Leg Extension Machine: Bad For Your Knees And Bad For Building Muscle?

QUESTION: I see a lot of people online making fun of the leg extension machine and calling it the most useless leg exercise there is. They all say that if you’re trying to build muscle, you shouldn’t ever waste your time using it. I’ve also heard some people say it’s really bad for you knees. Is any of this true?

ANSWER: It’s hard to think of a weight training exercise that gets crapped on and laughed at as much as leg extensions do. Alright, tricep kickbacks probably take first prize there, but the poor leg extension machine isn’t too far behind.

But just how warranted is all of the negativity that surrounds it? How useless is it for building muscle? How bad is it for your knees? Let’s find out…

Does The Leg Extension Suck For Building Muscle?

Please note that we’re talking strictly about muscle growth here. If all you care about is strength and “functional” performance, that’s a whole other story.

But if all you care about is building muscle on your quads, then my answer is simple… leg extensions can most definitely be useful!

Yeah that’s right, I said it. You mad, bro? Don’t be.

I know this goes against most of what you’ve seen or heard before. And I know that even insinuating this on your favorite weight training forum may get you laughed right the hell off of it. But, hear me out.

Here’s What I’m NOT Saying

Before we go any further, let me stop right here and make a quick attempt at preventing the inevitable misunderstanding of what I’m saying.

The fact that I just said the leg extension machine can serve a beneficial role in your muscle building routine is the kind of thing a lot of stupid people will see and immediately interpret the wrong way. To clarify, here’s what I’m definitely NOT saying:

  • That you should do leg extensions instead of exercises like squats, deadlifts, split squats, lunges or leg presses.
  • That it alone will build you awesome looking quads.
  • That it’ll even play a major role in making that happen.
  • That you won’t be able to build awesome legs without it.
  • That the leg extension should become a primary focus of your lower body training.

I’m not saying any of this. Not even a little. I disagree with all of the above just like any sane person would (or at least should).

However, this is where the not-so-sane people like to make an extreme jump from “leg extensions aren’t as good as squats for building muscle” to “leg extensions are completely useless and are a total waste of time for everyone under every circumstance 100% of the time.”

And that, my friends, is total bullshit. “Hardcore” enough to get you a ton of internet cool-points among your inner circle of clueless bros? Yup, probably. But still bullshit nonetheless.

Here’s What I AM Saying

You see, the leg extension machine is a basic isolation exercise just like every other isolation exercise. And just like how bicep curls, tricep extensions, lateral raises and dumbbell flyes can all play a beneficial role depending on your goals, training needs/preferences and the overall design of your program, so too can leg extensions.

However, just like all of these other exercise, that role should be a minor one.

It shouldn’t be your primary quad exercise. That role goes to exercises like squats, or split squats/lunges, or even leg presses. But what it can be though is a perfectly acceptable secondary exercise.

How could that be, you ask? Because the truth is that if a given exercise can successfully place some form of tension on a target muscle group, and that tension can be increased over time (aka progressive overload), then guess what? That exercise is capable of building muscle. Simple as that.

Which means, virtually EVERY SINGLE EXERCISE — be it compound, isolation, machine, free weight, body weight, whatever — is capable of stimulating muscle growth… even leg extensions.

Now, do you need to do it? Of course not. There are plenty of exercises to choose from that all have their own set of pros and cons under different circumstances. All you have to do is pick the right tool for the job. And for many “jobs,” it just so happens that the leg extension machine can be the right tool.

Here Are Some Examples

Since we’re only looking at this from a muscle building standpoint, we should care about making a few things happen. First and foremost, we want to get stronger over time. Progressive overload is and always will be the key here.

But, it’s not the only key.

For the best possible results, we want to make sure we’re training muscles, not just moving weight around. We want to generate some degree of muscular fatigue in addition to progressive tension. We want to do a certain optimal amount of volume within a certain optimal range of reps within a format that allows for a certain optimal amount of frequency and recovery to be met.

Taking all of this (and more) into account, there are plenty of times when an exercise like leg extensions makes perfect sense. For example…

  • Let’s say your lower body workout starts with squats and Romanian deadlifts in the 5-8 rep range. Sounds good to me. Now let’s say you want to get some additional higher rep (8-15 rep range) quad volume done in a way that DOESN’T add additional unwanted volume for the hamstrings/glutes. Split squats and lunges, while both great exercises, would not be the ideal tool for that job. Leg extensions would.
  • Let’s say you’re using a 3 day Push/Pull split, where lower body pushing (quads) is done separately from lower body pulling (hamstrings). This means you will be training quads and hamstrings with just one day off in between (with the 4 day version of this split, you’d actually be training them on back-to-back days). Many people in this situation may find it helpful to limit the amount of posterior chain usage (hams, glutes, lower back) during that “push” workout. Which exercise combo do you think will be better for this… back squats and barbell lunges, or maybe front squats and leg extensions?
  • Let’s say that your lower body day began with heavy deadlifts, and by the time you finished those your lower back was super fatigued. Is this a good time to put a bar on your back and bang out some squats and split squats to get your quad work in? Or would sitting down and doing some leg presses and extensions (and thus significantly taking the load off of your lower back/spine) be a better choice in this case?
  • Let’s say that, like me, you’re very posterior chain dominant with your lower body compound exercises. Your hams/glutes get a ton of work, but your quads aren’t getting the training stimulus you want. Should you just do more of the exercises that are going to pose the same issue, or would an isolation exercise like the leg extension machine be a better choice considering it takes the posterior chain out of the movement completely and allows 100% of the training focus to be on your quads?

Those are just a few examples that come to mind. Now, is the leg extension the one and only useful option in all of these cases? Of course not. But, it still IS a useful option nonetheless.

The Big Point

Leg extensions are a potentially useful isolation exercise just like any other. I don’t love it, I don’t think there’s anything too great about it, and I certainly don’t think it’s something everyone needs to be doing (which is why it isn’t included by default in the majority of the routines I design).

But the fact that it’s generally inferior to squats, split squats, lunges, leg presses, etc. for building muscle doesn’t make it a useless waste of time by any means. Under the right circumstances and when used correctly, it can play a beneficial role in many people’s programs.

I’ve personally included it in mine at many different points over the years. Hell, it’s there right now.

So if it suits your goals, needs, preferences and fits right in your routine, feel free to use it. I occasionally do, and I see absolutely no reason not to. Well, unless of course it happens to hurt your knees…

Are Leg Extensions Bad For Your Knees?

This is like asking if flat barbell benching is bad for your shoulders. Or if dips are bad for your shoulders. Or if skull crushers are bad for your elbows. Or if straight barbell curls are bad for your elbows.

And on and on and on.

The answer is never universally yes or no. The answer is always maybe. Here’s why…

Sometimes It’s Just You

For some people, some exercises WILL be bad for some body part. No matter what adjustment is made to the way it’s done, certain exercises just cause issues for certain people for any number of reasons specific to their body.

However, this is NEVER true for everyone. That’s because for an equal number of other people, the same exercise(s) won’t cause any problems at all. No matter how many years they spend doing it, or how heavy it gets, or sometimes even how shitty their form gets… they’re perfectly fine. No pain. No discomfort. No negative effects whatsoever.

Leg extensions are just one exercise out of literally hundreds that fit this description. It’s bad for some people’s knees, but just fine for others. Just like how dips are bad for my shoulders, but perfectly fine for others.

All it takes is some common sense to understand that if an exercise causes pain for you, you should avoid that exercise. But if not… feel free to keep it around.

Sometimes It’s How You Do It

In addition to your body itself, sometimes it’s your programming of the exercise and the way you’re doing it that causes the problems.

For example, doing leg extensions too often, too heavy, and/or with horrible form (or all of the above) may be bad for your knees and cause pain, but doing it less often, lighter and/or with something close to good form may solve all of your knee problems.

Just like how flat barbell benching “bodybuilder style” with your elbows flared out to the sides and the bar lowered to your upper chest will be “bad” for WAY more people’s shoulders than the same movement done with your elbows tucked in a bit and the bar lowered closer to nipple level. Or how doing heavy chin ups 3 times per week might destroy some peoples wrists and elbows, but doing it once a week might be fine.

Speaking of which…

How To Do Leg Extensions WITHOUT Hurting Your Knees

Like I said before, for some, the leg extension machine will always be bad for your knees no matter what adjustments you make. For other people however, those adjustments might make a world of difference.

Here now are a few simple tips that may make leg extensions more knee-friendly…

  • Do it slow and controlled.
    As is the case with most isolation exercises, it really isn’t suited for being done explosively, or with significant momentum, or cheating to grind out more reps. This is an exercise best suited for being kept slow and controlled throughout the eccentric and concentric portions of the movement so the weight isn’t just swinging up and dropping down.
  • Keep the weight lighter, the reps higher and the progression slower.
    Just like I mentioned recently for skull crushers, I like to keep leg extensions in the 10-15 rep range. While you definitely still want to increase the weight when you can, this keeps things a bit lighter and allows you to keep pushing for reps longer. This turns it into more of a “pump and fatigue” exercise than a “progressive tension” exercise, and that’s likely what it’s best suited to be.
  • Play around with the settings.
    Most machines allow you to adjust the seat, the back pad and the pad down by your ankles. All of this (along with the direction your toes are pointing) plays a role in the exact angle your knees are at and exactly how your quads are being trained. Sometimes even the smallest adjustment can make a significant difference. Experiment to see if you have a sweet spot.

In Conclusion…

This isn’t at all meant to be a “pro-leg extension” article. It’s more of an “anti-being-totally-anti-leg-extensions-100%-of-the-time” article.

I’m not saying you should use it or avoid it. All I’m saying is that anyone claiming it should ALWAYS be used or ALWAYS be avoided is probably wrong about 50% of the time. This would be true for every other exercise, too.

The same goes for people claiming the leg extension machine is completely useless for building muscle. That’s just not true at all, as there are plenty of situations where it can definitely serve a beneficial purpose.

Having said that, you still need to remember that we’re only talking about a quad isolation exercise here. Even if it’s going to be a useful part of your lower body training, it’s going to be a very small secondary part of it. Exercises like squats, deadlifts, split squat/lunge variations, leg presses and more should get the majority of your time, effort and focus.

And this also assumes it doesn’t bother your knees. If it does, then it shouldn’t be a part of your program.

Like I said before, leg extensions are no different than any other exercise. It’s just one of MANY tools that can be used for certain purposes. If a purpose presents itself and it fits in line with your goals and needs (and you can do it safely), then feel free to put this tool to use.

But if not, or if it happens to hurt your knees, then avoid it and/or just eliminate it from your toolbox altogether.

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56 thoughts on “Leg Extension Machine: Bad For Your Knees And Bad For Building Muscle?”


  1. I already told you this but again, we need more objective, common sense and less fanatic and extreme opinions about everything. “This is totally BAD”, “That is absolutely PERFECT”. Come on people, nothing is absolute. The only absolute thing about training your body is that every body is different and will react different to the same exercise.

    I just can’t understand why people are so fanatic, with everything. They can’t put themselves in the middle and use the common sense, they HAVE to choose a side. I guess this is just our nature.

    I try to avoid extremes and fanaticism and I’m pretty sure you, Jay, try to do the same. That’s why I love your articles.

    Keep the good work, don’t be tempted by the evil forces of fanaticism =P

  2. My wife, son and I have all had knee surgery. Mine was simple meniscus repair, while my wife’s and son’s were ACL reconstruction after injuries. What our surgeon said after each and every surgery was that if we were intent on doing leg extensions, in order to preserve knee integrity, it should always be a secondary exercise, always at lower weights/higher rep ranges, and the starting point should be somewhere around 45 degrees. He told me that starting with the legs at a 90 degree angle (lower leg perpendicular to the floor) put incredible stress on the joint, especially at higher weights.

    Prior to my meniscus repair, I was a “pile on the leg extension weight” guy, and I believe it was a large contributing factor to my meniscus tear.

    • Yup, I agree… and the surgeon’s recommendations are surprisingly good (doctors + diet/exercise = usually horrible advice).

      Although, I’ve seen people whose knee problems were caused (or at least partially caused) by leg extensions, and others whose knee problems were fixed with them. It’s a weird individual thing, I guess.

      • If you two are interested in ACL/PCL loading then have a quick look at Brinckmann et al (2002) it basically shows the relationship between knee flexion angle/ distance below thetibial plateau and which structure is loaded ACL/PCL or nether

  3. I had to stop doing Barbell Squats because my shoulders are F**ked! I can’t even do a standing shoulder with the bar alone. I opted for the leg press, which I hate and the hack squat machine which I loathed. Front Squat Machine is decent, but I doing 500lb. plus on them. When I go to do leg extensions, I’m doing the full rack at 20+ reps!
    It’s pretty cool to see how much work I put into my body.
    IF only you were around when I picked up my first dumbbell.
    You are a scholar. DJA

  4. Well said as always. I hate it when those “experts” dismiss any exercise that does not work for them indvidually or hurt one of their joints.Some even dismiss all machine work marking it as sissy exercises.
    You think hating on leg extensions is bad? I’ve heard some of these supposed experts dismiss leg presses and only considering squats and squats variations as quad building exercises.
    Honestly , one of the best things I learned from this site and your book (right after progressive overload and calories in vs calories out ;)) is to stop doing an exercise once it starts hurting one of your joints no matter how much it is recommended and praised.

    • Even though squats are my primary exercise, I have been doing leg extensions for months now so that I can built mass on my quads. Had not the slightest problem with it, I think its a useful exercise provided one uses the right amount of weight. Also, I combine machines with traditional barbell exercises as well as basic bodyweight exercises such as push Ups, dips and chin Ups.

  5. I love doing these right after squats.
    What’s wrong with Kickbacks? I thought they were one of the best triceps exercises.

  6. I had the anterior cruciate ligament reconstructed from my right knee and my orthopedist surgeon told me to avoid leg-extension for the rest of my life. After reading this article, I might give the leg-extension a try and see if I feel any pain…

  7. Great artical,as usual.I love doing leg ext,and never had knee problems,if anyone seen Zack Kahn’s road to recovery,they will know he detached both acl tendons doing hack squats,and after surgery, he used leg ext alot to get back to health,it wasn’t until later in the recovery he used squats.

  8. can you give an example of how can someone incorporate leg extensions in your muscle building routine without removing squats lunges and leg presses? also since hams/glutes get hit significally from lower body compound push exercises why do you recommend same number of reps/sets for hams and quads?

    • To add in leg extensions there WITHOUT removing other quad exercises would require changing the layout of the routine.

      As for the second question, it can go both ways. It depends on the needs of the person and the overall design of the routine. Basically, if it feels like keeping things even is too much for hamstrings, direct hamstring volume can be reduced. But if not, don’t.

          • I meant in the context of cutting down volume from hamstrings as they get enough from squats/lunges/leg press plus they’re quite smaller than quads.

          • If you feel hamstrings are getting too much indirect volume from quad training, I’d reduce hamstring volume without increasing quad volume.

            Another option is to replace one of those hamstring-heavy quad exercises with leg extensions (e.g. swap split squats for extensions… quad volume stays the same and indirect hamstring volume is reduced.)

  9. My name is jay, and i’ve been working out for 4.5 months and i have been doing a full body routine and gains are great. My question is: in the beginner full body routine you listed the only muscle group that gets 3x a week frequency is the back. I am aware that shoulders and tris are hit with bench and bis with rows, legs with deads etc… but you constantly talk about 3x a week of direct training is the most optimal routine for a beginner with any goal, but the beginner routine you have listed does not follow this routine:
    Frequency of body parts
    Legs: 1.5 a week
    Back: 3x a week
    Chest 1.5x a week
    Shoulders 1.5x a week
    Tris 1.5x a week
    Bis 1.5x a week please explain…

    • First, don’t worry about it.

      Second, you’re pushing, pulling and doing a lower body dominant movement 3 times per week. In some cases, including this one, that’s full body.

      Third, don’t worry about it.

  10. Just starting your 4 day split routine. I use barbell/dumbells at home. I also have a squat cage for safety. As I don’t have a leg press machine I just do extra sets of squats. For some reason split squats/lunges always hurt my knees. Would leg extensions be a suitable replacement for split squats in your routine???

  11. Thanks i just need clarification man. I was curious on how that routine was full body. You give out the best information dude, especially that article on how bro splits suck, thanks for that. Oh, and that article on volume was superb on how you clarified volume with specific frequencies.

  12. I started working out at the beginning of July weighing 155 pounds, and i know weigh 167 lbs. I have been doing a full body routine, and everything, but im just not happy with my results, i want things to move faster etc… I’m going to sound whiny, but can you reassure me im in the right direction, give me a lil motivation etc.
    Thanks for your time

    • The only way to do that would be to look over your entire diet and workout along with the full breakdown of the progress you’ve made both in the gym and in terms of body composition.

      Unfortunately, if I do that for you, I’d have to do it for the other dozens of people who ask me to do the same thing on a weekly basis. Just no time for it.

      Here’s a better idea, though. Read everything I’ve written here and you’ll be able to answer your own question.

  13. In your routine, why do you have Romanians but not regular Deadlifts and why not Squat for both A and B workouts instead of using a leg press?

  14. Another excellent article.

    The problem is people just don’t seem to stop and think about things. Of course you can progressively add resistance to leg extensions, so you can build muscle. It’s simple.

    I’ve had a hip problem that has gotten worse so I can only do extensions and curls for thighs. Yeah it sucks, I love squats, and I know I’ll never build monstrous legs isolation exercises, but it’s better than not working legs at all.

  15. I’m an old man now, but when I was younger, I did 2 leg exercises:

    1) Leg extensions.
    2) Leg curls.

    The problem with squat exercises is not only does your thighs get bigger, but your butt gets bigger also.

    I wanted bigger legs, WITHOUT the bigger butt.

    • Exactly right. If you just want to train your quads without adding extra volume to your glutes/hamstrings OR having those muscle groups take over the lift… the leg extension becomes ideal.

      I wouldn’t recommend it as a primary quad exercise, but it certainly has its uses.

  16. Awesome article! Matter fact…..awesome website dude; you call it like you see it with absolutely no bullshittin. It’s really helping me pick and choose from routines I’ve done in the past so I can make my own routine with all the elements that I think are necessary for my goals. I’m basically using exercises from 3 different workouts (high school weight training, a former professional powerlifter’s routine for me and his sons, and p90x), but constructing the order, volume, and frequency based off your articles. So far so good, my powerlifting buddy is the only person I’ve ever heard say the word “volume” in the weight room, but I never understood it until I read your explanations. None of my buddies know about that, we always just figured the more the merrier when it comes to lifting. I’m confident my new routine will be a good fit but I’ll probably show it to a personal trainer friend just in case.
    My cousin has been telling me for weeks that leg extensions are “super bad for your knees”, I’m definitely showing him this. Also I got a roommate who is full of “you shouldn’t jog after you lift, you’re not supposed to lift like this, this workout sucks, etc.” I always tell him there’s no one answer, it all depends on you and your goals. If he had the attention span, I’d show him your website. Thanks again man!

  17. I had been squating for over 15 years before I had my first knee injury from squat. The sport doctor who repaired my tedon told me that I should warm up my knees with leg extension. I followed it and did 5 sets of progressive sets before heavy squating and leg press. I also did them afterward. I had not any knee problem ever since. There is no other exercise that would add defintion to my quad. Likewise, I also did dumbell bench press to warm up my shoulder before heavy bench and I never had any shoulder problem. These are parts of the Weider pre-exhausting principles.

  18. Awesome article and advice. I’m all for functional training and started to shy away from machines but realize machines like the leg press and others have served MY body well. I’m looking into incorporating some leg extensions and was told to stay away from it by a few “functional workout” fanatics. Glad I know better and I’m now doing what works for MY body. Thanks again.

  19. Speaking of squats, where do you prefer the barbell? Front lumbar or the back resting on the traps?

    I have heard with the bar on the front there is less stress on the back, while putting more emphasis on the hamstrings.

  20. Are dumbbell kickbacks any good? I don’t have any cables, so I can’t do any pushdowns. I did skullcrushers for awhile, and now the long head of my triceps is more developed than the lateral head. Your triceps guide said “elbows at your sides” exercises place more emphasis on the lateral head. Are dumbbell kickbacks good enough to use instead?

    • It’s not something I love, but it’s a potentially useful triceps isolation exercise especially if your options for others are limited.

      Also keep in mind that if you get some bands, you can do pushdowns like that.

  21. I am 53, a professional piano mover, and our job includes carrying upright pianos to heights by two men. That is hard on the knees at times, and it has reduced my ability to develop a big squat. I love squatting, so having knee problems eats me.
    Finnish Mr. Universe 1980, Jorma Räty, advised me to skip squats for a while when i was training at his gym in the 90s. He had me do high reps ofleg extensions instead. In a couple of weeks my knees felt a lot better. Now, in 2015, I took his advice again and did 2×100 reps of extensions today.
    How can anyone say ext’s are useless when most of the champions have used them in some way? Mike Mentzer had his trainees do them (1 set), Arnold did them, Lee Labrada built his quads WITH them!
    Joe Weider had them included in his system, so did Bill Pearl and Vince Gironda. But those three didn’t know any better, right? In fact, we still have wisdom of Pearl so why not ask him if in doubt.

  22. I used leg extensions as part of rehab instruction from a physio of Quad tendonopathy. Squats actually made the problem flare up as primarily over use injury from snowboarding.

    I use both legs to lift then when at full extension pull one leg away leaving one leg to do all the work (as heavy and as slow as possible) on the eccentric part of the exercise.

    8 year chronic problem never felt better, thanks mainly to leg extension work!

  23. Hi Jay!. I only feel my quads on the back squat when I’m doing high reps. But I want to follow your advice EXACTLY how it is written and do the Primary movements in the 5-8 rep range. The problem is I always feel my hams in that rep range and not my quads. I looked at your excerise list and you have Machine Hack Squat on there but not Barbell Hack Squat. My question is can the Barbell Hack Squat (not the machine) be used as a primary quad compound movement? Or can it even be used to replace leg extensions and spit squats in your routines? Does fit better as a Tension exercise? A Tension + Fatigue exercise? Or a Fatiques exercise? Thanks coach!

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