Do you find yourself leaning too far forward when performing back squats and it hurts your low back?
Well, a simple trick you can use to determine the reason for this excessive forward lean is putting your heels on two 25 pound barbell plates or a 2” x 4” and trying to squat again.
If your torso becomes more erect and you can squat deeper with ideal form, the issue is most likely lack of ankle mobility, specifically tightness of a muscle called your soleus. Instead of squatting on plates, get to the root of the problem by stretching your soleus against a wall with your knee bent.
You can stretch as much as you want and when you get proper ankle range of motion back, then you can start doing back squats again with proper form and decreased risk of injury.