Breathing while working out you would think would come natural, but it does not. Must of us hold our breath during strength training or heavy exertion which is called Valsalva maneuver. The Valsalva maneuver involves forcibly exhaling against a closed glottis (entrance to the throat), which increases your pressure in your chest. This raise in chest pressure can couple with adverse effects that could become life-threatening.

Breathing throughout the performance of exercise should be continuous and natural, and it should be performed with an open mouth. As an exercise becomes more difficult and the lactic acid begins to accumulate in your muscles, causing that “burning” sensation, you should deliberately breathe faster, or hyperventilate.  This step will help you break the urge to hold your breath and employ the Valsalva maneuver.

Two good articles to check out on the importance in breathing when lifting weights are www.livestrong.com or www.answers.com