If you find it challenging to do a deep squat because your trunk(upper body) is falling or leaning forward every time you go down you need to work on several things.
Tips | September 23, 2013
1. Try elevating your heels! If you get more depth with elevated heels this is an indication of a tight soleus and gastrocnemius (back of your calves). Work on lengthening those muscles and shortening(strengthening) your anterior tibialis muscle to create an equal length tension relationship.
2. If your calves are not tight, chances are your hip flexor muscles are tight and need to be stretched/lengthened. Tight hip flexors are very common and usually due to habitual motions such as sitting down for extended periods of time.
3. Work on your intrinsic core muscles. Work on strengthening the deep muscles in the core that are responsible for most movements and stabilization. Unless you are competing in a fitness competition I don't recommend knee raises etc for core movements because you are constantly flexing your spine and working your anterior muscles which will cause shortened hip flexors, Poor posture, and probably low back pain. The best core exercise is doing a plank, simple!