Yawning during weight training

Oh my gosh, I do the same thing! I used to have a personal trainer and felt so bad anytime I'd yawn infront of him because I didn't want to look disrespectful.

Lack of oxygen...

Googled and found this, maybe it helps!

Causes Of Yawning

According to an old hypothesis, yawn is considered as a reflex action triggered by the brain on the account of lack of oxygen and excess buildup of carbon dioxide.
According to dictionaries, yawning is a process triggered by fatigue, drowsiness, or boredom. Although this theory doesn’t explain why we yawn when we are not bored or tired.
Many people say that may be yawing serves health purposes as it causes us to draw more air inside our body and to make our heart, race faster.
It is also speculated that yawning is triggered from the part of brain which we are yet to discover about.
It is said that people yawn by stretching their mouths wide and inhale deeply in order to supply oxygen to the lungs and eventually, to the bloodstream.
As per a recent hypothesis, yawning is an action, which helps the body to regulate temperature.
According to a study, the chemicals named neurotransmitters, present inside the brain, make people yawn. It is also believed that the more these chemicals are activated in the brain, the more is the frequency of yawning.
Many people say that yawning is a behavior similar to stretching, which is done to flex the muscles and joints.
Some theories also believe that as even cats, dogs, tigers, fishes and almost all animals use, it may be some form of communication, which we are yet to understand.
It is believed that yawning is something inherent in human beings. The practice started with our ancestors, who used this behavior to show their teeth and intimidate others.

Interesting Facts About Yawning

Generally the average yawn last only for six seconds. The heart rates at the time of yawning can rise up to 30 percent.
More than 55% people yawn within five seconds of seeing someone else yawn.
Even reading about yawn can make you yawn. It’s the same with hearing too. The moment you hear someone yawn, it may trigger yawn in you too.
It is also seen that Olympic athletes often yawn before their competition starts.
It is also a very interesting find that blind people yawn more after hearing about it, mainly the audio tapes of other people yawning.

Yawning isn't a 'bordom' thing. Your body does it when it doesn't have enough oxygen. When you push your muscles they need more oxygen, thats why your heart pumps more blood (full of oxygen) to them. Without oxygen your muscles become tired. You are probably just yawning because your muscle are wanting more oxygen. BSN makes a drink mix called N.O. Explode... I personally don't care for it. But what it does is oxygenate your body to help you to do more of a workout and not tire as easily and not 'bulk' up. For me the flavor is hard to swallow and it makes me light headed, BUT many many people swear by it.

I just posted about this a few days ago. I'm a yawner, too.

http://www.myfitnesspal.com/topics/show/129864-am-i-really-that-lazy?hl=that+lazy#posts-1762054