From Fellrnr.com, Running tips
There are two components to running ability; fitness and economy. Fitness is the ability of our bodies to generate energy for running and is the focus of a lot of our training. Economy is how far and fast you can run with a given amount of energy. Good economy is a critical part of running, and [Cadence] is one element I focus on.
In an ideal world, we’d be able to easily measure our running economy and see if things are improving. If we could measure our breath, find out how much O<sub>2</sub> we consumed and how much CO<sub>2</sub> we produce, we’d know how much energy we burned (and from fat or carbohydrate). Sadly, this is not practical, so the best measure we have of energy consumption is our heart rate. This is far from perfect, as heart rate can vary for other reasons besides supplying O<sub>2</sub> for energy production. However, I believe it is a useful approximation.