WALNUT CREEK, Calif. — When the artistic swimming team competition begins on Friday at the Tokyo Games, the goal of the swimmers 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.

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.

But the sport has in recent years 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.

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.