there is always this group of people in the gym who go to squat legs with confidence every time. But after squatting for no more than a few sets, they start to experience lower back pain. Even after doing an adequate warm up, the stiffness in the lower back makes it difficult to improve this condition, it's really not that they don't love squatting.
If you really love to work your legs, squats may not be your only option, there are many variations of the unilateral squat that you can choose from. For example, the bulgarian split leg squat, or the pistol squat. These movements are great for building stable lower body and core strength.

by unilateral deep squat we now simply mean a squat where the feet are not side by side: The unweighted leg can be a few feet behind the body (as in a lunge), or on a bench behind you (as in the bulgarian split-leg squat), or in front of the body (as in the pistol squat).
The reaction force perpendicular to the ground (under the control of the leg muscles) is greater in the unilateral deep squat than in the conventional two-legged squat. As a result, unilateral squats can produce significantly higher barbell speeds, which is a great way for people with lower back pain to generate more energy.

also good for leg development
you must be confused that the free squat is a two-legged strengthening movement. Yet the single leg side squat is not even half as good as the deep squat for leg training?
Firstly, a lower weight with higher reps will definitely result in a stronger pump. Unilateral squats will also stimulate your hamstrings, calves, glutes and abdominals just as much as double leg squats.

of course, the quads may not get as much stimulation as a normal double-legged squat, but you can do some leg flexion extensions while recovering your back muscles and doing leg flexion and the quads will still get plenty of development.
Thinking of the back
on the one hand, unilateral squats are not as good as double-legged squats: They activate the erector spinae muscles. When it comes to lower back pain, unilateral squats are not a bad thing - they are a good thing. Although the unilateral squat puts stress on the abdominal muscles, putting much less pressure on the lower back than a double-legged squat. Reducing the stress on your lower back means you can start increasing the weight of your unilateral squats (rather than not doing them at all).

so, if it's leg day and you have back pain: Firstly, go to the doctor and make sure you can work out safely. If you can train, do this planned workout (adjust to your body and needs).
Example of a unilateral deep squat workout
1. Unilateral squat with barbell or dumbbell
choose a barbell or dumbbell according to familiarity and comfort with the movement, and choose the right distance between your feet. Choose a conservative weight and work up to 4 sets of 8 reps per leg with 90 seconds rest in between.

2. Dumbbell arrow squats
keeping the core engaged, do 4 sets of 8 reps on each leg.

3. Dumbbell single leg romanian hard pull
do 4 sets of 8 reps on each leg on one side with 1 minute rest between sets. Make sure it is a hip-driven movement rather than placing the bend towards the floor and make sure you do not lower it too low, even above knee height. Maintain constant tension to give the hamstrings a more frantic stimulation.

4. Unilateral leg curls and extensions
keep the weight light to ensure that the lower back is not under too much stress. Do 4 sets of 15 reps each without borrowing: If you can do 15 full reps, can you challenge yourself with a bigger weight?

5. Seated calf raises
standing calf raises are tempting, but the body may not be able to handle the extra strain. Complete 4 sets of 15 reps of seated calf raises. Slow down and emphasise the top contraction of the movement without borrowing any strength.
For those without injuries, this movement can be very good for training balance, stability and gives us a lot of extra points for free squats.

it is true that leg training is a bit tough, but when the results are good, everything feels worth it. The effort you put in is not in vain. One or two leg training sessions per week will give you visible breakthroughs that.