important for the correct tracking of the patella or kneecap and can help increase knee joint stability.

However, it’s important to remember that running and sprinting involve more than just the quads; the hamstrings are essential too. Also, the knees do not work in isolation, and the hips are also involved.

So, while leg extensions can be helpful, they are just one of the exercises you can do that’ll help your running or sprinting performance. Lunges, squat jumps, Bulgarian split squats, etc., are equally valuable.

Can I only do squats and deadlifts and skip leg extensions and leg curls completely?

You sure can! Where squats emphasize your quadriceps, deadlifts emphasize your hamstrings, which makes leg extensions and leg curls somewhat redundant. A lot of powerlifters train almost exclusively with squats and deadlifts and rarely isolate their quads or hammies.

Old-school bodybuilders built massive legs without leg extensions and leg curls because those machines weren’t even invented back then.

That said, if you want to work the front or back of your thighs without training your hips, leg extensions and leg curls could be a valuable addition to your workouts, especially if you do them after your squat and deadlift workouts.  

So, while you don’t have to do them if you don’t want to, there is no reason to avoid leg extensions and leg curls.

How can I do lying leg curls without a machine?

The easiest way to do lying leg curls without a machine is by doing dumbbell leg curls. This is an old-school exercise and how many golden-era bodybuilders used to train their hamstrings.

To do this exercise, lie on your front on an exercise bench, so your knees are resting on the edge. Place a dumbbell between your feet and hold it in place by squeezing your legs together. Bend your knees and pull your feet toward your butt, and then extend them again.

Alternatively, you can do lying leg curls with a resistance band wrapped around your ankles and anchored to something like a heavy dumbbell or the upright of a squat rack.

You can also go leg curls using a suspension trainer or gymnastic rings.

What’s the difference between a seated leg curl and a lying leg curl?

Not a lot! Both exercises involve knee flexion and target your hamstrings. However, when you do seated leg curls, your hips are flexed, which places your hamstrings in a slightly stretched position at the start of each rep. As such, your muscles may be under tension for longer.

With lying leg curls, there is no such stretch, so tension only tends to come on near the mid-point of each rep. Some lying leg curl machines get around this by having a cambered bench that lifts your hips.

Both exercises are similarly effective, so try them both and see which one you prefer.

How to do these exercises without machines?

There are many ways to train your quadriceps and hamstrings without resorting to leg extensions or curls. You’ll find lots of ideas in these articles:

• 10 Best Quad Exercises of All Time
• 13 Best Quad Exercises You Can Do At Home
• Five Must-Have Hamstring exercises for Jacked Legs
• Best Bodyweight Hamstring Exercises

And don’t forget to stretch your quads and hamstrings, too:

• The Seven Best Hamstring Stretches for a Healthier Lower back and Knees
• The Seven Best Quadriceps Stretches for Healthier, More Mobile Knees

Leg Extension vs. Leg Curl: Final Thoughts

In many vs. type articles, there is a clear winner. But that’s not the case here!

Leg extensions and leg curls work completely different muscles and, as such, are equally valuable. So, the real question is should you do these popular isolation exercises, or are squats, deadlifts, etc., all you need?

The truth is that leg extensions and leg curls are valuable but not compulsory exercises. Instead, they should be viewed as supplements to and not replacements for compound leg training. Your quads and hammies do not work alone, so it doesn’t make all that much sense to train them that way. In almost every everyday movement, the hips are involved too.

That said, if you want to focus more attention on your quadriceps or hamstrings for localized strength or hypertrophy, leg extensions and leg curls are hard to beat. But, you’ll undoubtedly get better results if you combine them with compound leg training.

Either way, you should always choose the best exercises for your current training goal. Bodybuilders often include leg extensions and leg curls in their workouts, whereas powerlifters and weightlifters do not. It’s a matter of meeting the needs of the exerciser.

Include leg extensions and leg curls in your program if you think they’ll help, or skip them if the time and effort needed to do them outweigh the benefits.