Are you in the middle of putting together a program for yourself? Have you already built a program you love, but are looking to add something to build those legs? Leg Extensions (or knee extensions, as they are sometimes called) and Leg Curls are two common movements that can add great benefit to your training.
Ultimately, it comes down to a combination of what your training goals are, how much time you have in which you can complete your sessions, the availability of these machines, and of course, the ability and health of your knee joints.
So, should you add leg extensions and leg curls to your leg workouts?
If increasing strength and hypertrophy of your quadriceps and hamstrings, along with stronger, healthier knee tendons and ligaments are part of your overall goals, then leg extensions and leg curls belong in your workout.
Let’s dive a little bit deeper into some of these individual benefits to paint a clearer picture of how and why they can be of benefit.
Which Muscles Do These Movements Train?
The leg extension trains the 4 divisions of the quadriceps which are the Vastus Medialis, Vastus Lateralis, Vastus Intermedius, and Rectus Femoris.
The leg curl trains the 3 divisions of the hamstrings which are the Semitendinosus,Semimembranosus, and the Biceps femoris.
They are excellent for muscle growth.
When people think of big legs, most often they think of the big compound movements like squats, deadlifts, and their different variations.
While these movements are no doubt able to pack on some size, there are some details about the leg extension and leg curl that perhaps make them just as good, if not better at building muscle.
When it comes to building muscle, there are a few variables that come to mind such as load (weight on the bar, dumbbell, or machine), volume (sets x reps), and frequency (how often you train a muscle).
Aside from those, I’d like to focus on two others, stability and isolation.
Stability and isolation are very important and often overlooked factors when choosing a movement for the sake of building muscle. Stability is important because it allows you to execute a movement with effort and intensity on the target muscle, while not having to spread your attention elsewhere such as keeping yourself balanced and making sure other parts of your body are also moving in sync. You also run much less risk of other muscles significantly jumping in and not getting as much tension on the muscles that you are actually trying to work.
Let’s compare a barbell back squat to a leg extension.
In a back squat, you are making sure the bar is centered and stable, that your feet are spread evenly, and that your torso is in the right alignment from top to bottom. You need to stay tight as you descend and ascend, and stay centered to keep the weight distribution even in all directions.
With the leg extension, you are seated, back pressed against a pad, hands gripping onto handles, allowing you total body tension as you focus completely on just your knee joints and pumping out quality reps, giving nearly all of the work to the quads.
See the difference?
The same can be said when we compare a stiff-legged deadlift with a seated or lying leg curl for working the hamstrings.
During an SLDL, you have a number of things to focus on such as keeping your spine neutral, making sure you are pushing your hips back while keeping a fairly straight leg while also keeping the weight itself in the correct position throughout each rep.
Contrast that with a seated or lying leg curl, where like the leg extension, you are able to stabilize your body much better, and focus on knocking out quality reps with the intended muscle.
This is not to say that you can’t effectively build muscle with squat and deadlift variations, you absolutely can.
This is to shed light on the fact that movements with greater stability and isolation play a huge role when choosing movements for hypertrophy.
They are great for unilateral training.
Unilateral training can be great for bringing up a weaker limb or side in an exercise or movement pattern.
When we are doing bilateral movements, in which both limbs are being used to move a load, we oftentimes inadvertently let our dominant side take over. This is especially true when the weight gets heavier and the reps become more fatiguing.
Leg Extensions and leg curls allow us to take advantage of the stability and isolation factor we mentioned earlier, and work each limb individually ensuring that even the weaker limb is getting its share of the training stimulus.
Here is a tip when doing your unilateral training:
When starting a set, use your weaker limb first and see how many reps you are able to achieve within whatever your rep goal system is, and then match that number with your stronger limb.
This is a good way to ensure that you don’t further increase the strength gap between both sides.
They can help with other big lifts like the squat and the deadlift.
Even if your main goal is a bigger squat and deadlift, leg extensions and leg curls can help you on your path.
In the squat, the quads contribute greatly during both the concentric (raising) and eccentric (lowering) portions of the movement. The deeper into a squat you go, the more the quads have to work to control the descent as well as drive you out of the bottom when you’re ready to stand up. The hamstrings act mainly as stabilizers throughout the movement.
In the deadlift, the quads are activated at the very start of the movement when first pulling the bar off of the floor up until about the knees. From that point on it’s our glutes and hamstrings that drive hip extension toward lockout.
It would make sense to think that a bigger muscle has more potential to be a stronger one, and these two exercises can definitely accomplish that.
A common knock on these movements is that they aren’t functional, or that just because you can move a lot of weight on them, doesn’t mean you’ll be able to squat and deadlift a ton.
And there is truth to that. Yes, if you want a strong squat and deadlift..you have to squat and deadlift. But growing the muscles heavily involved in those movements can carry over to better performance and strength.
They can contribute to greater knee health.
Knees and knee health often come up when discussing these two movements, often in a negative way.
However, doing these exercises can strengthen the tendons and ligaments, as well as the muscles around the knee when we gradually expose them to stress and subsequent recovery.
Performing these motions at the knee also helps to release synovial fluid into the joint which acts as a lubricant.
A caveat to this is that there are no pre-existing conditions, dysfunctions, or injuries present. It is always wise to consult with a qualified physician before taking part in physical activity.
Proper and responsible loading and execution of these, and any movements, are crucial. Do not train when injured, or through any pain that you think might cause injury.
Overall, leg extensions and leg curls can be a great addition to your leg days. Strength, healthy knees, and bigger legs are all bases that are covered by using these exercises.
If you’re on board and are looking to add these pieces of equipment to your own gym, here is an article I wrote comparing 5 great leg extension/curl machine combo options.
If you have these pieces of equipment available to you, the time to add these exercises into your program (or even swap them in place of an exercise you’re currently doing to add some variety), and the physical capacity to do them safely, feel free to give them a shot and see how they affect your training.
Before you go, check out TigerFitness for all of your protein and supplement needs. I highly recommend MTS Whey (Chocolate Chip Cookie is a must), and pretty much any flavor of The Outright Bar.
Disclaimer: None of the information in this article is intended to be medical or health advice. Always consult with a qualified physician before taking part in any exercise or fitness regimen. This post contains affiliate links. If you decide to purchase a product using any of the links provided, I will receive a small commission at no extra cost to you.