Hypoxic chambers are kinduv the thing in professional bike racing, triathlon, and running. Top athletes in all these sports use the chambers. The chambers are designed to duplicate different levels of oxygen like what you'd find naturally at different altitudes.
Lots of endurance athletes live at altitude (Boulder for example) hoping to gain benefits from adjusting their bodies to training in an oxygen depleted environment. The theory is your body will naturally compensate and make more red blood cells to more effectively transport the limited amount of oxygen (more or less the jist of it-I'll try and find something more scientific). Normal people who live at altitude have a higher hematocrit (red blood cell count) than those who live at sea level. So this proves the theory that training at altitude is better, right? Actually no...
Here is the main problem: training at altitude isn't as good as training at sea level. There is more oxygen at sea level so you can get a better more intense workout-think of it like this; at sea level there is more gas for the engine so you get more power from the engine; at altitude there's less gas so your engine can't produce as much power. HOWEVER recovery at altitude has added benefits that recovery at sea level doesn't. When you train at sea level and then recover at sea level your body doesn't need to produce more red blood cells (RBCs), it just has to maintain status quo-your body need x amount of RBCs so your body produces that number. When you train at sea level and then recover at altitude your body needs to produce more RBCs to help efficiently transport the limited amount of oxygen; you need x amount of RBCs to recover from training + y to deal with decreased oxygen levels at altitude. Top cycling/tri/running pros will train normally (at sea level) and sleep in these chambers. In Europe I heard an Italian cycling team would have their training rides along the coast and then drive up to altitude and stay in a resort in the high mountains. Top pros prefer this to taking EPO as the cycling federations have finally developed EPO tests (though not very good ones) and it has almost the same effect while being more natural too. Exogenous EPO administration has to be done very carefully or you can kill the athlete! It is said that this increases your wattage at anaerobic threshold 10% and you get a 5% boost in hematocrit levels. This is huge for an athlete-this really benefits endurance athletes and anyone working out would notice better endurance.
Significant benefits can be gained from living and training at altitude for 3 weeks. After that the effect drops off. The decreased training intensity from training in an oxygen depleted environment is offset by the boost in RBC production.
There are a few companies who make chambers, but tents are supposed to be the nice thing. You can drape the "altitude tent" over your bed and sleep at 10,000 ft.
On thing to consider people tend to lose muscle mass when training at altitude!
Here's a link to check out:
http://www.sportsci.org/jour/0001/pf.html
FHG