heart rate and RPE

Why Heart Rate Is Not a Good Training Tool for Ultrarunning


By Jason Koop,
Head Coach of CTS Ultrarunning,
Author of “Training Essentials for Ultrarunning”

If you are going to use interval training to accumulate time at intensity and target specific areas of your fitness, you need a way to figure out how hard you are working. Prescribing intensity based on either heart rate or pace is notoriously difficult in ultrarunning, and after trying all manner of methods, I found the greatest success in a remarkably simple, nontechnical, yet scientifically accurate method: rating of perceived exertion.

In some sports it’s easy to figure out how hard an athlete is working. A cyclist with a power meter can directly measure workload in watts, determine how many watts he or she can produce at lactate threshold, and then create intensity ranges based on percentages of lactate threshold power. Ultrarunners don’t have it so easy. For a long time, runners have tried to use heart rate to gauge intensity, intensity ranges based on percentages of lactate threshold heart rate or the average heart rate recorded during a 5K time trial. Others have used pace ranges based on time trials or goal race paces, or a combination of heart rate and pace ranges.

Heart rate and effort

The heart rate value you see on a watch is an observation of your body’s response to exercise. It’s not a direct measure of the work being done; instead, the work is being done primarily by muscles, which in turn demand more oxygen from the cardiovascular system. Because that oxygen is delivered via red blood cells, heart rate increases as demand for oxygen rises. It’s an indirect observation of what’s happening at the muscular level, but in the absence of a direct way to measure workload, heart rate can provide valuable information.

Research has shown conclusively that there’s a strong correlation between heart rate response and changes in an athlete’s workload, and that research allowed sports scientists and coaches to start creating heart rate training zones back in the 1980s. But as sports science has evolved over the past 30-plus years we have learned that many factors affect an athlete’s heart rate, and those factors reveal that heart rate response is not reliable and predictable enough to be an effective training tool.

Factors Affecting Heart Rate

Core Temperature

As your core temperature increases, heart rate at a given exercise intensity will increase. Your circulatory system carries heat from your core to your extremities to aid with conductive and radiant cooling.

Caffeine and Other Stimulants

When you consume caffeine, either from your morning cup of coffee or from a caffeinated gel during a training session or race, your heart rate increases.


A race is an exciting event, and that causes an adrenal response that increases your heart rate. Other emotional responses, including frustration, anger, and anxiety, can also affect heart rate.

Hydration Status

Although heart rate changes due to hydration status are often with or concurrent with impacts from core temperature, your heart rate can increase from dehydration with or without a rise in core temperature. As your blood volume diminishes, your heart needs to beat faster to deliver the same amount of oxygen per minute.


Most athletes train within a small range of elevations in their local area, but goal races may feature dramatically different elevation profiles. Your heart rate response to any level of exertion will be different when you compete at between 10,000 and 12,600 feet during the Leadville Trail 100. Heart rate and respiration rate increase at elevation, starting at about 5,000 feet above sea level, because the reduced partial pressure of oxygen in the air you’re breathing means there are fewer oxygen molecules in each lungful of air.

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While many of the factors that impact heart rate act to increase it, fatigue often suppresses it. When you are fatigued, your heart rate response to increasing energy demand is slower and blunted. A tired athlete will see heart rate climb more slowly at the beginning of an interval or hard effort and will struggle to achieve the heart rate normally associated with a given intensity level.

Why Ultrarunners Should Embrace Perceived Exertion

As much as I embrace the role of technology in enhancing the precision of training, there’s an incredibly simple measure of workload that continues to hold its own against new gadgets and software applications. Rating of perceived exertion (RPE) is the ultimate in simplicity: It is nothing more than a scale of how hard you feel you are exercising. There’s not one single piece of data collected, and you don’t need any special equipment. All you need is a numerical scale.

In the lab

In the physiology lab, I use the Borg Scale, which ranges from 6 to 20 (with 6 being no exertion at all and 20 being a maximum effort). Why 6 to 20? Borg’s research has shown that there’s a high correlation between the number an athlete chooses during exercise, multiplied by 10, and his or her actual heart rate at that time. In other words, if you’re on a treadmill during a lactate threshold test and tell me that you feel like you’re at 16, there is a pretty good chance your heart rate is around 160 bpm. This isn’t absolutely true of all athletes, but you’d be surprised at how accurate the 6 to 20 scale tends to be.

In the real world

Outside the lab, however, the Borg Scale isn’t as helpful for athletes, most of whom find it easier to relate to a simpler 1 to 10 scale (with 1 being no exertion at all and 10 being a maximum effort). Using this scale, an endurance or “forever” pace would be a 5 or 6, a challenging aerobic pace would be a 7, lactate threshold work occurs at about 8 or 9 (lactate threshold intervals on climbs are a solid 9), and VO2 intervals are the only efforts that reach 10. Just as the Borg Scale multiplies the perceived exertion number by 10 to correlate with heart rate, the number chosen on the 1 to 10 scale, multiplied by 10, seems to correlate closely to the percentage of VO2max that an athlete is currently maintaining.

You can learn more about Jason Koop’s Training Essentials for Ultrarunning here. 

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Comments 17

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  5. I wonder – much of what I’ve been reading about HR training has to do with using your HR monitor to keep your easy runs easy. Many of us have the tendency to hang out in moderate intensity too much, and the HR monitor can help you avoid that. For hard workouts, I know what it feels like and don’t need a watch to tell me, but it’s very easy for me to fall into an “easy” pace and actually be going too hard – thus not reaping benefits of recovery runs and getting burned out. Thoughts?

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  7. Thank you! This article answered some very specific questions I have been wondering about for a very long time. Jonathan Gaus

  8. I am not sure why heart rate is demonized so much. Sure, it’s not a perfect tool to train but is there a perfect tool ? No. GPS watches are not accurate and still a lot of people benefit greatly from them. Even RPE is not perfect as it can be subjective at times. What about the Stryd Power meter that Koops gives to some of the athletes he coaches, is it perfect ? No, but he still uses it.
    In a nutshell, yes, heart rate is not a perfect tool for training but it can help a lot.

  9. So question, I moved to Colorado a year and a half ago, and since I have been here I cannot get my heart rate up, it seems that the limit is my breathing. However, I recently did Austin Rattler, and my heart rate was able to get much higher. I lived in Memphis before moving here. I am 61 and have been working with a coach for a number of years. He is puzzled as well. So my question is, why isn’t my heart rate higher. BTW I regularly ride with a power meter and have noticed about a 10% reduction in power as well.

    1. Hi Steve,

      This is a well know phenomenon. A person is unable to stress the anaerobic system at altitude. Training at altitude is great for increasing aerobic capacity, but is not optimal for anaerobic threshold training.

  10. Being from the old school of heart rate was the best meter of response to your bodies effort, it is good to see how that has changed over the years. Like many things in our daily life’s that have changed, your own perception is the better tool than technology. But I am so use to my gadget!!!

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  12. Surprise…millennia of evolution have equipped us with myriad precision sensors funneled into complex integrative networks and dynamic arrays that are collectively better at quantifying overall work and stress than a single variable measurement. Huh. 😉

  13. Thank you for explaining the Borg scale! I never understood why “6 to 20”. And I agree – while not an ultra runner or ultra cyclist for that matter, RPE pretty much gives you the information you need and is very responsive to all of the variables that affect performance.

  14. I completely agree with this article. As a coach, I use a perceived exertion scale of 7 zones quite similar to your scale. The scale is based on physiological responses to each training zone, how they feel and how long you can hold them for. I coach multi sport athletes from sprint to Ultraman and this method works for all of them! I have used and tested heart rates, blood lactates and power as metrics but when it comes down to race day my athletes always go back to perceived exertion.

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