Gabe Mirkin, M.D.
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.
June 15, 2006