Oxygen transport from outer air to muscle cells and final tissue oxygenation are controlled by breathing. Physiological and medical studies proved that the more we breathe at rest, the less oxygenation of tissues. Hyperventilation reduces tissue oxygenation. Why?
While overbreathing cannot improve blood oxygenation (it is up to 98% saturated with O2 during light normal breathing), it reduces CO2 content in the body leading to two negative effects down the oxygen delivery cascade:
1) Blood vessels become constricted (CO2 is a potent vasodilator);
2) Oxygen release in capillaries is suppressed since CO2 is a chemical catalyser of this process. This effect is known in physiology for a century and described in many textbooks as the Bohr law.
Hence, the less we breathe (down to 2 l per minute for minute ventilation), the better oxygenation of the human body.
Since CO2 is the main factor in breathing control and it also participates in regulation of dozens of chemical reactions, including synthesis of proteins, lipids, hormones, and immune cells, humans cannot change their unconscious breathing pattern at once. The changes are gradual and usually take, even for healthy people, at least weeks or months.
Obviously, VO2max values are sensitive to the unconscious breathing pattern and body oxygenation at rest. By breathing slightly less (with air hunger) and improving body O2 reserves at rest, we can enhance endurance and VO2max within 10-15 minutes just before exercising.
The breathing exercise is done only on empty stomach, with straight spine (or no slouching) for diaphragmatic breathing, in cool conditions with good air quality. The first step is to listen to own usual breathing for about 20-30 s. Then instead of taking large inhalations, take a slightly smaller breath, using your stomach only, and immediately relax all breathing muscles and the rest of the body. Continue to breathe in this light mode, so that you gradually accumulate air hunger (or desire to breathe more air), while being totally relaxed (this is the most challenging part of this breathing exercise). In 1-2 minutes, if you do it right, you will notice that your arms and feet will be warmer (due to improved circulation), while your nose will be moister and cooler. If you continue to practice this shallow or reduced breathing for about 10-15 minutes, your heart rate also significantly drops, as you can easily check using devices, and this will also indicate your improved physiological state.
If an athlete learns how to change their unconscious breathing pattern or achieve very light unconscious breathing 24/7, then body oxygenation and VO2max will be greatly improved too. Currently there is only one medically proven therapy that produces these results: the Buteyko breathing technique practiced in Russia by about 200 MDs. In order to learn it, one requires to regularly practice shallow breathing exercise, daily measure own body oxygen content using a special breath holding time test, breathe through the nose during almost all or all training sessions, prevent sleeping on the back at night, and do many other things that are part of the Buteyko method.