Why Do Leg Extensions Burn So Bad? (4 Factors to Consider)

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Last updated on November 3rd, 2022 at 02:01 pm

Have you ever wondered, “Why Do Leg Extensions Burn So Bad?”

Personally, I can’t think of another exercise where I “feel the burn” as much as I do with leg extensions.

Now, don’t get me wrong, you can certainly feel the muscles working with pretty much all exercises you perform (when done correctly with good form of course).

You definitely know that you’ve performed a movement with adequate intensity and you may even experience DOMS the following day.

But still, there is no other exercise that gives you a burn like leg extensions.

And weirdly, you don’t even have to do that many reps before you start to feel your quads on fire.

So, what exactly is going on here?

Why Do Leg Extensions Burn So Bad?

The main reason that leg extensions burn so bad is because they are a single-joint exercise. Many other quad-focused exercises, such as squats, lunges, leg press, etc. will involve you using other muscles during the movement. However, leg extensions are solely focused on working the quads alone. Furthermore, you will typically keep constant tension, do reps at a slower pace, and are more likely to perform drop sets with leg extensions. The intensity of exercise can deplete the muscles of oxygen and lead to lactate build up.

1. Leg Extensions Are a Single-Joint Exercise

A Very Muscular Pair of Legs

The first thing to consider is that leg extensions are a single-joint exercise.

Basically, they are solely focused on working the quads, and the quads alone.

Now, I don’t know about you, but much of my training is focused around compound movements.

So, movements that involve using a number of muscles at the same time.

Much of my lower body work will involve squat and deadlift variations.

And even the single-leg work I perform will work a variety of muscles at the same time.

Let’s take the “King of Lower Body Exercises” as an example, namely the squat.

The squat is typically viewed mainly as a quad-focused exercise, but this isn’t always the case.

This is why I spoke of “variations” above, as I would rather perform front squats if I wanted to focus more on my quads.

Additionally, the glutes, hamstrings, and adductors take up a lot of the strain when you perform conventional barbell back squats.

Obviously, I’m not taking anything anyway from the barbell back squat.

It truly is a fabulous exercise.

However, you could say that the weight that you’re squatting is distributed among a wide variety of muscles.

But, when it comes to leg extensions, all the muscular stress is placed upon the quads.

2. You Are Applying Constant Tension With Leg Extensions

Something else to consider is that you apply constant tension to the quads during leg extensions, or you should be.

Don’t be one of those people who allows the pulley to drop and the weight stack to come crashing down with every single rep.

Once again, using the barbell back squat as a comparison, you never really apply constant tension to the quads during the movement.

In fact, you could even view the top of the squat as “active rest”.

Okay, I understand that (hopefully) you have a barbell that weighs 1.5 to 2 times your body weight resting on your upper traps.

And even just standing there with that load on your back is still working the body in some way.

But, you certainly aren’t applying any tension to the quads at the top of the movement.

However, with leg extensions you never really release the tension on your quads until the final rep.

Speed and Volume

I would also say that it’s likely that you’ll perform leg extensions at a slower pace than you would barbell back squats.

In fact, there’s almost a sense of urgency to rack the bar when performing squats.

Now, I’m not saying that you rush through a set of squats with poor technique.

But, I’m sure we can all agree that due to the muscular stress applied to the body during squats that we typically perform them at a faster pace than leg extensions.

So, in effect, not only are you applying constant tension during leg extensions, you are also usually doing so for a longer period.

Another point to consider is that it is far easier to perform drop sets with leg extensions than it is with barbell squats.

Don’t tell me you’ve never removed the pin, reduced the weight, and then carried on repping with leg extensions.

It’s definitely not as easy to do this with squats.

So, I would say that the speed of reps and the volume of work with leg extensions will have an effect on your burning quads.

How to Do Leg Extensions With Perfect Technique

3. Your Squat Technique Can Affect Leg Extensions

I’ve already mentioned that barbell squats aren’t solely quad-focused.

However, how you perform your squats could have an impact on overall quad strength.

Let’s say for example that you perform your squats with a powerlifter technique.

So, basically your stance is wider and you tend to sit back more at the bottom of the movement.

This actually makes the squat more biased towards the posterior chain.

In other words, your glutes and hamstrings are being worked harder than your quads.

So, it will very much depend on what other quad-focused work you do.

You may also be someone who uses the leg press machine with feet high or feet wide.

So, once again, even though the quads are being used to some extent, the movement is far more focused on the muscles of the posterior chain.

Not to put too fine a point on it, but depending on your other lower body training, your quads could be weak in relative comparison to your glutes and hams.

Therefore, when it comes to performing a single-joint exercise solely focused on the quads, i.e. leg extensions, you’re going to feel the burn much more than you normally would.

I will say that this is probably quite unusual, and more so for men than for women.

I don’t wish to stereotype, but many women tend to have stronger lower-body posterior chain muscles, simply due to how they train.

Whereas, men typically focus on what they believe is the “power look”, which can blow up the muscles on the front of the body, and leave the muscles at the back of the body lagging behind.

As I say, this isn’t always the case, but I’m willing to bet it’s not too far off from the truth.

Anyway, if your training is more focused on the lower-body posterior chain muscles (man or woman) then you may feel leg extension burn much more.

4. Your Leg Muscles Are Depleted of Oxygen

A Car Window With Colourful Writing That Says, 'Trippin' On Lactic Acid'

This is something that I went into in more detail in my article, “Why Do My Legs Shake on the Leg Press?

However, during intense exercise the muscles can typically become depleted of oxygen.

And this is especially true of lower-body exercises.

The reason being that the lower body is home to the largest muscles in the body, so there is the ability to exert more force.

As I explained in my leg press article, this isn’t something that you need to be concerned about.

I know that the thought of a “lack of oxygen” conjures up all types of dangerous connotations, but it is slightly different for the muscles than it is for the respiratory system.

Basically, the harder you push yourself and the more intense a particular set is, the more likely that the muscles will become low on oxygen.

Quite often when you try to push through a set when the muscles oxygen levels are low the body will produce lactate.

And the body has the ability to convert lactate to energy without the need for oxygen.

With that being said, lactate can build up in the bloodstream quicker than your body has the ability to burn it.

This is known as acidosis and will create that burning sensation you feel during an exercise like leg extensions.

I’m guessing most of us have experienced this build up of “lactic acid” at one point or another.

Lactic acid build up isn’t actually a bad thing, and it can be quite beneficial for building muscle.

However, you should also be wary that it means that you are fast approaching muscular fatigue.

Plus, a build up of too much lactic acid will cause an imbalance of the body’s PH levels.

If you overdo it this could lead to headaches, nausea, abdominal pain, cramps, exhaustion, diarrhea, etc.

The Truth About Lactic Acid & Muscle-Building

Final Thoughts

Leg extensions are a single-joint exercise, which solely focus on the quads. This explains why you generally “feel the burn” more than many other leg exercises. You will also apply constant tension with leg extension and typically perform the reps slower and at higher volumes. “Leg burn” can also be caused by weak quads or a build up of lactate in the muscles.

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