The main reason that leg extensions burn so bad is because they are a single-joint exercise, so you 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.
Leg Extensions Are a Single-Joint Exercise
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.
“Leg extensions are a good isolation exercise to target the quads, but they shouldn’t be your only leg exercise. Compound exercises like squats and lunges are essential for building overall leg strength and athleticism.”
Arnold Schwarzenegger (Bodybuilder & Actor)
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.
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 quad