Here's about the best, non-technical, discussion I've found:
Q: Why is the valsalva maneuver used in weight lifting?
(3 answers - asked 23 months ago)
A: It often occurs in weight lifters that require a rapid and maximum application of force for a short duration.
But, it can be dangerous. Read the following:
Valsalva's Maneuver
By: Erik T. Nason, MS, ATC/L, CSCS
OK class the new medical term for the day is Valsalva maneuver. No Kenny, it is not a dance. It involves something that we all do everyday. That something is breathing. Most of us take breathing for granted and therefore give the saying don�t know what you got until it�s gone a brand new meaning. Breathing as we all know is very important. So why do we forget to do it when we are weight lifting or performing a strenuous activity. Have you ever been spotting your friend as he or she is doing a bench press and you notice that their face is turning really red. Almost like their head is going to explode. Well that is not right. This is Valsalva maneuver in progress. The Valsalva maneuver is defined as increased chest and abdominal pressure by forcing a exhalation against a closed mouth (or glottis). Let me explain...
First let's review the breathing process. We start with an inspiration followed by an exhalation.. During inspiration, the chest cavity increases in size because of the raising of the ribs and lowering of the diaphragm as air moves into the lungs. During exhalation (breathing out) the ribs swing down and the diaphragm returns to a relaxed position. This reduces the volume of the chest cavity and air rushes out.
The chest is made up of a group of muscles that surround the rib cage. These muscles are involuntary muscles, meaning that you don�t have to think to use them�they just work. The muscles are mainly used to force air out of the lungs. Just think of your hands placed around a balloon with a valve on the end of it. If you squeezed the balloon between your hands the air in the balloon would be forced out the valve. The chest muscles do the same thing. They increase the chest cavity pressure forcing out air from the lungs. Now here's a little more anatomy. At the end of your wind pipe (in your throat) there is a trap door called the glottis that keeps food particles from going down the wrong way. By holding your breath you are actually keeping that trap door closed and trapping the air in the lungs and/or chest cavity. If, however, the glottis (trap door) is closed after a full inspiration and the chest muscles are maximally contracting (because they want to force the air out), this will increase the chest cavity pressure higher then atmospheric pressure. This forced exhalation against the closed glottis (or trap door) is termed Valsalva maneuver. The Valsalva maneuver commonly occurs in weight lifters and other activities that require a rapid and maximum application of force for a short duration. (For example, an individual bench pressing, squats, )
The increased chest cavity pressure can cause some physiologic consequences. There are several veins that pass through the chest cavity that are carrying blood back to the heart. This blood is under very low pressure. When a Valsalva maneuver occurs, the blood flow in these veins are actually diminished greatly. With exercise involving straining, blood return to heart is minimal and therefore oxygenated blood leaving the heart is also minimal, which causes a lack of oxygenated blood flow to the brain. This can produce dizziness, spots before the eyes, and even fainting. Blood pressure is also altered during the Valsalva maneuver. With the onset of a Valsalva maneuver, at the start of a lift, blood pressure rises abruptly as the chest cavity pressure increases. This can be dangerous for individuals who have been diagnosed with a heart illness or are being treated for high blood pressure.
Here is an example of proper breathing during a bench press. During the down phase of the bench press the individual should inhale. As the individual pushes the weight back up to the starting position, the individual should exhale. The individual should not hold his or her breath during the up phase. This is where the Valsalva maneuver would occur.
All in all, the next time you go to the weight room remember not to take breathing for granted. Focus on proper breathing. Remember holding your breath during a strenuous exercise is not only an improper lifting technique, but most of all it can be dangerous to your health.
Sources:
http://www.ask.com/web?q=valsalva+maneuver&qsrc=0&o=20036&l=dir
