An often overlooked but limiting factor in most people’s squat is their upper back mobility. If you have limited mobility into extension in your thoracic spine, your hips will compensate to counterbalance your forward torso, so you do not fall over. This will often result in you pulling your knees backward too far, creating a vertical shin position.
Our body is great at compensating and will try to find mobility in some other segment in the body that is not intended to move as much when squatting.
Compensation then puts compensatory strain/stress on your body’s other areas, such as your low back and knees. Due to the COVID-19 pandemic, most people have been more sedentary and sitting more often. As a result, your thoracic spine has spent more time in a flexed or slouched position vs a neutral or extended position. This may have created an extension mobility problem that you need to address before your workouts.
There are several ways to stretch your thoracic spine into extension but two of my favorite stretches are the open book stretch and thoracic extension at a bench.
Checkout out React’s YouTube page for more details about these stretches.