Squats with a barbell. Take care of your back!

The squats are very easy to overload the lower back and thereby get injured. It is important to understand exactly how this happens and for what reasons. When you sit down and begin to rise, it is necessary that the rod be raised simultaneously with the straightening of the legs. The error is that you can raise the ass, and the rod will lag behind in recovery and then you will have to raise the bar through the back.
Please note, when sitting down, begins to rise, the pelvis goes up, and the post is late. And with each repetition, when the force in the legs is less, this error becomes more noticeable.
Often this is due to the lack of flexibility of the back and hip joints. To sit down, you have to lean forward. By itself, a strong forward with a heavy barbell on the shoulders of the already very dangerous.
If you do not have the flexibility to place them under heel stand and then the back will be more vertical.
And don't forget about the belt.