I've seen 77kg lifter back squat 290kg for a double like nothing. ol lifting does involve your knees getting in front of your toes, but i feel with with proper training,. it shouldn't be a problem. just listen to your body. as far a missing a lift, not a big deal, just go where the weight doesn't. not many of the best miss cause it is so routine to them. nico is right though, ironmind has some cool videos u might want to check out. i have video of a 94kg lifter 18 year old kid clean 517, but missed the jerk. that weight would have been a new world record, but the kid still holds the old one of a 512 c + j( what a shame, yeah right)