If you’ve ever trained or raced at high altitude it can be a physically humbling experience along with a decrease in performance. Headaches, light headed, sometimes nausea. The decrease in performance at altitude is largely due to the altitude-associated decrease in VO2max. Because there is less oxygen available to the muscles at altitude your maximal cardiac output is lower so your VO2max is lower than at sea level. The reduced VO2max at altitude is greater with higher level athletes because they exhibit a greater decrease in hemoglobin’s oxygen saturation. In other words, the more an athlete has to begin with, the more he or she has to lose.