When a person lifts heavy weights and either holds the breath or performs the Valsalva maneuver [i.e., forcefully straining against a closed glottis (the opening between the vocal chords)], air becomes trapped and pressurized in the lungs. This high intra-abdominal and intra-thoracic pressure causes an immediate, short-term increase in blood pressure and also inhibits the return of blood to the heart by compressing the large veins that lead back to the heart. Holding the Valsalva maneuver for an extended duration can result in lightheadedness, dizziness or, in some instances, fainting. Because of its negative cardiovascular effects, the Valsalva maneuver should not be performed by most fitness enthusiasts. However, there is some application for its use in conditioned athletes, as many competitive weightlifters perform the Valsalva maneuver during the lifting of heavy loads to help stabilize the trunk.
From looking at the research on the Valsalva maneuver and its undesirable influence on blood pressure and venous return, some experts recommend that during a resistance-training exercise you should exhale during the most challenging part, or the exertion. This serves two purposes: (1) it allows you to breathe continuously during the lift (i.e., inhale on the less-challenging part and exhale on the exertion), thus reducing the increase in blood pressure that accompanies resistance exercise, and (2) it provides added stabilization to the spine during exertion because exhaling activates a deep abdominal muscle, called the transverse abdominis, which functions to support the lumbar spine (lower back). Please note that this style of breathing is recommended for lifting moderate to heavy weights. During low-intensity stabilization exercises, such as those recommended for maintaining a healthy back (e.g., bird dog and side bridge), breathing normally and naturally without thinking too much about the timing of the inhale and exhale is recommended. In this way, the breathing during the exercise activity mimics breathing during the activities of daily living, which helps the brain translate exercise to real-world application.
Lastly, there are some specialized forms of exercise, such as Pilates, that encourage specific breathing patterns during specific movements because the breath actually enhances performance of the movement. For example, reaching forward to touch your toes in the seated position (part of the Pilates roll-up movement) requires forward spinal flexion, which compresses the rib cage. Thus, exhaling during this portion of the movement facilitates the exercise because it compresses the ribs even more, and if you exhale most of the air from your lungs, you will actually be able to move forward further into flexion (albeit only slightly). The opposite is also true in that if you lie prone and lift your chest off the floor, you are performing spine extension, which expands the ribs. In this instance, inhaling is recommended because the ribs are in a good position to open even more as you inhale.
Of course there are exceptions to each individual and each exercise, but I wanted to provide a basic foundation for the different concepts behind breathing during exercise. I haven’t even mentioned the most important muscle in proper breathing mechanics, the diaphragm! For an excellent article on the respiratory muscles (including the diaphragm) and improving your exercise performance through proper breathing, check out the link below.