Leg Extension: How-To & Benefits (With Video)

June 5, 2022

Isolation exercises are often forgone for compound exercises when building muscle. Typically, they are not seen as "functional." However, if your goal is to build the most enormous quads possible, the leg extension machine must be part of your training routine. Here are the ins and outs of the leg extension and how you can use it to maximize your workouts.

What Is The Leg Extension?

The leg extension is an open-chain exercise isolating the quadriceps muscles through knee extension. It is typically performed with a leg extension machine. However, those without machine access can do them with a dumbbell between their feet or a sandbag on top of their feet.

These variations aren’t as effective as the machine leg extension since the load at the bottom of the movement is near non-existent. In contrast, the machine is designed to provide resistance through the entire range of motion.

Leg Extension Muscles Worked

The leg extension is unique to other quadriceps exercises. It targets the rectus femoris muscle, whereas exercises like the squat target the vasti muscles [1,2]. That doesn’t mean the vastus medialis, lateralis, and intermedius aren’t engaged when performing the leg extension.

It's a great overall quad builder. And you'll need to perform leg extensions or similar open-chain exercises for complete quadriceps development.

Leg Extension Benefits

Leg Extension Machine

There are two main benefits of the leg extension:

Target The Muscle That Isn’t Emphasized When Squatting

As mentioned in the muscles worked section, the leg extension targets the quadricep muscle that isn’t well activated during squatting exercises.

The rectus femoris shows greater muscle activation during leg extensions compared to squats.

Therefore, if growing massive quads is your number one goal, leg extensions are necessary for overall quad development. If your goal is purely strength orientated, then the leg extension is not necessary but can be helpful for hypertrophy of the quads.

Low Systemic Fatigue

This is where I believe the leg extension is most applicable. High rep squats are typically a go-to option for building big legs. It's highly effective, and Super Squats' program was built purely around this principle.

However, the fatigue generated from high rep squatting is unlike nothing else you’ll experience in the gym. Not to mention the beating your upper body takes along with your legs. Using the leg extension to blast your quads localizes the fatigue to only your quads.

The rest of your body doesn't take a beating, helping you recover faster. Hard sets of 20 on the leg extension and intensity techniques like drop sets will make you sore. Still, you don't experience fatigue like high rep squatting.

How To Leg Extension

Forget the guy you’ve seen swinging his legs violently on the leg extension. This is how to perfect the leg extension:

  • Adjust the back support so that when sitting against it, the back of your knee is comfortably around the seat with your legs hanging straight down.
  • Adjust the foot pad, so it sits comfortably on your ankle joint. Not on your shin without touching your foot.
  • Adjust the foot pad, so it pushes your feet under the seat. This will place maximum stretch on the quads maximizing muscle growth.
  • To initiate the leg extension, extend your knees to full lockout. Pause at the top position for 1-2 seconds. This is where the gains are made.
  • Slowly control the eccentric phase to maintain tension on the quads. Don't relax and let the weight fall; otherwise, you're missing out on great muscle growth stimulus.
  • I find holding the handles by the seats and pulling yourself down helps increase the tension on the quads and loads you lift.

Common Leg Extension Mistakes

These are the common mistakes I’ve seen with the leg extension.

Going Too Heavy

Ego lifting on the leg extension machine defeats the purpose of the exercise. It’s not for pure strength development. It’s for isolating the quads to maximize hypertrophy. You must control lowering phases, concentric phases can be explosive, but you should pause the weight briefly at the top.

Swinging With Momentum

This is often a symptom of ego lifting, where swinging and momentum are used to move the weight stack instead of being under control. Less tension is placed on the quads, so you don't get the same growth response.

Incorrect Seating Adjustments

If you're not set up correctly in the leg extension machine according to your body proportions, you will have a hard time getting the most out of the exercise. As mentioned in the "how to" section, the back of your knees should wrap comfortably around the seat with the foot pad directly in the crook of your ankle.

What Are Leg Extensions Good For?

Leg Extension Muscles Worked

Leg extensions are great for getting big quads, strengthening the quads in isolation, and in some cases, reducing knee tendon pain. Done for high reps close to failure, leg extensions will pack mass onto your thighs. If you have niggles or an injury that prevents you from squatting, the leg extension can be an alternative while you recover.

Finally, using the 2 up, 1 down leg extension is a lesser-known exercise to repair the patella tendon and reduce knee tendon pain.

Do Leg Extensions Make Your Thighs Bigger?

Leg extensions can make your thighs bigger if you:

  • Lift close to or at failure
  • Eating in a caloric surplus

These are the prerequisites for leg extensions to grow your quads. Lifting close to failure is essential as leg extensions are performed with lower loads. Research shows a high level of effort (close to or at failure), and high training volume is needed to maximize muscle growth with these exercises [3].

Do Leg Extensions Slim Thighs?

If you hold some body fat around your thighs and eat at a caloric deficit, your thighs will become slimmer and more muscular. Leg extensions will help maintain quadriceps muscle mass while you shed body fat through dieting.

How Many Leg Extensions Should You Do?

I recommend performing between 10-20 reps per set regardless of whether your goal is to build muscle, get stronger, or lose body fat.

Why?

Because loads are lighter when performing the leg extension than compound leg exercises, you'll need greater total volume by performing more reps close to failure. This maximizes metabolic build-up to signal a hypertrophy response.

This rings true for those who want to get stronger too. After your main strength exercises, you might use leg extensions to get more quadriceps training volume to add more mass. Greater muscle mass increases the potential for greater force production.

Finally, the main goal of weight training when losing weight is to maintain as much muscle mass as possible. So hypertrophy training protocols should be used on the leg extension machine.

Should You Do Leg Extensions Before Squats?

Leg Extension Benefits

Typically, the leg extension is performed after squats. This allows you to squat with heavier loads when fresh increasing mechanical tension on the quadriceps and stimulating more significant muscle growth. However, you've likely seen high-level bodybuilders perform the leg extension before other leg exercises like squats.

This is because they are so damn strong that performing the leg extension first fatigues the quads limiting the loads used when squatting. This reduces the stress on the lower back and upper body while still maximizing the tension on the quads.

Are Leg Extensions Better Than The Leg Press?

Like squats, leg extensions are better after the leg press to maximize the loading when leg pressing. Sometimes, leg extensions are performed first as a warm-up before the leg press, which can be a viable option if you need a greater warm-up for your knees.

However, in my experience, lying leg curls warm up your knees better and reduce lingering knee pain for the leg press when performed first.

Are Leg Extensions Good For Your Knees?

A decade ago, leg extensions were vilified as knee shredders. However, experience and research state otherwise. For example, the 90° knee position during the squat shows a 33% greater knee joint stress than the highest joint stress at the top of the leg extension position [4].

Are Leg Extensions Good For Your Knees

As you can see in the graph above, the squat shows significant increases in knee joint stress compared to the leg extension indicated by the blue and orange lines.

Are Leg Extensions Safe?

Leg extensions are safe as it's easily controlled with a machine. It is an excellent exercise for beginners to develop a mind-muscle connection with the quads and for intermediate to advanced lifters to safely build their legs without squats if they suffer from bad knees.

Are Leg Extensions A Waste Of Time?

Leg extensions are not a waste of time if your goal is to maximize the size of your legs. However, they may be a waste of time if you are a strength sports athlete and regarded as a waste of time for non-strength athletes.

I never program the leg extension for my sporting athletes. It serves very little purpose to an overall training program for athletic development. However, some strength sports athletes may benefit from the leg extension for building the quads.

Summary

The leg extension is best performed on a leg extension machine to build the size of the quad muscles. Specifically, it targets the rectus femoris muscle, which isn't well activated during closed chain lower body exercises like squats and leg presses.

References

1. Mangine, G. T., Redd, M. J., Gonzalez, A. M., Townsend, J. R., Wells, A. J., Jajtner, A. R., ... & Hoffman, J. R. (2018). Resistance training does not induce uniform adaptations to quadriceps. PLoS One, 13(8), e0198304.

2. Ema, R., Sakaguchi, M., Akagi, R., & Kawakami, Y. (2016). Unique activation of the quadriceps femoris during single-and multi-joint exercises. European journal of applied physiology, 116(5), 1031-1041.

3. Lasevicius, T., Schoenfeld, B. J., Silva-Batista, C., de Souza Barros, T., Aihara, A. Y., Brendon, H., ... & Teixeira, E. L. (2022). Muscle failure promotes greater muscle hypertrophy in low-load but not in high-load resistance training. The Journal of Strength & Conditioning Research, 36(2), 346-351.

4. Powers, C. M., Ho, K. Y., Chen, Y. J., Souza, R. B., & Farrokhi, S. (2014). Patellofemoral joint stress during weight-bearing and non—weight-bearing quadriceps exercises. journal of orthopaedic & sports physical therapy, 44(5), 320-327.

About the Author

I am a professional strength & conditioning coach that works with professional and international teams and athletes. I am a published scientific researcher and have completed my Masters in Sport & Exercise Science. I've combined my knowledge of research and experience to bring you the most practical bites to be applied to your training.

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