Does Altitude Training Improve Sea Level Performance In Endurance Athletes?

Physical training at altitude is thought to invoke similar physiological changes to those caused by endurance training at sea level. The adaptations considered most important for performance enhancement include increases in blood haemoglobin and myoglobin, improved buffering capacity and improved aerobic enzymes in the muscles.

It is postulated that the stress of hypoxic exposure, in addition to training stress will compound the training adaptations the athlete experiences and therefore be an advantage over sea level training (Wolski et al 1996). 

Conclusion

Residing at altitude allows a number of potentially beneficial physiological, ventilatory, haematological and metabolic adaptations to occur. These adaptations are postulated to improve endurance exercise performance. While is widely supported that training at altitude enhances performance at altitude, there is much less support for the view that altitude training will improve sea level endurance performance.
It is now theorized that the reason that many studies found no improvement in sea level performance after altitude training is due to the fact relative deconditioning appears to occur while training at altitude, which may offset the potential beneficial changes resulting from altitude exposure.
Therefore a new strategy has been introduced where the athletes live at altitude but train at sea level. This approach allows both the beneficial adaptations of acclimatization to develop, as well as provide the opportunity to train without reducing power output during exercise. Therefore, this strategy is accumulating support from scientists, athletes, and coaches alike, as the most advantageous method for enhancing sea level performance in highly trained endurance athletes.
In conclusion, well trained athletes whose development has included adequate base preparation with regular, high intensity interval work, and have reached a plateau in their training response at sea level are likely to derive the most benefit from altitude training using the LHTL approach.

http://podiatry.curtin.edu.au/resources/educational-resources/exphys/00/altitude.cfm#conclusion