At this level, O2 is probably only 11% of the atmosphere
Michael Phelps sleeps in an oxygen controlled environment every night. Top class athletes also sleep and train in oxygen depleted chambers or sealed rooms. Why do they do it ? About 50 years ago, there were some astonishing physical test results from people who lived and exercised in high altitude conditions, namely the mountains. Scientists were baffled as to why these people, Kenyans, Nepalese to name a few. Since the early '90s the marathons of the world were dominated by Kenyans who lived and trained in hilly regions and ran easily 20 - 30 km to school.
After years of research, it was theorised that the lack of oxygen will affect the cognitive and physical functioning of the brain, and other body parts. Studies have shown many years ago that with much lowered oxygen levels, people struggle even to perform the simplest of physical and mental tasks like speaking, simple arithmetic and memory. Even walking up a flight of stairs resulted in tremendous effort. The physiological reason is that the body is naturally conditioned to operate at optimum levels of oxygen in the air at around 20.8%.
A conditioning chamber can control oxygen to 11.2% or equivalent to 5000 meters above sealevel.
Hence, the mature adult body requires oxygen in the bloodstreams and the red blood cells are the main transporters of this to all the other body parts where aerobic respiration takes place whereby the oxygen is then broken down in the muscle fibre, energy, work done and motion are hence the result.
In long term reduced oxygen levels, the body adapts brilliantly and the speed of transport of the red blood cells to the various body parts is increased so as to maintain the same level of work done or energy. That is why if you are in an oxygen depleted room, you will find yourself panting, taking shorter inhalations, perhaps gasping as you perform the same run on the treadmill at the same pace of running as you would if you were in a normal open air room. Studies have shown that at 3000 meters above sea level or about 14.5% of Oxygen in the air even the simple run at say 6 mins 30 seconds per kilometer will make you feel like you are running at 6 minutes pace ! You will be putting in much more effort and your blood vessels will be increasing the oxygen distribution by up to 30%.
A practical example is if you went up to Everest base camp at 3000 meters, even 100 meters walk is a stretch for the weekend warriors and they end up panting through the mouth.
The effects of your body's increased productivity (up to 30%) by your red blood cells' activity can then be an enhancing effect if you then return back to normal oxygen levels of 20.8% - your performance will definately be enhanced. The enhanced performance can last days up to 1 week depending on the duration you live, train and go about your daily life.
So the top sportsmen and sportswomen will record even better results using legitimate and accepted scientific technology.