Almost blacking out wile squating

Maybe up and down too fast, or the bar was pinching something. Or like the previous poster, it was just too heavy.

If nothing else, talk to a dr just for peace of mind

if its not the weight being too much and your sure your breathing correctly i would say it might be some sort of circulation problem. i would bring this up to your doc next time you go just to be safe

I'm confused, you're new to lifting but you're squatting 390x5?

I've been lifting since January, my squats just have improved a lot. So I'd consider myself new still.

Okay, apparently I need to get my crap together. Been lifting for a year and I can only do 200x5. Edit: of that year, only the past 3 months squatting properly with a barbell. But still. 390x5 is insane in that amount of time.

squatting, lifting, bearing down, etc. can cause pressure on you vagus nerve, which can in turn effect your heart rate, causing dizziness and fainting. there are many activities that can cause this, be careful, maybe squat and rise a little slower, make sure your hydrated, and pay attention to your body. have you ever noticed, when having a sports physical done, many times, the dr will have you squat and rise, while listening to your heart. i dont think its something to be over concerned with, but do be careful.

Are you this guy's brother?

http://www.youtube.com/watch?v=k8RcDb_wZfQ

squatting can cause pressure on you vagus nerve, which can in turn effect your heart rate, causing dizziness and fainting. there are many activities that can cause this, be careful, maybe squat and rise a little slower, make sure your hydrated, and pay attention to your body. have you ever noticed, when having a sports physical done, many time, the dr will have you squat and rise, while listening to your heart. i dont think its something to be over concerned with, but do be careful.

This makes a lot of sense. I'll have to work on my rise. I'm also doing the high position of the bar, I don't know how to do the low position. .. er, I never had them check my anything wile squatting. That must be for girls, the Dr just grabs my boys and tells me to cough.

the dr checks all my kids squat and rise heart rate for sports, for my daughters and my son. my son has a heart problem, so, my dr has given me a thourough run down on the vagus nerve, because my son is extra extra sensitive to fainting. have you ever been on the toilot and felt dizzy or like you were going to faint after bearing down? thats your vagus nerve, runs through your entire torso, and is repsonsible for alot of unexplained dizzy spells and even fainting!

the vagus nerve is a cranial nerve, which pretty much governs your heart rate. you can even control it at times, by pinching the bridge of your nose between your eyes, which will slow your heart rate. people do this when they are stressed as a natural reaction to stress, not even realising why they are doing it; which is lowering their heart rate to relax! crazy huh!

bump for OP

I'm relatively new to heavy lifting, except for bench, but that's a different story. So I don't really know everything yet.
Like the title said. I was doing a personal best today at 3 sets of 5 reps at 390lbs. During the second set I got really light headed and felt like I was blacking out for a moment, but then it passed and everything else was fine.
I always concentrate on my breathing. Is this normal, or is there something I can do to prevent this?

Try using a weight that allows your knees to bend during the squat. Drop the ego lifting and settle for reality and you should be fine.