There is no clear consensus among gym goers and trainers concerning the correct way to breathe during a workout. Some people suggest exhaling during exertion and inhaling whilst returning to resting position. For example, with bicep curls this would involve breathing out during contraction and breathing in during relaxation. Other people suggest that holding your breath during a rep is a good idea, as it requires maximum effort. The most extreme versions involve holding your breath throughout the entire set. It goes without saying that this is a very bad idea and can have serious consequences for your heath.
Holding your breath increases the pressure in your torso and raises your blood pressure. If you hold it too long, especially during strenuous activity, you could faint and injure yourself. In more serious cases your ocular nerve could become detached and you could bleed from your eyes and go blind. In the worst case scenario you could develop internal bleeding in your brain, burst a blood vessel, get a stroke, go into a coma or die.
The other common technique that is taught in gyms is to exhale during the push. The main advantage of this technique is that it prevents you from holding your breath. However, if you breath in and out once with every movement, your intake of oxygen is dependent on how quickly you are performing the reps. This means that as you slow down near the end of the sets, you will not get enough air, as your reps have slowed to a crawl as failure sets in due to the secretion of lactic acid in the muscles. This results in an oxygen debt at the end of every set, and a resultant need to breathe heavily and for extended periods.
Perhaps not so surprisingly, the most effective breathing technique is just to breathe naturally. Choose your initial pace for the reps and don’t think about your breathing. If you feel yourself needing to take shorter or deeper breaths, listen to your body and don’t try to artificially control it.