A study has been done with cyclists that have cycled in the Tour de France in regards to their hearts. The study found that each person had a heart 20 – 40 percent larger than the average humans. This allows them to control their oxygen flow much better and handle more blood volume in the heart. No ordinary person would have a heart just this size, but it comes about with exercise. Not just any exercise regimen would work though. A cyclist exercises almost everyday and really to the extreme in order to participate in the Tour de France. They study was done on almost all riders that have participated in the Tour past and present. The interesting thing is that they discovered that a riders heart will shrink about one quarter in size after they retire from riding. This shows that riders have to build their hearts just to be able to participate in such an event as the Tour de France. The study also found that when an athlete starts to blood-dope, then they run the risk of clot or stroke because it can make the blood too thick. It might even ruin the natural advantage the rider might have had with their abnormally large hearts to begin with.