The upper back is not the true limiting factor during a front squat, the legs are.

Here's a simple way to test this theory for yourself: set the safety pins up really high in a squat rack so that you only have to move the bar a few inches to unrack it. Now set up in your front squat position, unrack the bar, brace hard, and just hold it for 30 seconds. Start with roughly 60% of your front squat 1RM. Then go to 75%, then 90%, then 100%. If you can do 100% then try 110%. If you can do 110% then try 120%, etc. etc. etc. and so on and so forth until you can't hold the bar safely or securely anymore. 

Now, if you can hold your 1rm on the front squat in a stable isometric front rack position for 30 seconds then guess what: your upper back is NOT the limiting factor in how much weight you can front squat, your legs are. You start upright and can support the weight easily, but when you squat down you likely lose positioning on the way up most likely in the form of your hips popping up and running away from your shoulders. 

This places the torso in a forward leaning position, which even if its just a SLIGHT forward lean, greatly amplifies the amount of stress that is going to be placed on the upper back. Suddenly the thoracic region has to create far more extension force just to be able to maintain rigidity, and as such it most likely folds over here.

But it's subsequent failure at this stage is not due to it being the weak point. It's subsequent failure is due to the LOSS OF POSITIONING that occurred FIRST, and the loss of positioning was due to your weak ass legs not being able to keep your torso upright driving out of the hole.

Ergo, the legs are the limiting factor, not the upper back.

So ya'll can stop bitching now about how "front squats aren't an effective leg builder because I suck at them." They are an effective leg builder. You just have weak legs. Get stronger legs and stop bitching.

Now am I saying this is going to hold true for everybody? No, of course not. I never am. There are going to be people who have a build that is so not conducive to squatting well that their positioning is always going to suck no matter what and the front squat is going to be limited by the upper back and is not going to be an effective leg builder for these people.

There are also going to be people who truly have upper back weakness. That may even be a lot of you at this very moment. I used to even be one of you! But this weakness is easily rectified, and rather than running away from it forever, why not fix it instead?

A simple way to do this is to end your lower body days 1-2x per week with a few sets of isometric holds in the front rack position (or if you have access to a yoke then you can try doing front rack carries instead, which are a hell of a lot of fun but aren't very safe to do with a barbell).

For the isometric holds, you just load up something challenging and perform 3-5 sets of 30-60 second holds. Make sure to set the safety pins in the rack as high as possible so that there is no danger here in the event that you lose control of the bar.

Breath rhythmically (do not hold your breathe), brace hard, and focus on maintaining good posture without leaning back.

If your upper back truly is the limiting factor in your front squats then it won't be for very long if you develop a truly strong front rack. Personally, I have front squatted 455lbs and I have held over 600 pounds in an iso front rack position for something like 45 seconds or 60 seconds. That is over 130% of my best ever front squat held for a period of time that was much longer than it takes to execute even a 5 rep set of front squats.

So obviously I cannot blame the upper back as being the limning factor in my front squat performance. Do I still round over up top sometimes? Yes, of course. But I don't blame the upper back. I blame the legs for being too weak to drive properly out of the hole.

Just as most people who have a "lockout weakness" on the deadlift don't actually have a lockout weakness, but rather they have an inability to maintain good position off the floor that creates a cascade up the chain resulting in an inability to lock the bar at the culmination of the lift.

This is the same thing.

The loss of position at the early portion of the front squat (hips popping up) creates a cascade effect that accumulates as you move up the chain, and ultimately results in a failure at the mid portion or end of the lift when the upper back fails. Then the blame game starts getting misassigned because it creates the appearance of a weak point, but this is a false weak point. 

Then don't! That's your loss and you are missing out on some immense and amazing gains in terms of leg strength, leg hypertrophy, hip and thoracic mobility, overall fitness, and mental toughness. But that's your prerogative!

But stop saying its a "poor leg builder" just because you wanna cope about how you suck at it, and instead of owning the truth you want to convince other people that it's a sub-optimal movement instead. That's an incredibly lame MO that stems from a myopic viewpoint of what lifting weights is.

So either fix the issue by putting your ego aside, starting light, and building your way up slowly. Or just stop talking about things you don't understand. End rant.

The golden ratio to shoot for is front squatting 80% of your high bar back squat. If you can do that then you're a damn good front squatter and leg strength is not an issue for you. I have personally taken lots of guys who were self-proclaimed "front squat haters" and turned them into front squat lovers. I even have a question on my intake questionnaire for clients that has them name any exercises they want me to exclude from their programming, and you probably won't be surprised to hear that a decent number of guys put front squats on that list.

And we abide by that for a while, but then after some time passes I convince them to give front squats an earnest go when I believe they will benefit from them, and after a few months, when we have built a respectable front squat, suddenly these guys who "sucked at front squats" and "hate front squats" don't have a problem with them any longer.  Some even grow to love them!

So just some food for thought! You probably have not heard this perspective before so it's good to be exposed to new concepts like this (at least, I like to think it is). If you can take the info here to heart it just might help you get to the next level in your training!