Dr John Hellemans wrote: Various forms of altitude training and altitude simulation are well proven methods to improve endurance performance in many sports. Road cycling is an endurance sport with a high dependency on the aerobic (oxygen dependant) energy systems. Your capacity to process oxygen in an energy efficient way is a deciding factor in cycling performance as it helps you to last longer at a higher average velocity.. If you can improve the amount of oxygen delivered to the working muscle (by increasing the redblood cells) and process it in the most energy efficient manner (by improving the off loading and processing mechanisms), you have an immediate advantage. Altitude training achieves just that. Also, many races, especially at international level, have mountain stages and being acclimatised to altitude will help performance at altitude.