We have seen this discussed before many times among strength coaches. "I have an athlete who rounds his low back at the bottom of the squat. What do you think it is?"
Many theories have been put forward. Hip mobility, core stability, or lateral hamstring tightness/stiffness. All of these could most definitely be contributing factors. But something I have never seen discussed on the matter is the thoracic spine, and cervical spine.
I started to think any the thoracic spine and cervical spine being limiting factors to the squat (and also the deadlift for that matter) when talking to Charlie Weingroff at Providence last June. Charlie was talking about the importance of "packing the neck" when squatting and deadlifting.
Charlie went on to say that looking up, and putting the cervical spine into hyperextension is not a good idea. Then it clicked for me. Think about it. Your spine is just one long line. We separate it into the cervical, thoracic, and lumbar regions for convenience, but what happens at one region of the spine will without question effect other regions of the spine.
So say we have an athlete with a slightly hyperextended cervical region, and he has a stiff t-spine, as he squats down he will run out of his neutral spine position at a higher point during the descent, then if we had another athlete who had packed he cervical region, and had a mobile t-spine.
The athlete with the stiff t-spine will now look to get more motion from another area to reach the depth required. This is where the rounding of the low back at the bottom position will be seen.
Another thing to look for with athletes who struggle with this is the hand position on the bar during a back squat. If they have trouble holding the bar, chances are the t-spine is so stiff that it is limiting their amount of external rotation at the humerus. These are the athletes that get 1's on the shoulder mobility of the FMS. If you switch the athlete to a front squat, they should be able to descend slightly further as external rotation will not be a limiting factor.
So when someone asks for your opinion on why does the low back round when squatting, ask the coach, "Do you get your guys to tuck their chin in?". "What is their shoulder mobility on the FMS like?". And "What is their ALSR Like"?.
I think if you get an athlete who scores 3 on the ALSR, but 1 on and SM, and they have a hard time getting into position when deadlifting or round their low back at the bottom of the squat, consider looking at the thoracic, and cervical regions of the spine.
I am not saying that the cervical and thoracic spine are the only cause of the low back rounding when squatting and even when deadlifting, but I think that it is something else to consider.