The Hahn paper was a review of literature that focused on moderate altitudes (1500 to 3000 meters/ 4950 to 9900 feet) and extended exposure of 2 to 4 weeks. Among the relevant findings of the review was that altitude training did not create enough of a stimulus to significantly increase red blood cell volume or hemoglobin mass. Hemoglobin is the part of the red blood cell responsible for transporting oxygen. They did note, however, that with acclimatization lactate clearance and production was altered to the benefit of the exercising athlete (e.g., lower production, higher clearance).
Another interesting study on the impact of altitude on performance comes from Gore et al. in 2007. They sought to identify which factors, other than EPO production and RBC volume, might account for differences in performance after altitude training. In the study they undertook a review of literature focused primarily on the Live High Train Low (LHTL) approach, where training occurs at low altitude and the athlete lives at high altitude via use of an oxygen tent or other mechanism.
More: Acclimating to Altitude Before a Race
Upon review they came to the conclusion that a genetic transcription factor known as Hypoxia Inducible Factor (HIF-1) acts as a global regulator of oxygen homeostasis, and is present in tissue in the body, acting to transcript, among others, the EPO gene and to influence factors like improved exercise function via glycolytic enzymes, glucose transporters and other non-hematological (blood-based) pathways.
In addition, they considered that increased ATP production per mole of oxygen (or decreased ATP utilization) might play a role as well. Management of muscle pH is another area where hypoxic training plays an important role. Specifically, the transport of lactate and the removal of hydrogen ions from the blood are dramatically improved with hypoxic training.
They did note a couple of key negatives from altitude training. Specifically cardiac output was lower, sleep was impacted and immune function can be suppressed even at moderate altitudes (2650m/8750ft). In a different study, Clark et al. (2007) looked at peak oxygen consumption at four different elevations: 200, 1200, 2200 and 3200 meters, and found that in their 10 well-trained cyclists peak VO2 declined 8.2 percent, 13.9 percent, and 22.5 percent at each of the higher elevations. Correspondingly, 5-minute power on a time trial effort decreased 5.8 percent, 10.3 percent and 19.8 percent at the same altitudes when compared to the baseline of 200 meters.