zatopek

We are trying to increase cardiac size and strength. Larger heart means greater stoke volume and more blood delivered more rapidly to skeletal muscle. In addition, stronger cardiac muscle equates to more efficient emptying with each contraction. And, I think, just as our skeletal muscles develop with training to contain more mitochondria, more, and more efficient enzyme systems for energy metabolism and dealing with muscle cell waste products, trained cardiac muscle develops similarly.
As a consequence of these changes, resting heart rate decreases because a greater volume of nutrient and oxygen rich blood can be delivered. VO2 max can be reached at a lower percentage of max HR.