While leg extensions and leg curls can both be used to strengthen and build the muscles of the leg, depending on your weaknesses, I would recommend prioritizing one over the other.
So, what is the difference between the leg extension and the leg curl? The leg extension isolates the muscles in the front of the leg (quads), while the leg curl targets the muscles in the back of the leg (hamstrings). Leg extensions can improve your squat, while leg curls can be used to improve your deadlifts.
In this article, I will explain when it is appropriate to implement either the leg extension or the leg curl. I will also cover the fundamentals of each exercise, muscles used, and their pros and cons.
Leg Extensions vs Leg Curls: An Overview
Lower body movements, like the squat, deadlift, and leg press, often involve hamstrings and quads but don’t isolate them. Incorporating isolation work for these specific muscles, like the leg extension and leg curls, can improve strength, increase muscle mass, and ‘bring up’ lagging body parts.
The leg extension is an isolation exercise where the main focus is on the muscles of the quads.
This exercise involves knee extension, which is a key movement that is involved in the squat, lunge, and running exercises.
However, the quads are often targeted alongside the glutes, calves, and core stabilizers in compound exercises. Consequently if you want to focus on quads, leg extensions become a key component to a successful program.
The leg curl is an isolation exercise where the main focus is targeting the muscles of the hamstrings.
This exercise involves knee flexion, which is a key for stabilizing the squat, building tension in the deadlift, and braking while running.
The hamstrings can be overpowered by the glutes, calves, and core stabilizers in compound exercises, whereas if you wanted to focus on hamstrings, leg curls are a great addition to your program.
Leg Extension vs Leg Curl: Pros and Cons
Leg Extension Pros:
- The leg extension isolates the quads. Compound movements like the squat target the glutes, hamstrings, and erectors which can take away from the quad activation. By isolating the action of knee extension, quads can get greater attention and work which has many strength and size benefits.
- The leg extension can help strengthen the squats. Quads are a prime mover in the squat, by strengthening the quads, we can therefore improve the squat exercise.
- The leg extension can improve knee and quad health. Healthy joints and muscles can stretch and shorten while moving through full range of motion. The leg extension moves through the full arc motion of knee flexion/extension which is fundamental to knee health.
- The leg extension puts no stress on the back or hip. Oftentimes when we have a hip or knee injury, we cannot do compound exercises such as the squat or the deadlift. However, we still need some sort of strengthening exercise, which is why the knee extension can prove to be a handy exercise to utilize.
- The leg extension can be done for many reps without fatigue. Dissimilar to compound exercises, as an isolation exercise, leg extensions aren’t very fatiguing, which allows for little consequence when incorporating them into your routine. Furthermore, you can do many reps, burnouts, and many sets with this exercise.
Leg Extension Cons:
- The leg extension typically requires a machine. While you might not have access to a machine to load up this exercise. Doing banded variations can provide similar if not greater benefit in improving knee extension and quad strength.
- The leg extension cannot be loaded heavy enough. By targeting only a single joint, the knee extension exercise cannot be loaded up as much as a squat exercise, which is multi-joint and involves bigger muscles such as the glutes and erectors.
- The leg extension can put greater stress on the knee. Isolation exercises have the potential to put greater stress on a single joint. In contrast, compound exercises allow for the assistance of other joints and muscles, therefore placing less stress on a single joint and muscles.
Leg Curl Pros:
- The leg curl isolates the hamstrings. While hamstrings are prime movers in exercises such as the deadlift and good mornings, they will often get overpowered by other muscles such as the glutes and quads. Leg curls focus on the action of knee flexion, which gives greater attention to the hamstring muscles by isolating them.
- The leg curl can help strengthen the deadlifts. Hamstrings are prime movers in the deadlift exercise, by focusing on the hamstrings, we can therefore improve the deadlift exercise.
- The leg curl can improve hamstring flexibility. Hamstring flexibility is fundamental in exercises such as the squat and deadlift. Leg curls strengthen the hamstrings in different ranges of motion, which can directly improve our positioning in the squat and deadlift.
- The leg curl puts no stress on the back or hip. While compound exercises such as the good morning and deadlift can place stress on our back and hip, we still need to strengthen the muscles involved. A leg curl can be used to strengthen the hamstrings while being placed in an uncompromising position.
- The leg curl is non-fatiguing due to its single-joint nature. Similar to the leg extension, leg curls isolate a single joint action of knee flexion, which creates many benefits while having little cost when added to a program. Because of this, we can implement burnouts, high repetitions, and dropsets to produce maximal fatigue of the hamstrings.
Leg Curl Cons:
- The leg curl typically requires a machine. Oftentimes, one will need a leg curl machine to fully load this exercise. If you don’t have access to a leg curl machine, banded leg curl variations can similarly load up the hamstrings.
- The leg curl cannot be loaded heavy enough. Heavier intensities are sometimes key to make breakthroughs in plateaus. While this might be the case, leg curls are somewhat limited in their ability to be loaded. Tempo reps can make up for this by producing a different type of loading on the hamstrings.
- The leg curl can put greater stress on the knee. Just like the leg extension, the hamstring curl isolates the small single-joint action of knee flexion, which can place great stress on the knee.
As well, sometimes the leg curl can cause your calves to cramp. If this happens to you, then check out my article: 4 Reasons Why Your Calf Cramps In The Leg Curl.
Leg Extensions vs Leg Curls: Muscles Used
The leg extension isolates the movement of knee extension which targets the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius muscles. While the leg curl isolates the movement of knee flexion which targets the gastrocnemius, soleus, biceps femoris, semitendinosus, and semimembranosus muscles.
The knee extension and leg curl exercises involve a different action of the knee, thus targeting a different set of muscles.
Leg Extension vs Multi-Joint Exercises: Quadricep Activation
If we are trying to most effectively target the muscles of the front of the leg which are the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius, then we need to compare muscle activation between different exercises.
A paper by Feldmann et al. (2008) compared rectus femoris (quad) activation between the squat, deadlift, lunge, step up, and leg extension.
So, what did they find?
They found that compared to other exercises, the leg extension was proven to be most effective in targeting the quads. While the squat, lunge, and step up targeted the quads a similar amount.
Takeaway: When compared to other resistance training activities, the leg extension was most effective in isolating the rectus femoris muscles (quads).
Leg Curl vs Multi-Joint Exercises: Hamstring Activation
During the leg curl exercise, the main muscles that are targeted are the semitendinosus, semimembranosus, and biceps femoris.
So, how does the leg curl fair with its other exercises for targeting the muscles of the hamstring?
A paper by McAllister et al. (2014), compared the leg curl with the RDL, good morning, and glute-ham raise.
Surprisingly to me, they found that the different muscles of the hamstring were most targeted during the glute-ham raise and the RDL.
Consequently, if your goal is to isolate and target your hamstrings then you are probably better off prioritizing RDL and the glute-ham raise into your exercise selection.
Nevertheless, the leg curl is still a valuable exercise by placing minimal stress on key areas by being a single-joint exercise and its non-fatiguing properties. It’s also less complicated to learn compared to the RDL and glute ham raise.
Related Article: 12 Best Glute Ham Raise Alternatives.
Takeaway: When compared to RDL and glute-ham raise, the leg curl was less effective in targeting the hamstrings, but is still a valuable non-fatiguing single-joint exercise to target the hamstrings.
If your hamstrings are extra sore after deadlifts, you might wonder if it’s good or bad. Check out my article that explains more!
I go into more detail about the glute ham raise in my article on Glute Ham Raise vs Nordic Curl: Differences, Pros, Cons
Leg Extensions vs Leg Curls: How to Perform
How To Do The Leg Extension?
- Set the back pad so that the knees are bent over the front of the seat.
- Set the leg pad so that it sits right above your foot during the entire exercise.
- Sit upright on the leg extension machine, while maintaining a neutral spine.
- Grab the handles on the side of the machine to ensure stability throughout.
- Firmly place your feet underneath the pad, with your toes pointing up.
- From here, simply extend your legs until your knees are locked out.
- To complete the repetition, flex your knees until back at the starting position.
How To Do The Leg Curl?
- Adjust the back pad a bit more forward so that there is clearance between knees and the seat.
- Adjust the leg pad so that it sits right behind your ankle.
- Sit upright on the leg curl machine, while maintaining a neutral spine.
- Fasten the support just above your lower thighs so that you are locked into the machine.
- Grab the handles on top of the machine to ensure stability throughout.
- From here, simply drive your heels back until your knees are fully flexed.
- To complete the repetition, guide the load back into knee extension.
Which Exercise Is Best For You?
While the leg extension and leg curl are both great additions to a training routine, you might implement one over the other depending on key variables.
When To Use The Leg Extension
- If you are in an offseason for olympic training or powerlifting and want additional quad accessories.
- You can’t perform squat or hack squat variations due to injury.
- You want to maximize the growth of your quads.
- You want to isolate the quads as much as possible because it’s a lagging body part.
When To Use The Leg Curl
- You can’t perform romanian deadlifts or good morning variations due to injury.
- You want to improve hamstring flexibility.
- You want to improve knee joint mobility.
- You want to maximize the growth of your hamstrings.
- You want to isolate the hamstrings as much as possible because it’s a lagging body part.
When To Use Both
- You want to improve total knee mobility.
- You want to build the hamstrings and quads, especially if you are a beginner.
- You have extra time to workout at the gym.
Coming Back From Injury: Leg Extension vs. Leg Curls
Leg extension: The Rehab Process
After a back, hip, or knee injury, we are often unable to perform the exercises that we once could. A simple question that I am often asked is, “what can I do?”
Knee extension is the most simple action that directly targets the muscles of the quads. For this reason, several goals should be made during the come-back from an injury:
- Restore range of motion in the knee.
- Strengthen and maintain the muscles of the quads.
- Improve flexibility in the muscles of the quads.
To break this movement down further, one should be able to do seated leg extensions without weight. If you are able to do so, then you can progress to banded knee extensions.
Banded leg extensions can be done by sitting on a box facing away from an anchor point. From this point, place the band around your ankle and extend the knee.
To load the knee extensors to a greater degree work your way towards a leg extension machine.
Furthermore, if you are unable to do full range of motion knee extension, then you can do partial range of motion leg extensions to slowly build towards greater ranges of motion.
Leg Curl: The Rehab Process
Hamstrings are responsible for knee flexion or knee stabilization by assisting the rest of the muscles during the bottom of a squat or the braking forces when cutting in sprinting drills.
So, strong hamstrings are necessary but are often neglected when it comes to isolation work, for this reason several goals should be made during the come-back from an injury:
- Restore range of motion in the knee.
- Strengthen the stability of the hamstring muscles.
- Improve flexibility in the muscles of the hamstrings.
To break this movement down further, you should perform laying face down leg curls with just the body weight. Push this activity to improve range of motion in the action of knee flexion.
That said, it is okay to experience discomfort in this exercise as long as there is no sharp pain. Focus on building range of motion and flexibility in the muscles of the hamstrings.
To progress this, you can anchor a band to a rack or a rail and place it around your ankle. This can be done either laying down or standing up facing away from the anchor point.
Finally, to load the hamstrings to a greater degree you can use a leg curl machine.
Do you experience knee pain while squatting? Check this article out!
Frequently Asked Questions
Here are some frequently asked questions for the leg extensions and the leg curls:
Are Leg Extensions Bad For Knees?
While normally leg extensions are completely safe and effective exercises, they can place a lot of pressure on the knees. If you experience discomfort, then you would benefit more from a lunge, squat, or a glute bridge.
Are Leg Extensions Better Than Squats?
While there is no replacement to the incorporation of a squat exercise in a program, the leg extension can target the quads to a greater degree. For this reason, the leg extension is a valuable supplementary exercise to a squatting movement.
Are Leg Curls A Waste Of Time?
Leg curls can be a valuable exercise in the isolation of the hamstring muscles. However, the leg curl is no replacement to a deadlift or RDL exercise, but can supplement these compound lifts nicely.
Lower Body Training Guides
Read our other lower body training guides:
- How Do Powerlifters Train Legs? (3 Powerlifting Leg Workouts)
- Do Powerlifters Do Isolation Exercises? (Yes, Here’s How)
- 18 Exercises To Improve Deadlift Strength (Science-Backed)
- Do Leg Curls Help Deadlifts? Yes, Here’s How
- 7 Best Leg Exercises That Don’t Use Ankles
Other Exercise Comparisons
- Kettlebell Swing vs Deadlift: Differences, Pros, Cons
- Bulgarian Split Squat vs Lunge: Differences, Pros, Cons
- Back Extension vs Glute Ham Raise: Differences, Pros, Cons
Whether you are recovering from an injury or seeking to isolate your leg muscles, both the leg curl and leg extension exercises are great additions to a program.
While the leg extension proved to be the most effective at isolating the quads, the leg curl seemed to be less effective at targeting the hamstrings than the RDL and the glute-ham raise.
For this reason, you could prioritize leg extensions as means of targeting your quads, while prioritizing the glute-ham raise and the RDL for targeting the hamstrings. However, the leg curl is still valuable for its non-fatiguing properties and single-joint nature.
Ebben WP, Feldmann CR, Dayne A, Mitsche D, Alexander P, Knetzger KJ. Muscle activation during lower body resistance training. Int J Sports Med. 2009 Jan;30(1):1-8. doi: 10.1055/s-2008-1038785. Epub 2008 Oct 30. PMID: 18975260.
McAllister MJ, Hammond KG, Schilling BK, Ferreria LC, Reed JP, Weiss LW. Muscle activation during various hamstring exercises. J Strength Cond Res. 2014 Jun;28(6):1573-80. doi: 10.1519/JSC.0000000000000302. PMID: 24149748.
About The Author
Javad Bakhshinejad was born and raised in the Washington Area. Currently, he is a student at Seattle University where he’s been pursuing an MS in Kinesiology, and has been a Strength Coach in the athletic department. He was a competitive bodybuilder for 8 years where he later transitioned to competitive powerlifting for 4 years. Currently, He has his own personal coaching business, where he works with powerlifters and bodybuilders.