Thanks for the links. I will try the tips. One question. Why are my harmstrings too tight when my back rounds. I dont feel it in the harmstrings. I thought the problem lies in my back, not in my harmstrings.
@ burton I postet a video of mine squatting.
Your back is rounding b/c your hamstrings are tight. And the reason you're not feeling much in your hamstrings is b/c you have not learned how to properly engage them in your squat. Focus on pushing your butt back instead of just sitting down. Feel the difference in your hams?