That heart-pumping, give-it-your-all feeling is one of the reasons many of us run in the first place. And a ramped-up heart rate during any type of exercise is not only normal, it’s necessary. But when is a high heart rate, especially on an easy run or jog, cause for concern?

As you increase your effort level from a walk to a jog and beyond, your muscles require more oxygen to produce energy. To get it there, your heart needs to increase your cardiac output—the number of liters per minute of oxygen-rich blood it pushes through your arteries, says Dr. Elaine Wan, Esther Aboodi associate professor of medicine in cardiology and cardiac electrophysiology at Columbia University Medical Center, College of Physicians and Surgeons and attending physician at New York-Presbyterian Hospital.

That figure is the product of your heart rate and one other factor: your stroke volume, or the amount of blood pushed out with each pulse. Regular training can boost your stroke volume over time, but in the moment, the only way for your heart to meet increased demands is to pick up the pace.

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Regular runners tend to have lower heart rates at rest and at every level of physical activity, from light to moderate to intense, says Dr. Ruwanthi Titano, a cardiologist with Mount Sinai Health System. In fact, runners often cruise through the first few stages of stress tests—cardiology exams that require increasingly hard efforts on the treadmill.

But sometimes when you’re trying to run easy, your heart rate may feel like it’s soaring out of control—or at least, out of proportion to your pace. Why does this happen, when it is a problem, and what can runners do about it?

In most cases—especially if you’re active and healthy overall—it’s probably nothing to worry about, says Dr. Eli Friedman, medical director of sports cardiology at Miami Cardiac & Vascular Institute. That said, there are a few red flags that should prompt you to seek treatment for a fast heartbeat, including when it comes with other symptoms such as shortness of breath, chest discomfort, irregular heartbeats, nausea, lightheadedness, or passing out.

Understanding Your Baseline

Thanks to the ubiquity of smart watches and other tracking devices, it’s easy to obsess over your heart rate—and even compare it to your running partners’. Resist the urge: “It’s very genetically driven,” Dr. Friedman says. “What one person’s heart rate is on a run can be completely different from somebody else’s.”

Besides genes, a few other factors play a role. On average, women’s hearts are smaller, and have to beat faster to supply oxygen and other nutrients to their bodies (though training can certainly begin to equalize this). Your height and amount of muscle mass also play a role, as can your breathing patterns.

Another reason Dr. Friedman cautions runners against obsessing over their heart rate: wrist-based heart rate monitors aren’t 100-percent accurate, especially if you have darker skin. The more you’re moving, the less reliable the data. So there’s a chance a high reading may not even be accurate in the first place.

If you’re wearing a chest strap, which directly reads your heart’s electrical rhythms, your numbers are more accurate. But that still doesn’t mean you should freak out if they creep up.

Reasons Your Heart Rate is High When Running

There are plenty of reasons your heart rate is elevated on a given run, and many are harmless and easily reversible.

Your run isn’t so “easy” after all.

When you’re first starting to run (or coming back after a break) any increase in intensity stimulates what’s called your sympathetic nervous system. That’s the branch responsible for the “fight or flight” reaction, the adrenaline surge that bumps up your blood pressure, heart rate, and breathing rate, among other changes, Dr. Titano says.

Over time, your body begins to adapt to the strain of training, decreasing the burden on your cardiovascular system. Stronger muscles do a better job of extracting oxygen from your blood, so you don’t need as high of a heart rate to fuel you, Dr. Wan says.

Your heart grows stronger, too, meaning it can push out more blood with every pump. And, you’ll improve what’s called your vagal tone, your ability to activate the parasympathetic nervous system, which calms your body down from stress.

Add it all up, and after about two weeks of consistent running, you’ll likely notice a difference in your heart rate, Dr. Titano says. Not only will your resting heart rate decline, you’ll see less of a spike at each new intensity, from an easy jog to an all-out interval. You’ll also find your heart rate comes back down more quickly after hard efforts.

Until then, slowing down can bring your heart rate back under control. Instead of targeting a specific pace you think “should” feel easy, go by perceived effort instead, suggests Amy Morris, an RRCA-certified running coach and director of personal training at CrossTown Fitness in Chicago.

Use the “talk test:” During a true easy run, you should be able to have a full conversation with a friend. About 80 percent of your training should stay in this range, with only about 20 percent harder or faster, Morris says.

You’re sleep-deprived.

Adequate rest is essential to reaping the benefits of training over time, and poor slumber can influence your heart rate the next day, Dr. Titano says.

Sleep deprivation interferes with your normal circadian rhythms, altering the amount of hormones like cortisol and adrenaline coursing through your veins. This makes your heart more sensitive to changes in intensity, causing more of a rise on the run than normal.

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You’re dehydrated.

When you’re low on fluids, the total volume of blood in your body decreases. Because your heart delivers less with each pump, it has to speed up to get the same number of liters to your muscles.

Electrolyte levels, too, play a role, Dr. Titano says. Minerals like sodium and potassium are essential for the proper function of the heart. When you’re parched, the pH level of your blood also changes, which trips up the wires of your heart’s electrical system in ways that can trigger faster heartbeats.

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You’re anemic.

Iron plays a key role in your body’s ability to carry oxygen from your lungs through the rest of your body. So when you’re low on this vital mineral—as many women runners are—your heart must work harder to compensate, says Dr. Raffaele Corbisiero, chair of electrophysiology at Deborah Heart and Lung Center in Browns Mills, New Jersey (and a marathon runner himself). Your doctor or a dietitian can diagnose anemia by testing your iron and ferritin levels, then recommend iron-rich foods or supplements to address it.

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You’re pregnant.

Nourishing two bodies, your baby’s and your own, requires more blood, and more effort to push it through your cardiovascular system, Dr. Wan says. What’s more, your hormones shift in ways that activate your parasympathetic nervous system.

Most pregnant women’s resting heart rate rises 20 to 25 percent throughout pregnancy, and heart rate when you’re running or otherwise exercising increases as well. That’s one reason the old advice to keep your heart rate under 140 doesn’t make sense—in fact, it wasn’t based on any solid evidence in the first place. Recently, researchers have developed new recommendations that can help keep current and future runners safe.

RELATED: Your Guide to Pregnant and Postpartum Running

Your thyroid is working overtime.

This butterfly-shaped gland in your neck releases hormones that govern your heart rate, as well as your blood pressure and cholesterol level. So when it’s not working properly, you may notice changes in your rhythm.

An overactive thyroid—hyperthyroidism—can cause an increase in heart rate, Dr. Corbisiero says. So could a modification of medications to treat any type of thyroid condition.

Hyperthyroidism can lead to arrhythmias and other heart issues, as well as thinning bones, eye disease, and problems with fertility or pregnancy. So, check in if you have a fast heart rate and other symptoms, including muscle weakness, trouble with heat tolerance, more frequent bowel movements, and tremors.

You’re getting back into it after being sick with COVID-19.

Dr. Titano and her colleagues have seen patients recovering from COVID-19. Many, including runners, have noticed their heart rate rising dramatically even just for everyday activities, like climbing the stairs or walking down the block. Some also develop a condition known as postural orthostatic tachycardia syndrome, or POTS, where their heart races and they may even pass out upon standing up.

In most cases, these don’t appear to be signs of permanent heart problems, she says. But if you have them, your doctor may run diagnostic tests to be sure, then put you on a protocol of hydration, electrolyte replenishment, compression stockings, and a conservative progression from exercises done lying down to more intense activities.

“It’s a pretty slow, steady process that takes a few months, depending on how long you’ve been sick,” she says. “But we’ve had very, very good success with this sort of graduated exercise program.”

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You’re taking certain medications.

While some drugs, such as beta blockers prescribed for other heart conditions, can slow your heart, others speed it up. Medicines like asthma inhalers, antidepressants, and treatments for attention deficit hyperactivity disorder can boost your beats per minute.

You’ve consumed caffeine.

There’s a reason that morning java gives you a jolt: caffeine is a stimulant, which means it increases adrenaline levels and your body’s sympathetic nervous system response. Between coffee, tea, pre-workout supplements, and gels, it’s easy to consume more than you realize, Dr. Titano says.

In most cases, this isn’t a problem. However, dosages of more than 400 milligrams per day might increase side effects like headache, anxiety, and heart palpitations (and overdoses can be dangerous). Some people are more sensitive and experience these with less caffeine, so monitoring your intake and how it makes you feel is a good idea for runners.

RELATED: Jitters and Java: Your Genes May Affect Your Response to Caffeine

You’re overloaded.

Your mental health isn’t truly separate from your physical. Stress or emotional pressure from other parts of your life also boosts cortisol and adrenaline, potentially increasing your heart rate, Dr. Corbisiero says. Overtraining, too, can elevate both your resting heart rate and your heart rate upon exertion.

You have an arrhythmia.

“If you think of the heart as a house, over time, you can have plumbing issues as well as electrical issues,” Dr. Titano says. As an electrophysiologist, her specialty lies in diagnosing and fixing faulty wiring, which usually appears in the form of abnormal heart rhythms or arrhythmia.

Some are harmless, but others (such as atrial fibrillation or supraventricular tachycardia) are dangerous, Dr. Corbisiero says. Your risk increases with age, or if you have other conditions like high blood pressure, diabetes, vascular diseases, or sleep apnea. But, they can occur in younger people without other illnesses, too. That’s why it’s a good idea to watch for other heart-related symptoms and talk to your doctor if you have concerns.

When to Seek Medical Help for a High Heart Rate

If your heart rate rises unexpectedly on an easy run, you can start by going through this list of reasons and eliminating any obvious explanations. If you’re sleeping well, hydrating, low-stress, and haven’t had gaps in your training, keep a close eye out for any other symptoms, including:

• Dizziness
• Rapid heart rate at rest, or during other low-intensity motions, such as changing position from sitting to standing
• Nausea or vomiting
• Feeling like your heart skips beats or beats irregularly
• Changes in your bowel habits
• Unexplained weight loss

If you notice these or other abnormal signs—or if your heart rate continues to rise, or stays at a high level over a prolonged period of time—it’s a good idea to check in with your doctor, Dr. Titano says.

Unfortunately, some health care providers can minimize women’s symptoms, especially if you’re young and seem otherwise healthy. So if you truly have the sense that something’s awry, be your own advocate in seeking answers, Dr. Friedman urges. (It also may help to seek out a physician who has experience treating athletes.)

“Far too often we find that women are told they’re depressed, they’re anxious, and all of these different things, when they actually have real heart disease. And if they’re athletes, they’re far more likely to be told they’re fine,” Dr. Friedman says.

“Be persistent—the same determination you would have in getting through the training for a long run for marathon prep, should be the same thing that a woman comes into the office with if they’re being given answers that don’t make them happy or comfortable.”

One other way to minimize heart risks on the run, for you and your training partners: learn CPR and how to use a defibrillator. The Red Cross offers in-person and online classes, and your running group may even coordinate a training.

“Knowing how to recognize cardiac arrest and perform CPR will undoubtedly save lives,” Dr. Friedman says. “You could be in a run club and all of a sudden somebody goes down. Knowing how to perform those life-saving techniques is invaluable.”