Leg extensions wreck the knees? What the science says

Categories: Videos & podcasts

Leg extensions have a reputation for being unsafe for the knees. Do leg extensions really wreck the knees? Let’s look at this from a scientific perspective.


00:00 Introduction

00:15 Suboptimal for vastus medialis?

00:48 Too much ACL stress?

03:35 Conclusion


Leg extensions have a reputation for being unsafe for the knees. Do leg extensions really wreck the knees? Let’s look at this from a scientific perspective. One concern with leg extensions is that they don’t train the vastus medialis well, the teardrop part of the quads near the knee. And this is supposed to cause imbalances in how the different heads of the quads pull on the knee and therefore cause knee pain. This is a myth. Leg extensions actually have a very similar ratio of VMO to VML activation as leg presses and squats. All of these exercises train the vasti very well. As I explained in my last video, The rectus femoris, the middle part of the quads is actually trained better by leg extensions.

Another common concern with leg extensions is that they put too much stress on the anterior cruciate ligament of the knees. However, a 2019 meta analysis concluded that open and close kinetic chain exercises – leg extensions being an open chain exercise, the squats being a closed chain exercise are equally rehabilitative after ACL reconstruction. Even in a study on individuals with an ACL graft. The researchers found that leg extensions did not put more strain on the anterior cruciate ligament of the knee than squats did. In another study, mathematical modeling of the knee Joint forces showed that while ACL joint forces were on average higher during leg extensions than during squats, posterior cruciate ligament forces were higher during squatting. So it’s simply a case of leg extensions and squats putting different type of stress on the knee joint, which is actually a case for why it’s better to do both a little bit, then try to do just one of them and do too much of that because then you’re overloading a specific type of the knee more. If you distribute the forces more or you do different exercises, leg extensions and squats in the same program, then you can shift the stress to different parts of the knees. Indeed, 2014 systematic review of the literature on ACL rehabilitation found that open and closed kinetic chain exercises should both be included in most rehabilitative programs. It’s also worth noting that the stress put on the ACL by leg extensions in the first place isn’t that high. ACL joint strain generally does not exceed 4.4% during leg extensions, which is below the 10% threshold of ACL failure we see in research.

Not to mention ACL Joint injuries aren’t that much of a concern for people that lift in the gym to begin with. ACL joint injuries are much more common in athletes, where the knees move a lot, internal rotation, lots of acceleration, deceleration, uncontrolled movements in gym type settings, ACL joint injuries are quite rare. Anecdotally, also, when do you ever hear of anybody having an acute “oh crap” type injury during leg extensions? Yes, some people get a little bit of overuse pain, but that’s definitely equally the case for squats, leg presses… pretty much any type of exercise for the quads. The more common type of injury we see in the gym is patellar tendinopathy, jumper’s knee, runner’s knee that type of stuff.

And we have a study on that which found that leg extensions are just as effective in athletes for jumpers knee rehabilitation as the closed chain and more functional jump squats in this case. So overall, the research simply does not find that leg extensions are inherently too stressful for the knees, or that they are worse in any manner, or that they are strictly worse than squats, leg presses or any type of more supposedly functional, close kinetic chain type movement. So overall, we can simply conclude that leg extensions being bad for the knees is a myth. If they hurt your knees, then yes, you should probably skip them. Maybe do high reps more controlled movements, try to do them with blood flow restriction and then for super high rep sets of 30, you can use a very lightweight. If you do get knee pain from leg extensions. That doesn’t mean you should just do them because Menno said it’s fine. In general when you experience pain during a movement, you should omit it or modify it. In the case of leg extensions, what typically works very well is to do very high repetitions. You can also do them with occlusion wraps or powerlifting knee wraps at a pain point of like four out of ten.

Doesn’t have to be super tight, should not result in bruising, tingling, that kind of stuff. Just very, very snug, if you will, at the top of the legs. And then you can use a very lightweight – use 30 RM with the occlusion wraps on that will be a very light weight indeed. Sets of 30. Try to still go to failure or close to failure and you can with very light weights still stimulate the quads very well which is very easy on the knees.

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About the author

Menno Henselmans

Formerly a business consultant, I've traded my company car to follow my passion in strength training. I'm now an online physique coach, scientist and international public speaker with the mission to help serious trainees master their physique.

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