Physiologically speaking, Lance Armstrong is built for endurance cycling. His VO2 max, a measure of the body's ability to transport and use oxygen for aerobic respiration, was approximated at 84 mL/kg/min. An average male would have a VO2 max around 40mL/kg/min. Yet, other endurance athletes also have VO2 max measurements comparable to that of Lance Armstrong.

Cardiovascular fitness is, in part, contingent upon the ability of the heart to maintain an adequate output to meet the metabolic needs of respiring muscles. It is therefore likely that the source of Lance Armstrong's fitness may be his heart. In the 1930's, a Chinese doctor named Tung observed X-rays taken of 46 rickshaw pullers. He noted that the heart size of these rickshaw pullers was bigger than that of the general public. This process is known as "hypertrophy" and reflects an increase in the bulk of muscle fibres of the left ventricle in response to chronic exertional demands on the heart.

As studies by Edward F. Coyle have shown, Lance Armstrong does indeed have a heart with a hypertrophied left ventricle. This is evinced by a resting heart rate of 33 beats per minute. His maximum heart rate is just over 200 beats per minute, allowing him to maintain a huge cardiac output to his muscular legs.

However, it is not solely the size of Lance Armstrong's heart which makes him the world class athlete he is. Dr. Coyle also postulates that the molecular make-up of his heart may be different to yours or mine. He hypothesises that Lance Armstrong's muscles have a higher percentage of Type 1 muscle fibres which have a different type of a myosin (a molecule responsible for muscle contraction). This altered myosin imparts more efficient respiration in which more chemical energy from ATP hydrolysis can be converted into kinetic energy for power production.

The great thing is that you and me can also get bigger hearts and increased type 1 muscle fibre percentages by regular, hard cardiovascular training. Of course the psychological strain of such arduous training is hard to comprehend, making Lance Armstrong's feat all the more momentous.