your hips probably aren't mobile enough....you can youtube a lot of hip mobility drills that will help greatly with that...the more your hips can move the less your lower back has to move when you squat...could be the reason for your back pain. also...looking at your posture, you could be in anterior pelvic tilt, which can cause a lot of back pain...stretch your hip flexors, work on your deep abdominals...there are some decent articles on tnation.com by eric cressey and mike robertson called neanderthal no more...should help with the posture aspect.

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