Unless that flip-flop feeling is accompanied by chest pain, dizziness, or fainting, an occasional heart palpitation is probably nothing to worry about—especially in young, healthy athletes and frequent exercisers. It can usually be attributed to something called a premature ventricular contraction (PVC) which is normal, says Dr. Jeff Brinker.
"Almost everybody has, at one point or another during the day, a PVC," says Brinker, a professor of medicine and cardiology at Johns Hopkins University. "Sometimes you don't notice them at all, but sometimes you do get that sensation of your heart skipping a beat."
A normal heartbeat works like this: The atria pump blood into the ventricles, which then contract, sending blood to the lungs and the rest of the body. A PVC occurs when the ventricles contract before they've been filled with blood.
We don't actually feel that "empty" contraction, however. Instead, we usually feel the stronger beat that comes after it. "There's increased blood and increased pressure in that next contraction, so we tend to notice it as a hard beat or a pounding," says Brinker.
PVCs can be caused by a variety of issues, including old age, medication, and rarely, by a serious heart condition. But in young adults, they're almost always innocent, says Brinker. They can be triggered by something as simple as caffeine or alcohol consumption, lack of sleep, or a stressful situation that causes your body to release its own adrenaline.
Exercise is one such stressful situation. While irregular beats tend to be suppressed at higher heart rates, they can and do occur at the beginning or end of exercise when adrenaline levels are high—especially if other PVC triggers are also at play.
Caffeine is one example. "I used to experience PVCs when I would drink coffee in the morning and then jog," says Brinker. "When I stopped drinking coffee first, I stopped noticing them." Soda and energy drinks can be culprits, too.
Palpitations that seem to get worse with exercise, on the other hand, may be a sign that the heart is under significant stress, says Dr. John Mandrola, a cardiac electrophysiologist and competitive cyclist. This stress could be caused by something serious, like a blockage, or something relatively simple, like overtraining.
It's not always easy to tell the difference between what's normal and what's not, but there are a few key indicators you should pay attention to. Your heart rate will fluctuate while you exercise, but you shouldn't notice very drastic or sudden changes. "It should go up on a curve, increasing slowly and then decreasing slowly," says Dr. Noah Rosenthal, chief of cardiology at University Hospitals in Cleveland. "A sign of something abnormal would be the sudden onset of palpitations, like a light switch, that's not associated with your level of exercise exertion."
An irregular beat associated with chest pain, dizziness, fainting, or trouble catching your breath should also be checked out by a doctor, since these can be signs of something serious. Atrial fibrillation, for example, is a common cause of irregular heartbeats and can lead to stroke or blood clots.
But "A Fib" is much more likely to occur in people over 65, or in those with other risk factors for stroke, like diabetes, high blood pressure, or known coronary disease, says Rosenthal. In young, otherwise healthy people, palpitations are almost always harmless or are caused by less common conditions. And people who do have actual heart rhythm disorder are "generally very symptomatic," says Rosenthal. "They really do have a feeling that something is wrong."
Bottom line: If you're worried about your irregular heart beat—or if it's accompanied by other concerning symptoms like dizziness and shortness of breath—talk to your doctor. He or she can perform an EKG or monitor your heart beat for a few days to learn what's affecting it.
If you're just curious about an infrequent strange feeling, try experimenting with your caffeine intake, sleep and stress levels, or your training intensity, to see if any of these factors make a difference.