Is the Tour de France winner endowed with a supreme athletic physiology by a genetic freak? Or does he just work harder than the rest?
There is circumstantial evidence for the freak theory. Armstrong has particularly long thigh bones, for instance, making him biomechanically suited to cycling. His heart is a third larger than the average male's. His maximum heart rate is over 200bpm. His VO 2 max (the amount of oxygen the body can use in a given period, a standard measure of aerobic performance) is one of the highest recorded at about 83 ml/kg/min.
All these are factors, but relatively insignificant ones. All professional cyclists, for example, will have much bigger and more efficient hearts than untrained individuals. Most will have VO2 max numbers in the 70-80 ml/kg/min range. Where Armstrong stands out is that he seems to have an innate capacity to train exceptionally hard, recover quickly and reap the fitness benefit.
One study found that between the ages of 21 (when he was already a pro) and 28 (after winning his first Tour), his muscular efficiency had improved by 8%. After surviving cancer, he also became more focused about managing his weight, so that this 8% was actually worth an 18% increase in his power-to-weight ratio - crucial for cycling up the Alps and Pyrenees.
And this was achieved with a workload that would make most elite cyclists break down. Even Armstrong's own teammates, who would be continually worrying about picking up a virus, marvelled that Lance never seemed to get sick. A natural athleticism combined with a remarkable immune system and an incredible work ethic: that seems to be Armstrong's recipe for success.
Sauce
I would be "Oh, that's a shame, but it does explain some things" But that doesn't mean I think he IS juicing.