HRM can be a useful tool if you know your body. If you do not, it does more harm than good. Targeting a specific HR because a popular source says so is listed in the Top Ten Training Mistakes at
http://asksasha.com/Running/Top-10-Training-Mistakes.htmland for a good reason.
Maybe some discussion of the common misuses of the heart rate readings would be helpful:
- Your max HR is not necessarily 220 - age or any other formula that uses your age. It is what it is, if you want to know it, run an all out 2 miles and see what it gets up to at the end. It may or may not drop with age. It does tend to drop as your fitness increases when you start hitting neuromuscular limits. That is your heart could beat faster, but your nervous system cannot drive your muscles hard enough, or maybe your muscles are not strong enough to use that much oxygen. If it does drop with age, it is probably due to the loss in neuromuscular fitness (Quality X).
- A certain percentage of max HR does not necessarily represent the correct effort. If it feels wrong, it is most likely wrong. If you are going to use HR to gauge intensity, first learn about your body. Race different distances with an HRM and learn how long you can hold certain heart rates for.
- HRMs are notorious for intermittent errors that last for as long as several miles. If your HR is normally 130 at 8:00 pace, and then one day you are feeling normal, the breathing is fine, you are just starting your run, the weather is cool, the pace is still 8:00 and your HRM shows 160, it is wrong, do not believe it, do not slow down to make it read 130.
- It is normal to experience an upward drift in HR during a run. So if you target a certain fixed heart rate, you will run a huge positive split in your tempo runs. For example, if I targeted HR of 160 in a 5 mile tempo, I would have to run 5:00 pace for the first 0.5, then slow down to 5:30 for a mile or two, and finish at 5:40 pace. That is not how I want to run my tempos.
- HR varies with the weather. When it is warm, the right intensity will often produce a higher HR.
- HR at the correct intensity is often lower on the downhill, and higher on the up. Somebody may argue that it means you should pick it up on the down, and ease off on the up, but try racing like that on a rolling course, you will lose to a less fit competitor that runs purely by feel. Same for headwind/tailwind.