What laboratory test is the best predictor of finishing times for a running race of 100 kilometers (62 miles, or more than twice the distance of a marathon)? A study from Yokohama, Japan suggests that it is an echocardiogram to measure the size of your left ventricular heart chamber (British Journal of Sports Medicine, Volume 40, 2006), which determines how much blood your heart can pump with each beat. This would be expected to predict how fast you can run for short distances in which you have to move so fast that the limiting factor is lack of oxygen. This study is surprising because most athletes believe that the major limiting factor for running very long distances is the amount of fuel you can store in your muscles.

Now we know that the limiting factors for ultra-endurance competitions are similar to those of shorter distances: the time it takes to move oxygen from blood in your lungs to your muscles. This is determined by how much blood your heart can pump and how much oxygen your blood can carry. Since 98 percent of the oxygen in your blood is carried by the hemoglobin in your red blood cells, the higher your red blood count, the more oxygen you can circulate. However, a more important factor is how rapidly your heart can pump blood to your muscles, and this is determined by the strength of your heart muscle. The longer and harder you train by running, the stronger your heart, and that's what a thicker left ventricle means. The runners who ran the most miles in training had the strongest hearts and the best finishing times. So if you want to compete in any sport requiring extremes of endurance, you have to spend a lot of time training and you also need to exercise very intensely once or twice a week to strengthen your heart.

Dr. Gabe Mirkin has been a radio talk show host for 25 years and practicing physician for more than 40 years; he is board certified in four specialties, including sports medicine. Read or listen to hundreds of his fitness and health reports at http://www.DrMirkin.com

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