Leg Extension vs Leg Curl: What's the Difference

Leg extensions and leg curls (aka hamstring curls) are effective exercises for building muscle mass in your legs. Although compound movements like squats and deadlifts simultaneously engage multiple muscle groups, incorporating isolation exercises like leg extensions and leg curls specifically target and develop your anterior and posterior leg muscles to help strengthen them and fix lagging muscle groups.

Leg extensions and leg curls have similarities, such as knee biomechanics, but they target different lower body portions. Many training programs incorporate both on leg day and we agree that that's the right approach. In this detailed comparison, we'll explore these exercises in depth to assist you in adding these exercises to your routine. 

What Is the Leg Extension?

The leg extension is an isolation exercise that uses the knee extension mechanism to target the quad muscles. It’s a machine-based routine that works with resistance with cable weight plates. 

The leg extension machine is a staple in all commercial gyms and is becoming very popular for home gym owners as well.

Benefits of the Leg Extension

The primary benefit of the leg extension is that it reinforces your patella (knee) and surrounding tissues. This encourages muscle growth in the quadriceps and improves overall knee joint mobility. Below are other important benefits of this routine.

Quad Isolation

Squats and deadlifts target the hamstrings and glutes alongside your quads. This can take away from maximum quad activation. Leg extensions isolate your quadriceps, ensuring they get the attention needed for increases in strength and size.

Carryover to Exercises Like Squats

Since leg extensions are weight-loaded movements, you can use them to improve your performance for compound lifts like the deadlift or squats since they build stronger quads. And your quads are the prime movers when doing squats. 

Improves Knee Health

Leg extensions help you improve your strength at the terminal point of your knee extension, leading to better knee health. With healthy knees, your muscles can shorten and stretch to complete a full range of motion. 

Better for the Hip and Back

Doing compound routines like squats or deadlifts with hip or back pain is next to impossible. Leg extensions can help you strengthen and build muscle while reducing your risk of injury. Compared to compound exercises, leg extensions put no strain on your hip or back when moving weight. 

Non-Fatiguing

You can do many reps and sets of leg extensions without burning out. This isolation exercise is non-fatiguing, so you can easily add more volume for leg growth without risking overtraining. 

Focused Hypertrophy

Leg extensions are among the best exercises to induce muscle hypertrophy in your quads (1). They'll beef up your quads and thicken your thighs. 

Muscles Worked by the Leg Extension

The leg extension is an isolation exercise that trains the quadriceps muscles. Your quads are made up of, from smallest to largest, the vastus intermedius, rectus femoris, vastus medialis, and vastus lateralis. Leg extensions help you define your quads and, ultimately, your thighs.

How to Perform Leg Extensions

1. Set the machine back pad to position your knee bent over the front of your seat. 

2. Adjust the leg pad so it’s just above your feet at the ankle for the duration of the exercise.

3. Sit upright and keep your spine neutral, then grab the handles on the side of the machine for stability. 

4. Firmly put your feet under the pad with your toes pointing upwards. This is your starting position. 

5. Extend your legs at a moderate speed to lift the weight and straighten your knees, then pause for a second or two.

6. Bend your knees moderately to return to the starting position and complete the rep.

Tips for Performing Leg Extensions Correctly and Safely

1. Do the routine slowly. Using momentum will allow you to lift more weight but shift the focus away from building your quads. It could also be dangerous and lead to injury.

2. Stay upright and keep your torso straight by using the handles. This keeps the focus on your quads, as this is not a full-body movement.

3. Pause for a second or more at the top. Squeeze your quads at this point to maximize the tension and induce more hypertrophy. 

4. Don’t use too much weight. This could stop you from doing a full range of motion, and you could also end up using momentum. Both of these reduce the focus on your quads, hindering muscle growth. 

5. Keep your toes facing upward. This dorsiflexion allows you to tense your quads even more at the top of the rep.

What Is the Leg Curl?

The leg curl (also know as a hamstring curl) is an isolation exercise that involves knee flexion and targets the hamstrings. This resistance exercise is machine based and is often used to reinforce your posterior chain mobility. It’s preferred because you can do this without the risk of injury or too much fatigue. 

This is also done most commonly on a dedicated leg curl machine in the gym.

Benefits of the Leg Curl

The leg curl effectively induces training stimuli in your hamstring muscles. This name refers to both the seated and lying variations of this exercise. Both recruit similar muscle groups and use similar pieces of equipment. Here are the benefits of including leg curls in your routine. There are a few leg curl variations depending on which piece of equipment you use: a seated leg curl, a lying leg curl or even standing position leg curl

Hamstring Isolation

Leg curls focus on the knee flexion that activates the muscles of your hamstrings by isolating them. While deadlifts and other routines like good mornings also activate your hamstrings, they could be overpowered by other muscles like your quads and glutes. Leg curls help increase your hamstring strength and size while improving the range of motion in your knee.

Carryover to Routines Like Deadlifts

The primary movers in exercises like the deadlift are your hamstrings. So strengthening the muscle of your hamstrings will improve your deadlifts. 

Improved Hamstring Flexibility 

You need good flexibility in your hamstrings when doing deadlifts and squats. Leg curls help to improve your flexibility by strengthening your hamstrings at different ranges of motion. This helps prevent hamstring injuries, which are among the most common sports injuries (2).

No Stress on Hip or Back

The leg curl puts you in an uncompromising position while building your hamstrings. Since they're stationary, it doesn’t place stress on your back or hips while you train. While other movements like deadlifts and good mornings put considerable strain on the hip and the back.

Non Fatiguing

Since leg curls isolate your posterior leg muscles using knee flexion, they provide lots of benefits at next to no energy costs. This allows you to go for high reps and dropsets, inducing even more hypertrophy and muscle growth in the hamstrings. 

Better For Mind Muscle Connection

Many exercisers can’t feel their hamstrings while doing the stiff-legged or Romanian deadlift. However, with leg curls, you can improve your mind-muscle connection to engage your hammies better. 

Muscles Worked by the Leg Curl

Leg curl exercises isolate the knee flexion that targets your soleus, gastrocnemius, semimembranosus, semitendinosus, and biceps femoris. All of these hamstring muscles surround and ensure stable movement of the legs.

How to Perform the Lying Leg Curl

1. Lie on the leg curl machine face down and align your knees with the lever arm’s pivot point.

2. Adjust the leg pad to rest against the lower part of your calves, just before the ankles. 

3. Grip the machine handles, press your abs into the bench, and brace. Avoid lifting your lower back or arching. This is your starting position.

4. Bend your legs to pull your heels towards your glutes.  

5. Slowly lower the weight by contracting your hamstrings to extend your legs and return to the starting position for a complete rep.

How to Perform the Seated Leg Curl

1. Sit on the leg curl machine and straighten out your legs - this will be your reference point.

2. Adjust the leg pad to rest against the top of your leg, just above the knee. This pad will help hold your legs down during the movement

3. Adjust the ankle pad so that your legs rest with the pad just at the bottom of your calf/top of your achilles.

4. Grip the machine handles and brace your core. Avoid arching your lower. Your leg will be straight out in front of you. This is your starting position.

5. Bend your legs to pull your heels towards your glutes.  

6. Lower the weight by contracting your hamstrings to and slowly extend your legs to get back to the the starting position for a complete rep.

Tips for Performing Hamstring Curls Correctly and Safely

1. Do each rep slowly to maximize the muscle tensions and eliminate momentum. Try to avoid using a kick but rather a slow and smooth contraction.

2. Bring your feet as close to your glutes as possible to go through the full range of motion. Using an incomplete range of motion affects the efficacy of this routine.

3. Don’t lift your hips off the bench while doing each rep; this stresses your back. Instead, ensure that you bring your feet to your glutes to make the routine maximally effective. 

4. Don’t go too heavy with weights, as you will find yourself using momentum. This takes away from the workload on the hamstrings. 

Leg Extensions Vs. Leg Curls

Need help to decipher which of these exercises to include in your workouts? Here are some more details comparing them to help you decide.

Kinetic, Mechanical, and Positional Differences

The leg extension and leg curl use different biomechanics to complete the exercises. While leg extensions are performed upright, the leg curl can be done in a prone or seated position. In addition, the leg extension involves knee extension, while the leg curl use knee flexion. However, both exercises work with resistance placed at ankle height and using your knee joint with a stable torso and hips to move the load. 

All of that was to say the main difference is leg extensions work the quads on the front of your thighs and leg curls work the hamstrings on the back of your legs.

Disadvantages, and Safety Mechanisms

Both leg extensions and leg curls aren’t functional movements and don’t have real-world carryover like other movements such as squatting. But they have safety mechanisms built into the machine to isolate the muscles in your quads and hamstrings safely.

However, leg extensions pose a risk of straining your knee due to the the load during extension. As a result, they’re best done either at the beginning of your workout with light weight as a warm-up. Or later in your leg workout after compound exercises such as squats to prevent injuries. Always use proper form and control your reps!

Comparative Discussion on the Muscles Worked

Leg extensions work the four muscles in your quadriceps — the vastus lateralis, rectus femoris, vastus intermedius, and vastus medialis. Leg curls, on the other hand, target the three muscles in your hamstrings — gastrocnemius, semitendinosus, and semimembranosus. 

Exploring the Synergy Between Leg Extensions and Leg Curls

Leg extensions and leg curls are most effective when done together. They help to train the entire thigh by working on the two opposing muscle groups — the hamstrings and quads. These are antagonistic muscles, meaning while one lengthens, the other contracts. Therefore, adding both exercises to your leg workout is best to prevent overtraining or undertraining either muscle, which can lead to injury. Including both in your plan will overall develop more muscular and stronger legs. 

The Synergy of Leg Extensions and Leg Curls

Leg extensions and leg curls work on the quadriceps femoris and the hamstring muscles on opposing sides of the femur bone. They act synergistically for most leg movements, such as jumping and sprinting. Using these exercises to strengthen these muscles means increased athletic performance and more weight lifted on squats.

Since these muscles are complementary, scheduling them together in your workout is best. That way, you can boost the growth of your leg's anterior and posterior portions. This gives you a well-rounded leg workout that can help you prevent leg injuries from training only on one side. 

Frequently Asked Questions

How can I do leg extensions and leg curls at home?

There are many ways to incorporate these exercises into your home workouts. You can rig a bunch of different set ups using a bench and a resistance band, cable machines, etc. These are all cheap options and space efficient options that may take advantage of the equipment you already have.

But if we're being honest, the most effective way to train these exercises is with a dedicated leg extension/curl combo machine. This dual function piece of equipment should be a home gym staple if you're serious about training your lower body. 

Are leg curls or leg extensions better?

The routine that is better for you depends on your workout goals. Leg extensions are the best if you want to focus on quad growth. Leg curls work better for hamstring development. 

Are leg extensions bad for knees?

Leg extensions don’t cause any more stress on your knees than routines like squats and deadlifts. That said, there is an increased risk of knee injury due to the strain on your knees while in the extension position. 

Are leg curls a waste of time?

Leg curls can be a valuable addition to your leg-building workout. While this exercise may not adequately replace movements like the deadlift, it can perfectly supplement this routine.

Leg Curl vs Leg Extension Wrap Up

The leg extension and the leg curl are valuable isolation exercises that should be included in your leg training routine. While they may be seen as accessory exercises, they play a crucial role in complementing your compound leg exercises. So, incorporate them both into your program to achieve adequate leg growth and strength and prevent muscular imbalances.

When comparing the two, it’s crucial to consider your goals before deciding which is best. If you aim to develop larger and stronger quads, then the leg extension exercise better suits you. On the other hand, if you want stronger hamstrings, the leg curl is the one to go with. The leg curl machine will better protect your knees and prevent common sports injuries like pulling a hammy. However, a comprehensive lower body workout will ultimately include both for optimal leg development. 

References

1. Ebben, W. P., Feldmann, C. R., Dayne, A., Mitsche, D., Alexander, P., & Knetzger, K. J. (2009). Muscle activation during lower body resistance training. International journal of sports medicine, 30(1), 1–8. https://doi.org/10.1055/s-2008-1038785 

2. Danielsson, A., Horvath, A., Senorski, C., Alentorn-Geli, E., Garrett, W. E., Cugat, R., Samuelsson, K., & Hamrin Senorski, E. (2020). The mechanism of hamstring injuries - a systematic review. BMC musculoskeletal disorders, 21(1), 641. https://doi.org/10.1186/s12891-020-03658-8