WALNUT CREEK, CALIF. — When the artistic swimming team competition begins Friday at the Tokyo Games, the swimmers’ goal will be to make their movements appear effortless. But while viewers will see smiling performers, sparkly suits and gelatin-slicked hair, a risk lurks beneath the surface: the potential for concussions.

Artistic swimming, formerly known as synchronized swimming, combines elements of gymnastics and ballet in the water. Teams of up to eight athletes swim quickly, closely and precisely together, coordinating with one another and the music. Often described as beautiful above the water, the sport requires constant furious activity below. It’s not unusual for teammates to kick or land on each other during their routines.

The artistic swimming world has long known it has a brain injury problem, but nobody knew how extensive it was. So in 2019, as a student researcher at Stanford, I conducted research into how common concussions are in the sport in which I once took part.

The answer surprised me: In a survey of 430 athletes, about one in four who have competed in the United States reported having at least one concussion.

“Yeah that’s actually a lot more than I expected,” Karina Boyle, 25, said in an interview beside the pool where she trained for most of her career. Boyle, who swam for national teams, is now retired. “But I know it can be a pretty brutal sport when you’re swimming so close to each other and it’s very active.”

That one-quarter estimate might be low. Fifteen percent of respondents said that they thought they had sustained a concussion from artistic swimming, suggesting the actual overall figure might be closer to 40 percent.

The survey, sent to current and former athletes who have competed in the U.S. at any level, was conducted in the spring of 2019, and took into account the number of years each of the swimmers participated in the sport, their ages, the ages at which they sustained their concussions and what sort of treatment they had sought.

In recent years, the sport has begun to reckon with its concussion problem. The United States is not a powerhouse in the sport — it sent only a pair of artistic swimmers to the Olympics — but U.S.A. Artistic Swimming, the sport’s national governing body, has taken steps to promote concussion safety. It now partners with Hammer Head Swim Caps, which makes silicone bathing caps with a thin honeycomb layer that offers some protection against a misplayed foot or arm, or an unforgiving pool wall.

The United States national team relied on the caps when practicing a dangerous throw that it planned to unveil at an Olympic qualifier in June. No other country had attempted the throw at that level.

The move, in which the person being tossed into the air lands back in the hands of the throwers, carries the risk that a minor error could end in serious injury for teammates below. In the early stages of practice, the American swimmers wore the helmet caps.

“Many times she did not land back in the hands, so we were cautious and we made sure to put the caps on before we tried it,” Anita Alvarez, a 2016 Olympian who was part of the team, said in a phone interview in July. Alvarez, 24, and her duet partner, Lindi Schroeder, 19, will represent the United States in the duet events at the Tokyo Games.

The long-term effects of head injuries have been studied in many sports over the years, from football to sliding sports, inspiring leagues and federations to adopt protocols to mitigate effects or prevalence. But studies of concussions within artistic swimming have been limited.

Concussions tend to be underreported in youth sports for many reasons, including athletes’ desire to continue competing, fear of letting teammates down or simply not recognizing symptoms, said Dr. Daniel Daneshvar, the director of the recently opened Institute for Brain Research and Innovation, which studies the effects of head trauma. Previous research indicates that more than 50 percent of concussions go unreported.

Alvarez, the American Olympian, remembers the summer of 2013, when three of her teammates slated to represent the United States on an eight-person team at the Pan American Games were concussed — Karina Boyle, Karensa Tjoa and me.

Boyle had been kicked in the head after a lift, a move in which at least one swimmer is launched into the air by her teammates.

Tjoa was in pattern with seven other swimmers, jumping backward when she felt a knee hit the back of her head. The rest was a blur.

“I just remember stopping — and in synchro you’re trained to never stop — so it was uncharacteristic of me to stop and swim to the side,” Tjoa said. She got out and rested with ice on her head for a bit, but when her coach asked how she was feeling, she knew something was wrong. “It felt different, kind of like I was still underwater somehow.”

She decided to compete at the Pan American Games after resting for a month, and at Junior Worlds the following year.

Now Tjoa, 25, is not sure she made the right choice.

“Every time I would try to get in, I got a really bad headache, I would feel dizzy,” she said, looking at the pool where she spent some of her final years in the sport before retiring in 2017. “And so all of these starts and stops, I think, inhibited my recovery, and maybe it took longer than it would have if I had just focused on recovering then.”

I started artistic swimming when I was 9. I moved from the East Coast to California for better training opportunities and qualified for a few national teams before being recruited to Stanford University.

I got my first concussion in 2013, when I was 16. One of my teammates attempted a back flip off my shoulders during a lift. Instead of jumping backward, she went straight up and came down on my head. It took months for me to recover.

Over the past 20 years, artistic swimming has required athletes to move faster and swim closer together, as performances are judged on the difficulty of the routine and technical merit.

U.S.A. Artistic Swimming began to address concussions in earnest two years ago, even as it pushed for proximity, power and speed. In addition to encouraging protective caps, the organization has partnered with TeachAids, which aims to help coaches better recognize concussions.

Heavy blows are always a worry, but repeated small hits also take a toll, said Dr. Daneshvar, whose institute was founded by TeachAids. Sometimes, he said, they can bring about chronic traumatic encephalopathy, commonly known as C.T.E., which has been recognized in retired professional football players“Individuals who don’t get a concussion, but they have these repeated hits in something like football, for example, you can see structural changes on imaging and functional changes on imaging throughout, even in the course of a season, in the brain,” he said.

Boyle was fortunate: She did not sustain another head injury after her 2013 accident. She returned to the sport a few months later to compete for her club team, the Walnut Creek Aquanuts, in Northern California, retiring at the end of that season to pursue a college degree.

Although she wasn’t free of headaches and nausea in her first few months, she ended her final season happy and healthy.

“It was a long process, but that was one of the best years of my synchro career,” Boyle said.