I've recently heard many coaches and trainers diminishing the benefits of using your heart rate as a valuable training tool. The reasons I hear are valid statements but shouldn't be used in dismissing it as an essential piece in your toolbox:
1) heat can affect your heart rate which will lead to higher readings even when intensity hasn't increased
2) outside stressors such as fatigue, dehydration can result in numbers which don't really correspond to your workload
3) faulty readings coming from the monitor
While I agree with the variables being issues at times, it's the years of proven success in coaching athletes of all abilities that has me still using it after 19 years in the field. Let's dig deeper into the reasons I feel heart rate based training is an ideal metric for runners. However, I need to state that in my coaching I don't rely on heart rate alone. I look at pace, vertical, and now most recently thanks to STRYD power meters, running wattage as well.
Your Body's RPM
I want to provide you with an analogy using your car and it's tachometer, the gauge that measures the engine's revolutions per minute. When you're going up a mountain pass, you're probably going pretty slow. Grinding away with 12% grades probably has you steady on the gas telling you that it's taking a lot to make the climb. You look at your measly speed of 30mph but then glance over at the 4000+ RPM's you're turning out. Just because you're going slow is not an indicator that your car isn't under an extreme workload. The tachometer is responding to the work being placed on the car, just as your heart rate responds to running stress and other sources of workload.
Your heart rate is similar to your car's RPM. Heart rate doesn't tell you what your pace is doing, but your pace doesn't consistently paint an accurate picture of workload either. An example would be when you're out running a hilly route and your mile paces are dropping on the route due to the vertical gain. Your 10 min miles become 12's yet are you slacking off? Not at all. It's just that the climbs are requiring more power forcing your pace to slow so you don't blow up. We don't stop looking at our pace even though variables such as hills can throw it off and we shouldn't throw out monitoring our heart rate due to present variables mentioned above.
In order to become efficient at lower heart rates, you have to train there. You may be asking why should you want to become efficient at low heart rates when you don't race at that intensity. Your answer lies in the cost per unit of work as well as proven benefits of training in lower zones. First let's discuss the cost of running. If a mile at a HR of 155 costs 165 calories, yet a mile at a HR of 140 costs 135 calories, which is more efficient? The lower HR of course. But that's at a slower pace you say… or at least that's what I said. But, as we train at lower HR's, our body becomes faster at those low HR's. That 10 min mile pace in zone 4 just became zone 3. We are doing the same amount of running at the same pace, just at a decreased cost.
This isn't black magic tricks, it's simple adaptations causing you to become more efficient. The body loves efficiency and seeks to master it in everything we do. If you do something in repetition, you become better at it. This is no different. At the same time you're creating a favorable stage for more adaptations to take place such as increased mitochondrial density (powerhouse of the cell) as well as improving capillarity leading to increased means for a higher volume of blood flow.
One of my favorite benefits of heart rate training and spending time in the lower ranges is the fact that you can run yet create less stress on the body. This leads to quicker recovery time, less injury, less fatigue, and more energy for all the other things in your life. We as American's tend to think "more is better" and "I gotta be fast today". That thinking will have you stressed, burnt out and possibly injured. I promise you it is not the healthiest way to go about things.
Use It All
As a runner, you should be paying close attention to the way your workouts make your HR respond AS WELL AS the paces you're running. And if you're into more metrics, grab a STRYD power meter and tackle the wattage aspects of training as well for the most accurate assessment of how hard you're working during your runs. Use it all. Don't dismiss. Include! The more data, the better.