My brother seems to be having a problem with his squats; he always emphasizes going all the way down, but what happens is as he approaches the very bottom of the lift, his back (seems to be his lower back) rounds pretty bad. However once he starts coming back up it straightens out. It seems to me like its caused by his effort to get his ass as low as possible, and what happens is he rounds his lower back and dips his ass straight down, kind of towards his heels, rather than sticking his ass out.
My guess is that it's a combination of this knees being too far forward (caused by him not sticking his ass out) and him not keeping his lower back tight the entire time.
Any other advice on what could be the problem? I'll take a vid and upload it if I need to. But in the meantime hopefully this description is enough to get some help. Thanks