Zone two training is having a moment. Many runners use heart rate monitors to ensure they stay at an easy effort on their runs. However, heart rate training can prove trickier than it seems. A question I hear from many runners is “why is my heart rate high on easy runs?”
This article delves into potential causes of high heart rates on run. However, if you ever have concerns about your heart rate or symptoms such as chest pain, it is best to speak with a medical professional.
Additionally, the assumption is that you are using a chest strap monitor or band on your upper arm. Wrist-based heart rate monitors are more prone to errors, including locking onto your cadence and producing an artificially high heart rate reading. (You can learn more about heart rate monitors and heart rate training in this article.)
You are Running Too Fast on Your Easy Runs
Many runners simply run too fast on their easy run days. When many runners start to run, they start at a moderate intensity. That moderate pace is fine if you are running a couple of days per week for general fitness. However, many runners evolve from training for general fitness to training for races – but they carry their pacing habits with them.
Learning to run truly easy on your easy days takes practice. A heart rate monitor can help you learn how to run in zone 2 – unless you constantly ignore what it says.
If you frequently wonder why is your heart rate high on easy runs, honestly assess your effort. Are you able to carry on a conversation? Do you feel a bit fatigued near the end of your easy runs? Are you running too close to your race pace? (Here’s how to know if you are running easy enough on easy runs.)
If in doubt, start by slowing down your easy days. While the culprit may be one of the other reasons listed below, slowing down on your easy days is a safe place to start. You can also use walk intervals to help keep heart rate low on easy runs as you build aerobic fitness.
You Never Calculated Your Actual Heart Rate
Your Garmin watch may be telling you that you never run in zone 2. Or, you may have followed a standard age-based formula, such as (220-your age)x0.7, to set your heart rate zones. If your heart rate is high on easy runs, but you have never set individualized zones, you may not actually know your appropriate zone 2 heart rate.
Age-based formulas are not universally reliable for runners, particularly given the precision required for heart rate training. There are two commonly used maximum heart rate formulas: the Fox equation (220-age) and the Tanaka equation (208-0.7 × age). Neither formula is ensured to provide your accurate maximum heart rate. You may get an accurate number, or you may be an outlier – and you never will know unless you test.
According to a 2018 study from top researchers in Frontiers in Physiology, both the Fox and Tanaka equations are not perfectly accurate for marathon runners. The researchers found that the average female marathoner had a maximum heart rate 5 beats per minute lower than predicted, while the average male marathoner’s was 3 beats per minute higher.
However, those averages do not represent all the outliers. Some athletes had maximum heart rates greater than 20 beats per minute higher or slower than the formulas predicted (SD=1.96). Again, you have no way of knowing if you are 3 beats per minute or 20 beats per minute faster or slower than the formula without testing your actual heart rate.
Instead of relying on age-based formulas, you can use a variety of field tests to measure your actual heart rate. You can perform either maximum heart rate tests or a lactate threshold heart rate test. The lactate threshold heart rate test involves a 30-minute time trial while wearing a chest strap. (You can read the full instructions of a threshold heart rate test here.)
Alternatively, you can run 800m repeats at maximum effort until your heart rate plateaus (more information in this article). Knowledge coach Amanda Brooks of Run to the Finish recommends using your heart rate at the end of a 5K race to set your maximum heart rate.
You Set Your Zones Wrong
Once you have your maximum heart rate, you still need to set your heart rate zones. However, you may notice that there are conflicting recommendations on what your easy zone heart rate should be. Polar, Garmin, and Maffetone all give different recommendations. So which one do you use?
For most runners, 75% of your maximum heart rate or 88% of your lactate threshold heart rate will provide appropriate upper limits to your easy zone. Remember that these are upper limits, meaning you will typically run below them. If your upper limit easy zone heart rate is 155 beats per minutes, that does not mean every easy run is right at 155 bpm. Rather, you want to stay under 155 bpm, whether that’s 140 or 150 bpm.
You are Dehydrated
Dehydration will result in a higher heart rate during exercise. When you become dehydrated, your blood volume decreases due to fluid loss. Decreased blood volume leads to reduced cardiac output (how much blood your heart pumps with each beat). To compensate for the reduced cardiac output, your heart rate increases.
Both acute and chronic dehydration can contribute to a high heart rate on easy runs. The difference is when the heart rate is high. If you are dehydrated when you start the run, your heart rate will be higher from the start.
It is possible to start out a run under-hydrated. Hydration is not simply a mid-run thing. Runners need more fluids throughout the entire day, and many struggle to stay hydrated. As a result, some runners start out a run with minor dehydration. As a result, heart rate is higher on easy runs from the start.
First, look at your daily hydration and pre-run hydration, especially if your heart rate is high on short easy runs. While you never want to chug excessive amounts of fluid, you may need to increase how much you drink before runs. Featherstone Nutrition has a helpful resource on hydration, including how much to drink before runs.
You Consumed a Large Amount of Caffeine
Caffeine is a well-established ergogenic aid for athletes. However, caffeine could interact with your heart rate on easy runs. Caffeine has multiple potential adverse effects, including increased heart rate and even heart palpitations. You may know how that feels if you have ever consumed too much coffee and feel like your heart is racing.
Some athletes are poor responders to caffeine and more likely to experience side effects such as increased heart rate. For these athletes, even typically tolerable doses of caffeine can cause higher heart rate.
Other athletes may experience higher heart rate in response to high caffeine intake. While some caffeine can improve performance, too much caffeine will increase the risk of side effects with no additional performance benefits. The recommended doses are 3-6 mg of caffeine per kilogram of body weight; more than that is likely to trigger reactions such as high heart rate.
Caffeine can be present in supplements such as pre-workout or running gels. Always check the label and choose third-party tested supplements to ensure you know exactly how much caffeine you are getting.
Your Iron Levels are Low
If your heart rate is high on easy runs no matter what you do, it may be worth getting your iron levels checked. Low ferritin and iron deficiency are common in endurance athletes. If you find yourself constantly wondering why is heart rate high on easy runs, and you also have symptoms such as fatigue, you could have low iron.
Iron is a mineral that is used in hemoglobin and myoglobin. Thus, iron plays a role in oxygen delivery and utilization. Low iron levels affect the oxygen-carrying capacity of the blood. As a result, your heart rate is higher as the body tries to get enough oxygen to the working muscles.
While you never want to supplement iron without getting your levels checked, you can order a simple bloodtest via Inside Tracker, Quest, or your PCP’s office to assess iron status.
For more information on iron for runners, listen to this episode of the Tread Lightly podcast.
You Have an Underlying Medical Condition
This cause of high heart rate on runs is less common, but it should not be ignored. In rare cases, high heart rate can be indicative of a medical condition such as an autoimmune illness. You may experience other symptoms, such as fatigue, that indicate something is not normal.
Never diagnose yourself based on high heart rate alone. Instead, if you are concerned, speak with your primary care physician about your concerns. They may order bloodwork or other tests.
It may take trial and error to determine why heart rate is high on easy runs. However, it is worth it, especially if you are committed to using heart rate training.