When the roller coaster at Coney Island first debuted in 1884, thrill seekers climbed aboard a ride that scaled a 15-foot hill and sped at four miles per hour.

Modern coasters can reach heights of 300 feet and speeds of 90 miles per hour. They drop riders suddenly or jerk them backward. Riders twist, fly upside down and return to the start within mere minutes.

Scientists are learning more about what happens to the body during roller coaster rides. The twists and turns are harmless for most people. But for a select few, the experience can be risky. 

Sudden Stops

Roller coaster harnesses are designed to keep riders in place and prevent them from falling off the ride. But the head and neck can still move freely, and the quick starts and sudden stops can cause excessive movement in the neck (hyperextension) and the neck joints (hyperflexion).

These jerking motions can leave some riders with whiplash. In rare instances, neck movements can lead to cervicocephalic arterial dissection and then stroke. This particular type of stroke is hard to diagnose because the main complaints — headache and neckache — are associated with many other conditions.

In one medical case study, an 11-year-old boy felt a sudden headache when the roller coaster he was riding came to an abrupt stop. He pushed through the pain and rode several other rides that day. In the coming weeks, the headache remained and his symptoms advanced to include dizziness, nausea and vomiting.

A CT scan revealed the boy had a cerebellar stroke. The boy was treated with anticoagulants and made a full recovery. The case study authors noted this type of roller coaster reaction was rare, and they only had a record of it occurring about twice a year. 

Heart-Pounding Drops

The excitement of roller coasters can get a person’s heart pounding. A person’s body releases neurochemicals like dopamine and adrenaline, and the experience can be thrilling and delightful. Studies have found such excitement can also lead to heart palpitations and arrhythmias.

In a 2007 research letter to JAMA, a group of German scientists described how they asked volunteers at an amusement park to wear a 12-lead Holter electrocardiogram while they rode a roller coaster. Fifty-five people agreed. All were over age 18, and none had a known history of heart conditions.

The volunteers wore the electrocardiogram for five minutes prior to the ride so the researchers could gauge their resting heart rates. When the roller coaster started moving, the volunteers pressed a button that recorded the ride had begun. After the ride finished, they kept the electrocardiogram on for another five minutes.

The ride began with a 30-second-long hill climb up 203 terrifying feet, followed by four seconds of free fall. Then, with 82 seconds remaining, the ride went as fast as 74 miles per hour. It went up and down six more hills and followed sharp, sudden curves.

Not surprisingly, the volunteers showed a spike in heart rate and blood pressure. However, the highest heart rate levels happened during the first 30 seconds as the ride went up the first incline, and the riders anticipated the coming drop.

During the initial incline, one person had an arrhythmic episode that resolved on its own. One person had four seconds of atrial fibrillation at the end of the ride, including palpitations, that also resolved on its own. Forty-four percent of the participants had asymptomatic sinus arrhythmias within five minutes of finishing the rides.

The authors concluded the volunteers had benign cardiac responses, but people with underlying heart conditions were at risk for arrhythmic episodes. 

Read More: How a Heart Can Stop After a Forceful Impact

Super Speeds

As scientists have learned more about traumatic brain injury (TBI), they’ve questioned whether the high gravitational forces on roller coasters could cause it. A 2009 study in The American Journal of Forensic Medicine and Pathology recruited volunteers to ride three roller coasters at a Six Flags theme park.

Four people agreed to participate — two adults in their twenties and two kids ages 11 and 13. The volunteers bit down on a mouth plate with sensors for each activity. The sensors recorded their movements on the roller coasters as they flew at speeds up to 85 mph and zoomed through loops, drops and other nauseating features.

The authors compared the head movements for each activity and found there was a low risk of TBI from a roller coaster. The roller coasters had an average head injury criterion measure (HIC15) of 4.1, which was six times less than what a person would experience in a TBI-inducing car crash. Similarly, the head impact factor (or HIP) was .36 on a roller coaster, compared to a 3.41 HIP from a car crash.

In 2017, a similar study in the Journal of Neurotrauma found little evidence that roller coasters could lead to TBI. But the authors also cautioned that although the risk is low, scientists don’t fully understand what roller coasters do or don’t do to the brain.

Read More: Traumatic Brain Injuries In Children, Especially Girls, Shows A 20-Year Increase