If you have issues with the lower back rounding in the bottom position of deep squats, it can be due to the body trying to compensate for muscular weaknesses, or it can be due to flexibility issues. In rare cases it is just a matter of the lifter knowing their body’s position in each portion of the lift, which is why the typical “gym expert” advice of “stop rounding your back” is occasionally helpful, but usually useless, because it does not address what is causing the problem. This is how I would remedy the problem in a trainee, and while the source of the issue is most likely just one thing, it is easier to just treat every possible source of the problem to make sure this form issue is fixed and stays fixed, as the weight being lifted progresses upward.
1) Weak Glutes-
Solution- Glute isolation exercises, such as hip thrusts, glute bridges, and reverse hyperextensions. (Reverse hyperextensions would be ideal although I still have yet to see one of these machines in real life, so I know the vast majority of people have no access to them. It is heavily used in elite athletics circles, but has not yet “trickled down” into normal gyms, as of yet).
2) Lack of Hamstring Flexibility– In this case the lifter is rounding their back and tucking their butt under their body to avoid overstretching their hamstrings. The deeper the lifter goes on a squat, the more stretched the hamstrings are. People with inflexible hams will automatically round their back to compensate for this lack of flexibility.
Solution- The standard standing toe touch stretch, but do it with your back arched (opposite of back bent). It should look like you are doing a toe touch while trying to stick your butt out as far as possible. Often individuals with tight hamstrings will still be able to do well in the sit and reach due to a flexible lumbar spine. This stretch takes the spine out of the equation (by keeping the spinal erectors tight and contracted). I, personally, can touch my forehead to my knee while doing a seated sit and reach (and therefore I can reach 8 to 10 inches past my toes), and while doing this stretch, my hands barely pass my knees.
The two issues listed above are the most common issues causing this extremely common squat form problem. The reason this is the first leverage and form based post I have written, is that this is the lift (deep backsquats) that is practiced with the most horrible form, and this is the most common problem individuals have with squatting.
3) Lack of groin flexibility. (A bit less common)
Solution: Do a stretch, where you descend into the bottom position of a squat (without weight obviously), and hold the bottom position while focusing on forcing the knees outward. You should feel a stretch in the same muscles that the “butterfly stretch” stretches. This stretch is most effective because you are stretching the inflexible muscle group while in the bottom position of a squat, so it is more likely to help with squat form related inflexibilities than a typical groin stretch.
If you incorporate the two stretches listed above as well as glute isolation exercises, your squat form will fix itself, and a pleasant side effect is that your posture (lumbar spine posture) will probably improve, and your squat poundage will increase due to the direct glute isolation work.