It's important to maintain the natural curves in your spine to prevent lower back strain.
Lumbar flexion in the deep squat because of mobility limitations in the hip joints and pelvis.
In a full squat, proper flexibility and mobility in the hip joints and pelvis are essential for maintaining natural spinal curves.
Don't Perform the squat up to the stage in which we cannot maintain the natural lumbar curve. In other words, adapt the squat to the trainee's capability.
Each trainee has their own domain of movement, and it could be not completely bending to the ground.