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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