hard to see because of the plate, but it looks like your in quite alot of spinal extension through the lower thoracics to the upper lumbars in the back squat video, cant really see whats happening to the pelvis and spine too well as you drop into the squat because of the plates and safety bars.
it looks like your standing with the pelvis in anterior tilt at the start, and arching your back to compensate. You can see it as well in your stomach, which is domed. it gets pregressively worse through the set as well.
hard to give specific advice about improving that, as its hard to know wether its a tech issue, or a weakness in your body thats causing you to squat like that. At a guess, your doing it to stop yourself collapsing forwards.
need to make sure the core and things like glutes have the capacity to engage properly, and are doing so, work on bracing during your lifts, along with working on the tech for the squats which others are probably better positioned to advise on that than myself
<message edited by dazc on 22 January 2012 12:32>