It means you're working hard
This is especially common with dead lifts because its so compound and the meat of the working muscles are in your lower extremities.
NSCA - CSCS
New to deadlifts?
I actually get a high from lifting in general, but after I crush a record I've set, I'm on top of the world.
Psalm 34:10 - "The lions may grow weak and hungry, but those who seek the Lord lack no good thing."
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I've had this happen to me. But I was at a higher elevation than normal and never lifted at that altitude. I guess it was the lack of oxygen.
On the hardest part of any lift (squats, deads, and even bench) I hold my breath just for a second, till the weight is moving. And usually during the pause at top I take two quick breaths, then the big one at the bottom.
I give a f**K!!
The valsava will create a higher pressure in the abdomen and thorax, and thus help to keep the torso rigid. If held only for the moment when this rigidity is most needed (ie: the first pull in a deadlift, getting out of the hole in a squat, etc.) and then released, it is helpful and not really harmful. If held through the entire lift, then it may become a problem.
To the OP, here's whats happening
At the time when it is held, blood pressure sky rockets. When the hold is taking place, blood flow (and oxygen) to the brain is minimal due to constriction of arteries and veins in the throat/neck area. When it is released, the pressure causes a fast draining of blood from the brain into the now open veins, and there is a moment where the draining from and supply to (the head) is mismatched. Thus, oxygen delivery to the brain is inadequate for a portion of time and you get that light headed feeling. Same occurs when standing up too quickly, especially in people with orthostatic intolerance.
The more I read about Rippetoe's advice and form, the more I really doubt the guy. Yes, you should have big air while pulling, but you should release at lock out and drop the weight down. Lather, rinse, repeat.
Could be beta endorphines similar to the "runner's high" runners get.
For example: tght hamstrings will cause a loss of lordosis toward the bottom of the rep, and tight hip flexors will cause excessive lordosis and forward leaning especially as the athlete descending.
How much work do you put into improving in the exercise, and how much work do you put into correcting these?
Just curious, seems to be an issue I used to face quite often. Coaches (especially football coaches and parents) who want their kids numbers to improve dramatically, and myself, who feels a strong base (skill (movement pattern), muscular balance, and motor programming) is more important, especially if the athlete plans to compete beyond high school.
I start off with the box squat. It teaches you how to keep the weight towards the hamstrings and hips while heavily re-enforcing the hip drive aspect. With people that have awful form, it is not uncommon to add 50 lbs in just a few sessions by showing them proper technique. After that, you look to see where the sticking point is whether it be weak hamstrings, mid-traps, quads, etc. I've had many clients who swear up and down that they can't do squats and within 10 minutes, I have them squatting. 99% of the population does not realize that the motion is at the hips and not the knees.