Why Do Leg Extensions Hurt My Knees? (5 Factors to Consider)

Ever wondered, “Why Do Leg Extensions Hurt My Knees?”

When it comes to building a great set of quads there are few better exercises at isolating the target muscle than leg extensions.

However, this doesn’t come without its problems.

Many people typically complain of knee pain whenever they perform leg extensions.

And it’s difficult to know whether this is an issue with technique or something completely different.

So, allow me to explain why leg extensions are hurting your knees.

Why Do Leg Extensions Hurt My Knees?

Leg extension will apply constant tension to the anterior cruciate ligament. So, if you’ve ever suffered a knee-related ligament injury you should probably avoid leg extensions. Furthermore, unlike many other exercises, the hamstrings do not support the knees during the movement. The leg extension can also be considered an “unnatural” movement, as the lower leg rarely moves forward from the knee in everyday life (unless you kick something). Additionally, you should avoid “kicking” the legs up with force or using too much weight.

1. You’re Applying Constant Tension to the Anterior Cruciate Ligament

One of the main issues with leg extensions is that you’re applying constant tension to the anterior cruciate ligament (ACL).

This is one of a pair of ligaments, along with the posterior cruciate ligament, that are found in the human knee.

So, if you have ever had any sort of cruciate ligament injury in the past then you should probably avoid leg extensions altogether.

And even if you haven’t, the ACL happens to be the most injured ligament of those found in the knee.

The ACL plays a crucial role in many sports-related functions.

It will come into action whenever you change direction, as well as a change of speed, acceleration, and when tension is applied to the knee.

However, even in a sporting environment the ACL is only under tension for a few seconds at a time.

So, if you regularly perform leg extensions, you have to appreciate that you’re putting this ligament under constant tension for prolonged periods of time.

Eventually, over time, this can lead to knee pain whenever you perform the movement.

2. There is No Engagement From the Hamstrings

Whenever we bend at the knee the hamstrings are usually engaged in order to provide support.

This occurs in various daily activities such as walking, sitting, standing, bending over, etc.

This “support” from the hamstrings also happens during many fitness or sports-related activities.

You can include running, sprinting, and even a movement that is considered more of a quad and knee-dominant exercise like squats also incorporates the hamstrings.

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However, when it comes to leg extensions there is no hamstring engagement, and therefore no support provided to the knee.

Okay, this does help you isolate the quads to far greater effect, which is why leg extensions are such a favoured exercise.

With that being said, this lack of hamstring support places a great deal of additional stress on the knee joint.

Now, I’m not saying that you should never perform leg extensions, or that they’re a terrible exercise, or bad for your knees (although many people would say these exact things).

But, this is something that you should definitely keep in mind.

Pretty much every other knee-bend movement you perform will engage the hamstrings to some extent.

So, if you perform leg extensions regularly, and have been doing so for a long time, this could explain your knee pain.

A Fix For Leg Extensions (Hamstring Activation)

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3. Leg Extensions Are “Unnatural”

Okay, I’ve spoken about the constant tension applied to the ACL and the pressure that this applies to the knee joint.

However, this is a very unnatural movement and not one that many of us mimic in everyday life.

If you think about it, we hardly ever extend our knees in order to straighten our legs.

It is very rare for us to start any movement with the knees already bent.

Our legs are typically straight to start with and then we bend them to perform various activities.

So, once again think of walking, sitting, running, etc. as examples.

Granted, we will perform a similar type of knee extension to stand up from a sitting position.

But, it’s not often we repeat this standing movement over-and-over again in such a short space of time.

In fact, the only way that leg extensions are similar to a “real-life” movement is kicking.

So, if your preferred physical activity is a ball sport that involves kicking or a martial art then I’m all for training leg extensions.

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I will also admit that leg extensions are great for aesthetic reasons, especially for building up your quads.

But, other than this there are many more “natural” quad-builders that more closely resemble everyday human movement patterns, e.g. front squats, reverse lunges, etc.

4. You’re Using Too Much Force

Going back to “kicking” for a moment, this is typically something I see quite often with leg extensions.

There’s no real control over the movement.

I cannot tell you how many times I’ve seen someone on the leg extension machine forcefully bringing their legs up.

In fact, it’s almost as though they are trying to jar their knees straight.

Leg extensions place enough stress on the knees and anterior cruciate ligament without forcing the issue even more.

I think the vast majority of exercises should be performed in a slow and controlled manner.

The only real exception to this would be explosive movements, but these are supposed to be done in this manner.

You’d be hard-pushed to complete a power clean without a real explosive hip-hinge movement.

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However, forcefully exploding out of a leg extension and then coming to a shuddering halt at the top of the movement will only ever spell trouble for your knees.

Leg extensions are best performed slowly and in a controlled manner.

How to Do Leg Extensions With Perfect Technique

5. You’re Using Too Much Weight

Something else I see all too often is people stacking the leg extension with as much weight as possible.

The exercise really is performed slowly now, but without much control.

I’m a great believer in medium-to-high rep sets for leg exercises.

You don’t have to go as heavy as possible to stimulate growth.

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If you think about it, the legs receive a lot of stimulation in our daily lives.

So, it makes perfect sense that when it comes to training them for muscle and strength, more volume is the key.

In fact, I would rather load the leg extension machine with a much lighter load and aim for 20-30 reps per set.

I can guarantee this will give you that “quad burn” you crave and it is far more likely to stimulate growth.

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Final Thoughts

The main reason that leg extensions hurt your knees is because they apply constant tension to the anterior cruciate ligament. Additionally, extending the knees is an unnatural movement pattern and there is no support provided by the hamstrings. You should also perform leg extensions in a slow and controlled manner, plus aim for higher reps, as opposed to stacking the machine with as much weight as possible.

Check Out Nick Nilsson’s Time-Volume Training Workout Program