My last post on heart rate was about training with a heart monitor, and I talked about running (or working out) to your 70% recovery heart rate (an "easy" day) three times a week and to the 85% threshold ceiling (a "hard" day) once. But why is this strategy important? (Here follows a VERY simplified explanation of muscle metabolism and biochemistry!)
Our body contains two types of energy source: glycogen and fat. When you begin a run or workout, your body "fires up" and starts to burn glycogen without oxygen (anaerobic metabolism). This warm-up phase lasts a maximum of 2 minutes, after which your body begins to burn the glycogen with oxygen (aerobic metabolism). After 30 minutes of your run or workout you begin to burn fat aerobically; this process is the most efficient way to produce energy and is what will get you through an endurance race. You still need glycogen as a back-up (for dealing with hills and a sprint finish!), so training your body to burn fat rather than glycogen (and therefore saving this vital energy source for "emergencies") is important.
Muscles are formed of two types of cells (or fibres): slow-twitch and fast-twitch. Slow-twitch cells have a high oxygen content (for aerobic metabolism) and burn fat; they are therefore most important for endurance. Fast-twitch cells create energy from glycogen without oxygen (anaerobic metabolism); a by-product of this process is lactic acid, which creates that painful burning sensation when you train hard. Increasing the amount of fast-twitch muscle cells will help you deal with this burning pain (sprinters have a lot of this muscle-cell type).
How the science fits with training
"Easy" training days at 70% recovery heart rate increase the number of slow-twitch cells, thereby teaching your muscles to burn fat (and saving your glycogen for another day). By increasing your muscles' fat-burning potential you are not only getting fitter but also boosting aerobic capacity, the ability to burn energy with oxygen.
But you do need some capacity for anaerobic metabolism (without oxygen, glycogen-burning); anaerobic metabolism is going to get you started on a run or workout and will propel you up a hill quickly, but will make your muscles burn from the pain of lactic acid build-up. A "hard" training day or interval training---when you run for short distances at your 85% threshold ceiling heart rate---helps your body develop more fast-twitch cells to deal with the pain of lactic acid build up.
"Hitting the wall" (basically, when your legs stop working) happens when you've run out of glycogen. So easy training days---teaching your muscles to burn fat---are really important so you have glycogen available when you need it.
So, that's a very simplified explanation of why it's important to run or workout at these two different heart rate zones. For more details I suggest you read Heart Monitor Training for the Compleat Idiot by John Parker (which a friend very kindly lent to me---he can have his copy back now!) or borrow a book from your library on sports biochemistry or muscle metabolism (there are lots out there).
My personal experience
I've been running with a heart monitor and following these principles for about 6 weeks. I have really noticed a difference in my ability. When I started running at my recovery heart rate my average pace was 11 min/mile and I was running 7 miles; now my average pace is 10 min/mile and I can run 9 miles. Also, I ran 10K in early April in just over 57 minutes. Last week I knocked 2 minutes off that time. Who knows where I will be in another 6 weeks?
If you have any questions please let me know in the comments. I will reply!