From The National Geographic:
His oversized heart can beat over 200 times a minute and thus pump an extraordinarily large volume of blood and oxygen to his legs. His VO2 max—the maximum amount of oxygen his lungs can take in, an important measurement for an endurance athlete—is extremely high. Early in his career Armstrong showed only average muscle efficiency—the percentage of chemical energy that the muscles are able to harness to produce power. Higher muscle efficiency means greater production of power. From 1992 to 1999, the year of his first Tour de France win, Armstrong was able to increase his muscle efficiency by 8 percent through hard and dedicated training. Coyle says Armstrong is the only human who has been shown to change his muscle efficiency.
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