Swimming: What is the difference between training in hypoxic air, like at altitude, and simply holding your breath in a pool?

James said, Secondly,  holding your breath and restricted breathing will not necessarily lead  to the same adaptations. My understanding is that the forced breath  holding that occurs during swimming will likely lead to greater  adaptations with regard to CO2 tolerance rather than O2 delivery. I'm  not sure about this, but I don't think swimming will confer any special  O2 delivery mechanics that cannot be produced from other forms of  aerobic training. He's on the right track. Please do not practice hypoxic swimming or breath-holding underwater! It's way too high of a risk and way too low of a reward. I work with special operations candidates on breath holding for underwater evolutions, and the underwater breath-holding scenario is very rarely done, only at the end of dry land breath hold training, and always, always, always with overwhelming emergency medical support poolside AND with instructors monitoring the underwater trainee like hawks! If you hold your breath for a long time, the pain you feel is a change in blood chemistry because of increased CO2 levels, not a lack of O2. Training in that way mainly causes an increased ability to deal with that level of CO2. Few people are willing to hold their breath long enough even on dry land to cause serious O2 deficiency and the resulting "passing out". If you could, like other answers have said, the stimulus is very short in duration and would not cause anything near the same adaptations as long term, less intense O2 deprivation. Swimming is a moderate to high energy activity which will run you into that change in blood chemistry much quicker than sitting still holding your breath. The potential for underwater blackout is increased. Hyperventilating to increase breath hold times does nothing but eliminate stagnant CO2 from "dead air" spaces and increase the time until the "burn" of CO2 excess. Many people doing it suffer from O2 deprivation unexpectedly and rapidly, and pass out underwater. If that happens to you, you'll stop breathing and require immediate emergency medical intervention in order to survive the ordeal. And don't forget, when this extremely dangerous activity is done in training, with all that medical support, that's because it's specifically necessary to train that way for certain careers and tasks underwater. Otherwise it would never be done. You can hold your breath to your heart's content for athletic purposes, just don't do it in water.

At altitude, the oxygen content of the air is much lower.  Your body compensates by producing more red blood cells(RBC's) to provide your body the same amount of oxygen as you would have at sea level.  Hence, the US training facility in Colorado at high altitude. EPO does the same thing, stimulating the production of large number of red blood cells and enhancing endurance. Athletes have been known to sleep in partial vacuum systems to simulate oxygen deficiency to increase red blood cell numbers without using drugs. [1] Holding your breathe in swimming does not induce the same effect.  You still have the oxygen content of the air available to you and your body compensates to produce the required RBC's.  The training in swimming does increase volume of air consumed and the efficiency of the body to utilize it.  There is in an increase in mitochondria and the ability to break down lactic acid[2].  There are still debates about all the mechanisms affected and improved because of endurance training.  [1]http://www.tri-ecoach.com/art26.htm [2]http://en.wikipedia.org/wiki/Endurance_training

sorry, I am no expert in human pulmonary system, so can't give you what you are looking for, however, to start a discussion my two cents are here: if you train for longer period in altitude, your body becomes more adapted to the lack of oxygen, not just because you are training there, but because you also live there 24/7 during your training. So, in other words, your body learns to cope with this environment and becomes more efficient in oxygen consumption at least temporarily. This effect is then carried to the sea level when you return, but goes away as you body does not need to be that efficient any more. I don't believe that it is possible to replicate altitude training at sea level with hypoxic swimming or with snorkel restriction etc. It is just a marketing/psychological scam on swimmers ;). Which usually works to the benefit of the swimmer (I'd hope). To second Andrew's request, it would be great if someone with the appropriate expertise could set the swimming community straight :).