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Are You Scared? The Science of Running on Adrenaline

The same process propelling the heroine away from a monster in your favorite scary movie also works to get you across the finish line.

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When a murder of crows join in the apocalyptic uprising of their brethren, the only sane thing to do is run. The same goes if you find yourself in a dark, empty library and a headless man appears. And sure, these things would never really happen outside of the minds of Alfred Hitchcock or Stephen King, but in the world of fiction, they mastered the art of the racing heart.  

Even as a simple spectator to those chase scenes, your muscles likely tense up with the biological urge to run the opposite direction. That’s because what happens to your body while watching a scary movie isn’t all that different from what happens when you go for a casual run. And even if you’re not terrified while doing so, there’s still a bit of that fight or flight mechanism in exercise. Adrenaline, also known as epinephrine, is one of the hormones that mobilizes us. 

So what exactly is adrenaline? Scientists have known about adrenaline for a long time, since around the end of the 19th century. “[epinephrine] and its partner noradrenaline—norepinephrine—are two messengers/hormones that rise in circulation during any substantial stress, be it physical or psychological,” says Katarina Borer, professor emerita in the School of Kinesiology at the University of Michigan. Adrenaline is produced in the medulla in the adrenal glands and is released into the bloodstream to elicit responses from parts of the body when necessary. 

How Adrenaline Helps You Run

According to the Endocrine Society, “adrenaline causes a noticeable increase in strength and performance, as well as heightened awareness, in stressful times.” 

For runners, adrenaline is a nifty little hormone that does a lot. “This molecule helps increase force of contraction of the heart muscle, increase blood flow to working muscle—to a point—and reduce blood flow to non-working muscle and it activates the mobilization of substrate molecules to be used by those tissues,” says Allan Goldfarb, kinesiology professor at the University of North Carolina Greensboro. He adds that epinephrine also works with norepinephrine to “provide greater blood flow and control blood pressure.”

In general, it makes your body more efficient. A recent Harvard Medical School study, for example, found that epinephrine exposure, among other hormones, increased proteasome activity, a marker that the body is breaking down and eliminating ‘misfolded’ proteins that can cause disease if they accumulate.

Adrenaline is released when you increase the intensity of the run, both in speed and time spent running. “This is why your heart rate at a certain workload is elevated more at the end of the exercise than at the same workload at the beginning,” says Goldfarb. Both hormones are also activated from caffeine, which is why coffee has been shown to enhance running performance when consumed pre-exercise.

Here’s how it helps in different body systems:

Adrenaline and the heart.

To begin, adrenaline increases heart rate as well as the force of contraction of the heart. Norepinephrine and epinephrine both cause this reaction. Every time your heart contracts it’s pushing out blood, so with more contractions and more forceful contractions you’re getting better blood flow. And remember, the heart is a muscle, so these increased contractions also push it to become stronger and more efficient. 

Adrenaline and the blood vessels.

“This is where norepinephrine and epinephrine differ,” says Goldfarb. Epinephrine acts as a vasodilator, smoothing muscles to dilate blood vessels, while norepinephrine constricts. This dilation allows your blood to flow more freely and to move to areas where it’s more needed during activity. 

Epinephrine also triggers a change in your non-working muscles by restricting them from taking up energy sources that you need to keep moving. That leaves more glucose for your working muscles. 

Adrenaline helps you breathe better.

You’ve surely noticed that you breathe harder when running than you do at rest. Adrenaline is a player in that function. When activated, it causes your air passages to dilate, similar to dilating blood vessels, so you can take in more oxygen. This extra oxygen is needed for your muscles to get you where you need to go. 

It helps you preserve fluids.

Epinephrine and norepinephrine also help balance necessary fluids during exercise. “Both also stimulate the immune response and body fluid preservation,” says Borer. This is through reducing urine production to make up for fluid loss through sweat. 

The higher intensity of the workout or the longer the workout, the more epinephrine will be released. So again, this is why you’d find your heart rate higher at minute 30 of your run than at minute five, even if you’ve kept your pace and effort the same. “It would drift up because of the chemical,” says Goldfarb. 

“It basically amplifies the body’s homeostatic functions in preserving energy, water balance, allows some overheating due to increased metabolism, activates immune response against potential injury,” says Borer. 

The more fit you get as a runner, the less adrenaline is released, which subsequently saves you even more energy. “That’s what runners get when they get more efficient,” says Goldfarb. “They don’t have to get their heart rates as elevated. Every time your heart constricts it’s using energy.” Internally, you become more efficient when your body relies on adrenaline less. 

Exhausted female athlete taking a break after exercising in foggy forest.
Photo: Getty Images/skynesher

Adrenaline’s Role in Fear

It is generally assumed that the adrenal response is the same no matter the stimuli. The difference is whether or not you’re able to act on what is causing your adrenaline to rush. Adrenaline is directly associated with the fight or flight response. The idea is that when something threatens and scares you, the physiological effects of epinephrine and norepinephrine prepare you to either fight or run from the threat. 

But what happens if you get superficially scared and don’t physically act on it, say, when you’re watching a horror movie? “What happens to your heart rate? It goes up. What happens to your blood pressure? It goes up,” says Goldfarb. “But you’re not going anywhere.” If you were to run or walk away, you’d release the chemicals to vasodilate and lower blood pressure. Not doing so can leave you feeling angry, irritable, antsy, or tense with that pent up energy.

Holding on to that tension is likely to result in slight hypertension, putting strain on your heart. “It doesn’t mean you’re going to have a heart attack,” says Goldfarb. Eventually your blood pressure will return to normal. But in combination with weak or clogged vessels, it’s not the best recipe for heart health. Goldfarb speculates that this is probably why young people are more inclined to watch scary movies, “They think it’s cool and they can handle it.”  

But in the role of exercise, adrenaline has mostly a positive effect on the body that we can celebrate. “Runners should appreciate the fact that epinephrine and norepinephrine will take care of their energy production, thermoregulation, and respiratory needs without their having to worry about it,” says Borer.