Smart run training relies on an athlete’s ability to gauge exercise intensity. Runs will have a designated intensity zone to achieve a certain physiological response: easy runs, moderate runs, threshold runs, interval runs, and more (you can read a detailed overview on the different types of runs here). Heart rate zones have become a popular method for determining training intensity. This article will explore how to use heart rate training, including understanding the nuances and limitations of its application.
Is heart rate training essential? No. Many runners can train effectively without using heart rate zones. For some runners, heart rate training is helpful in calibrating training intensity. It is up to you (and your coach if you work with one) to determine the appropriateness of using heart rate zones in your individual training.
The Science of Heart Rate Training
To understand heart rate training, let’s look at what your heart does during exercise. When you run, your body needs to send oxygen to your muscles to produce energy for muscle contractions. Oxygen is delivered to your muscles through your blood. Your heart pumps out that oxygen-rich blood.
The amount of blood pumped from your heart per minute is called cardiac output. Cardiac output is determined by stroke volume (how much blood is pumped with each beat) and heart rate (number of heartbeats). Your heart rate reflects your workload since your body increases heart rate (and stroke volume) to meet the demands of exercise.
For runners, heart rate measurements offer a proxy measure of several internal physiological processes. Generally speaking, heart rate increases as oxygen consumption increases. Heart rate is a proxy for blood lactate levels, oxygen consumption rates, mechanical workload, and more.
However, it is not a perfect measure. In several scenarios (which we will explore), a runner’s heart rate could increase without significant changes in blood lactate levels. Additionally, heart rate is not a predictor of performance in the same way that velocity at VO2 max, lactate threshold, aerobic threshold, critical speed, and running economy are. While measuring heart rate can be helpful for understanding exercise intensity, it is not the
What Happens to Your Heart Rate When You Run?
At the start of exercise, a runner’s heart rate increases rapidly. This rapid increase in heart rate is triggered by your sympathetic nervous system. Sometimes, the nervous system can even start to increase heart rate in anticipation of exercise.
Corresponding to this, your body initially relies more on glycolysis (carbohydrate metabolism that can occur without oxygen). After the first few minutes of adjusting to the workload, heart rate stabilizes until intensity increases again (or cardiac drift occurs).
During longer runs, you may also experience an increase in heart rate independent of increases in pace and RPE. This phenomenon is called cardiac drift (or aerobic decoupling). Dehydration and elevated core temperature contribute to cardiac drift. Dehydration will cause a decrease in stroke volume, which means that heart rate has to increase to maintain the same workload. (You can read more on the science of cardiac drift in this article.)
Understanding Heart Rate Zones
Heart rate training zones establish a correlation between a certain heart rate range (usually a percentage) with training intensity. A common system used is the five-zone heart rate training system. (You can learn more about the five-zone system including zone 2 training in this article.) The Polar system for zones is as follows:
- Zone 1: 50-60% MHR
- Z2: 61-70% MHR
- Z3: 71-80% MHR
- Z4: 81-93% MHR
- Z5: 94-100% MHR
Some other models of the five-zone system exist, such as this model defined in a 2014 study in the International Journal of Sports Physiology and Performance. Exercise physiologist Dr. Stephen Seiler describes the science behind this five-zone model in this Youtube video.
- Zone 1: 55-72% MHR
- Z2: 72-82% MHR
- Z3: 82-87% MHR
- Z4: 87-92% MHR
- Z5: 93-100% MHR
Seiler’s model delineates the zones based both on physiological markers and practical considerations. This model makes a bit more sense than the Polar model (physiology doesn’t exactly happen in clean 10% margins), especially for trained athletes.
As you will read more about a bit further down, you can also calculate a five-zone model based off of lactate threshold heart rate.
It is vital to understand that physiology happens on spectrums, not sharp zones. A 2 bpm change in heart rate may shift you into zone 4, but it does not automatically mean that you are at your lactate threshold.
It is important to remember that not every runner’s heart rate zones are the same! Using heart rate training appropriately requires individual calibration. You cannot use someone else’s data from Strava or Instagram. Instead, you need to calculate your own heart rate zones based on one of the following models.
Maximum Heart Rate Model
The most common approach for calculating a runner’s heart rate zones is by using a maximum heart rate model. Maximum heart rate can be calculated using an age-predicted formula; however, this formula has a significant standard deviation (+/-10-12 bpm within one SD). That wide standard deviation does not make it a reliable predictor of heart rate if you are serious about heart rate training.
Instead, you can calculate your individual maximum heart rate. There are multiple ways to determine maximum heart rate (MHR). If you have been using a chest strap, you can review your data. The highest number you consistently hit in interval workouts or near the end of short races (such as a 5K) can be used as your maximum heart rate.
You can also perform a field test to determine maximum heart rate. Field testing will not be fun, but it will give you a reliable number. You will want to use a chest strap for this. Dr. Jack Daniels recommends a field test of repeated 2-min uphill runs or 800-m repeats on the track. Alternatively, you could run a mile time trial and observe your highest heart rate.
Lactate Threshold Heart Rate Model
The lactate threshold heart rate may be more sensitive to the physiology of a trained endurance athlete. As the name implies, you calculate your heart rate zones off of your heart rate at lactate threshold. This article guides you through how to perform a field test for lactate threshold heart rate (LTHR). Once you have determined your LTHR, you calculate your zones using the following model:
- Zone 1: Less than 80% of LTHR
- Z2: 80–88% of LTHR
- Z3: 89–95% of LTHR
- Z4: 96–99% of LTHR
- Z5: 101%+ of LTHR
Heart Rate Reserve Model
Heart rate reserve uses the difference between an individual’s maximum heart rate and resting heart rate to determine functional capacity for work. As with the maximum heart rate model, you will want to use your individual maximum heart rate over an age-predicted formula.
The Karvonen method determines your heart rate reserve (HRR) and corresponding heart rate zones. This method involves a bit more math than other formulas:
- To determine your HRR: max heart rate minus resting heart rate
- Sample: 185 max heart rate minus 45 resting heart rate = 140 HRR
- To determine heart rate zones: (multiple HRR by the intensity) plus resting heart rate
- Sample: 70% of HRR = (140 HRR x 0.70) + 45 = 143 bpm
Heart rate reserve zones are meant to correspond approximately with a percentage of VO2max. As a result, the percentage of HRR will differ from the percentage of max heart rate.
What Factors Can Affect Heart Rate?
The biggest limitation of heart rate training is that additional factors beyond oxygen consumption rates can influence heart rate. If you are wondering why heart rate is high on easy runs, one of these factors may be at play.
Your sinoatrial node in the heart generates electrical impulses that affect your heart rate. The autonomic nervous system (both parasympathetic and sympathetic) regulates the sinoatrial node, and therefore also affects heart rate. The parasympathetic nervous system inhibits the sinoatrial node, while the sympathetic nervous system stimulates the sinoatrial node. Both are simultaneously active in a normal healthy person, which means your SA node (and therefore your heart rate) has both stimulatory and inhibitory influences acting upon it simultaneously.
The nervous system activity affects heart rate by releasing certain catecholamines (neurohormones) such as norepinephrine. Norepinephrine signals an increase in heart rate, both in response to exercise and to other factors including emotions. (Think of your heart rate may spike high before a big presentation.) (Norepinephrine also increases blood glucose levels to support the demands of exercise.)
- Digestion: Energy is required to digest food (a process known as dietary-induced thermogenesis). If you recently ate or are ingesting food on the run, your heart rate will respond accordingly. (Fasted training may keep your heart rate lower, but it is not recommended due to its effects of the endocrine system and no clear correlation to improved performance.)
- Body temperature: Increases in core temperature will elevate heart rate. Conversely, you will also see your heart rate drop when your core temperature is lower. (That said, do not deliberately underdress and risk hypothermia to make your heart rate lower.)
- Ambient temperature: The hotter it is outside, the higher your heart rate since your body works harder to control core temperature. On the other extreme, peripheral vasoconstriction and a slight decline in VO2max in sub-freezing temperatures can also elevate heart rate.
- Humidity: Humidity environments make sweating a less effective cooling mechanism. As a result, heart rate increases as the body regulates core temperature via radiation.
- Dehydration: When you get dehydrated, your blood thickens and stroke volume decreases. Chronic and acute dehydration can both cause heart rate to increase in order to maintain cardiac output with lower stroke volume.
- Certain medications: Medications such as beta blockers can alter how heart rate rises in response to exercise. If you are on beta blockers or another medication that affects heart rate, you will not want to rely on heart rate training.
- Caffeine consumption: Caffeine is a central nervous system stimulant. As a result, it can increase heart rate at resting and exercise levels. Individual response to caffeine may affect just how much caffeine affects heart rate.
- Anxiety/stress: The catecholamines that regulate heart rate are altered by anxiety or stress. Norepinephrine is also responsible for stimulating your brain in response to fear or arousal. So when you are nervous, stressed, anxious, or even excited, your heart rate can go up.
Will My Heart Rate Change with Training?
Runners’ heart rates will typically change as they adapt to training. Let’s look at the science:
- Cardiac output is how much blood your heart pumps in one minute. Heart rate (beats per minute) and stroke volume (volume of blood pumped from the left ventricle with each contraction
- As you become more aerobically fit, your stroke volume will increase. As a result, more cardiac output can occur at the same or lower heart rate.
- The nervous system becomes trained to increase stroke volume and regulate heart rate during exercise.
- You can run at the same effort with lower heart rate because your stroke volume is higher. (Factors like the improved running economy will also affect your heart rate at a given pace.)
Understanding the Types of Heart Rate Monitors
There are two popular types of heart rate monitors: chest straps and wrist-based. Chest straps rely on sensors to measure heartbeat via electrical activity. According to the American College of Cardiology and supported by many research studies, chest strap heart rate monitors are typically more accurate.
Wrist-based heart rate monitors are rising in popularity and are included on many GPS watches now. However, wrist-based heart rate monitors are less accurate, with an error margin of anywhere from 1% (2018, Digital Health) to 13.5% (2019, Journal of Sports Sciences), depending on the brand. Wrist-based heart rate monitors utilize a bright LED light. This light is refracted off of blood flow beneath the surface of the skin. The watch then calculates the data through an algorithm to measure heart rate. Factors such as humidity, sweat on your skin, the tightness or looseness of the watch strap, and any built-up grime on the sensor can affect accuracy. Athletes with wrist tattoos or darker skin may experience less accurate data from their wrist-based sensors.
Should You Use Heart Rate Training as a Beginner?
Different coaches may express varying opinions on heart rate training for beginner runners. Personally, I do not recommend it if you are a new (<6-12 months) runner. Why?
Your physiology changes in response to running. The responses to endurance training are both acute (more immediate) and chronic (long-term). Within the first six to twelve months of being a runner (or returning to running after a long hiatus), your body undergoes significant chronic adaptations. One of these adaptations is changes to your stroke volume, as noted above. Your stroke volume improves with long-term training. This adaptation both increases overall cardiac output and allows you to maintain a lower heart rate at any given submaximal intensity.
Beginner runners do not have that increased stroke volume. As a result, their heart rate will be higher during exercise. You may find that your heart rate is higher even at a sustainable effort.
If heart rate training helps you as a new runner, use it. But if it creates obstacles for you or makes running more difficult, do not bother with it. In the first few months of running, focus on creating the habit and adapting your musculoskeletal system and cardiovascular system. Do not push every run, but also do not worry about numbers on your heart rate monitor (unless medically instructed).
Can You Use Heart Rate Training for Hard Workouts?
Heart rate monitoring can be used on tempo runs, since those are prolonged and continuous efforts. However, there are downsides. Worrying too much about your heart rate can hinder your ability to gauge your perceived effort. If you are using a less accurate heart rate monitor such as a wrist-based monitor (see below), you could end up training too hard – or not hard enough. Heart rate zones for your aerobic threshold, lactate threshsold, VO2max, and other hard workout zones are narrower than your easy pace zone, meaning a margin of error is heart rate reading is more likely to affect your workout.
Heart rate monitoring becomes even more complicated and less practical on interval runs. Your heart rate doesn’t instantaneously jump up when you start running faster. Cardiac lag occurs, meaning that you will be running hard for a good portion of time before your heart rate monitor reflects it. This is especially true for short intervals; your heart rate will not register the appropriate reading for a majority of the interval.
Should You Use Heart Rate for Racing?
Anxiety and excitement around racing can make your heart rate spike (even if the effort does not feel that high). A higher heart rate during racing is common. I encourage runners not to look at their heart rate during racing and rather focus on their perceived exertion and ability to push themselves for that day.
Is Heart Rate Training Right for You?
Heart rate training can be an effective tool for gauging exercise intensity. However, like any tool, it requires calibration and an understanding of its shortcomings.
If you have trouble slowing down on your easy runs enough, you may benefit from heart rate training. Opt for a chest-strap monitor if possible and focus on your breathing and effort as well. As with paces, you do not want to over-rely on data during a run. Any piece of technology will have a margin of error.
Do not use your heart rate monitor to cast judgment on your fitness. Use it as a tool to aid in the calibration of training intensity if it makes sense for you. However, even if you use heart rate training, you do not need to use it all the time. If heart rate training is not for you, you can try using the Rate of Perceived Exertion Scale or talk test.