Navy divers -- in particular, Navy SEALS -- practice breathing exercises to perform better during their free-diving expeditions, according to Stew Smith, a former Navy SEAL. Breathing exercises look to make the body more efficient in oxygen uptake and utilization so that a person can stay submerged in water for longer periods. These exercises can be dangerous, so it's highly recommended that you practice these exercises with a trained professional present. Consult your doctor before trying these exercises.
Power Breathing Technique
Inhale deeply and allow your lungs to fill completely with air. Then exhale deeply, drawing in your abdominals while keeping your lips pursed together. Remember not to blow air in to your cheeks or out of your nose. The direction for airflow should be directly in and directly out of the mouth and lungs. This exercise trains the muscles in the core that assist with breathing and can help you get a quicker, deeper breath when necessary.
This exercise aims to train the lactic acid system of the body, which doesn't require the utilization of oxygen for energy production. Training this system teaches the body to perform at higher lactic acid levels and potentially blunt the desire for taking a breath, which is important underwater. Take a deep breath and then perform an all-out sprint for 25m without taking another breath. Repeat two more times.
This exercise requires that you travel to an area of extremely high elevation or spend time in a hyperbaric chamber. These two places reduce the partial pressure of oxygen, which means the body doesn’t have access to the same amount of oxygen at sea level. Now perform your regular cardiovascular training at one of these two locations. This causes the body to adapt by creating more red blood cells, which carry oxygen, and improves other physiological processes that increase the body’s efficiency in utilizing oxygen.