Running twenty-six miles pushes a body up against some of its fundamental limitations. If they didn't know how to recognize those limitations and work around them, students would inevitably become exhausted and quit before they reached the finish line. What Carda gives them is the advantage of being a smart runner.
On one morning, for example, Carda prompts students to consider their heart rates. "I think everybody understands that when we exercise, our heart rates go up," he tells a room full of thin, athletic bodies assembled before him in a lecture hall at the McClain center. The reason is fairly simple, he notes: muscles work by burning oxygen, and when we exercise, we increase the demand for fuel. Automatically, the heart responds by beating more frequently to increase the supply of oxygenated blood to the muscles. "It's a pretty remarkable system," Carda explains.
But not a perfect one. Carda illustrates that the total amount of blood the heart can pump out is limited by the fact that at some point a heart can't beat any faster - a factor controlled by a person's age and health. To some extent, the heart can make up for that ceiling by beating harder, thus pumping a greater volume of blood out with each beat. But that quantity, known as stroke volume, is also limited, he says.