Breathing is of critical importance to a successful strength training program. Optimal breathing patterns will minimize the risk of injury while maximizing the benefit of the athlete's time in the weight room. Specifically, inhalations should occur during movements where the body moves out of or away from the fetal position; while exhalations should be reserved for movements that move the body toward or into the fetal position.
This is exactly how the body works. Try it: take a big breath in and notice that you get taller as your spine elongates into extension. This is the exact technique we used when my son went to Disney World for the first time. Not blessed by age (he wasn't quite five years old) or genetics (his dad is 5'4"), he was just under the minimum height for some of the better rides. But when I had him take a big breath in--presto! He was good to go! If only I had known about this trick when I was his age...
Now blow the air out of your lungs and feel how you get shorter as you literally compress into flexion. In a properly functioning body, inhaling is coupled with axial extension, abduction, and external rotation. Exhaling is coupled with axial flexion, adduction, and internal rotation. And lifting with proper breathing mechanics will help you be stronger during the lift.
The one exception to this rule is when lifting at intensities that necessitate holding one's breath. The body does this naturally as a way to stabilize the diaphragm so the muscles of in the Inner Unit have a solid foundation from which to apply force and support/protect the axial skeleton. Failure to do so would send excessive loads through the spine, eventually resulting in injury. Thus, using a heavy back squat as an example, optimal breathing for a safe and successful lift would proceed in the following order:
1--INHALE to charge the thoracic cavity.
2--Gently draw in the belly button to activate the TVA.
3--Descend in a controlled manner with knees tracking over toes and your pressure through the heels.
4--Once at parallel or at the appropriate depth based on the ability to maintain a lumbar lordosis, begin the ascent. EXHALE through pursed lips after passing through the sticking point as you return to the start position.
5--Begin as explained in step one and repeat for the designated number of reps.