need help w/front squats

[cmdubs]

I tried these for the first time last night and liked the way they worked the entire core, but everytime I went into the hole when I was going heavier the middle of my back rounded. I didn't really notice until I felt a twinge in my middle erector on my last set, then it hurt like biatch the rest of the night. What causes this? I think I could make my back stay flat, but my torso would have to go forward and the bar would roll off my shoulders. How can I fix?Anybody??

 

[from zero]

1) Use lighter weight.
2) Strengthen your core.