Running with a heart rate monitor can be helpful. Many guides to using these talk about various "zones" and how you might want to stick in various ones for different elements of your running. However there is one thing to remember when doing this and that is that your heart rate will drift up during a sustained period of effort (including during a race). To illustrate this, I have passed the information from my heart rate monitor for the tempo session I ran on Thursday - 4 laps of Lancaster Park - 5.6 miles. The top plot is altitude, so you can see where I ran up to Lancaster Park at the beginning. The middle plot is pace - the scale on this is not very helpful and makes it not that easy to see what is going on. The bottom plot is heart rate. You can see my heart rate increase as I ran up the hill. It then drops as the group paused to sort out who was doing what. Then I ran the tempo. The middle plot shows (as far as you tell) that I ran pretty steady pace. However, what you can see is that my heart rate increased steadily during the session. My average heart rate for each of the 4 laps was: 147, 156, 160, 164. The first average will be lower because my HR was slower at the start of the lap. However, you can then see that my HR was increasing by around 4 beats per lap.
What are the implications of this? Well suppose I had run a marathon at the same pace I ran this tempo. If my heart rate increased by 4 beats every 1.4 miles, by the end it would have been around 224. This is above my maximum heart rate - i.e. I couldn't sustain the pace for that long. What you need to do is be able to run at a pace that will lead to an increase in HR during the run that will not take you over your maximum!! So, a HR increasing by say 1 beat per mile in a marathon, with you starting off at around 150 would mean ending the race at around 176 - do-able. Trying to run at a pace that allows us to run fast for the entire distance of a race is all about getting this right. HR increasing, but not too fast.
How do you do this? The best way is learning to listen to your body and getting plenty of experience of running at different paces so you know what these feel like. This is why we do short efforts, longer efforts, tempos and long steady runs, as well as doing races every so often. This all helps you know during a race whether you are likely to be able to maintain the pace you are running right to the finish. A HR monitor can help by giving you some extra feedback, but there is no substitute for listening to your body. This is because additional factors (e.g. how well hydrated you are, how much sleep you have had, how stressed you are etc etc) all also affect your HR. So HR monitors are interesting, informative and helpful, but don't try to totally rely on them.
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