CHATTANOOGA, Tenn. — Late one Saturday evening in June, two men in their 20s stood across from each other, shirtless and swaying, in a mixed martial arts cage in Exhibit Hall B of the Chattanooga Convention Center. The mat was sticky, a dark canvas of blood and foot sweat. Something in the combatants’ eyes made them look both terrifying and terrified, wolflike and rabbitlike at once.

The bout was one of 12 that evening in the B2 Fighting Series 166, an amateur event, and Dr. Danielle Fabry, a primary care physician with a private practice in Nashville, had been hired to make sure no one got seriously hurt. Stationed by the cage door, she had the best seat in the house.

Combat sports run on the excitement of an unstable equilibrium. In a perfectly matched fight, combatants trade blows until the final bell, bringing their bodies as close as possible to their limits. One mistake, though, and it ends violently. This combination of uncertainty and danger has helped transform mixed martial arts over two decades from a siloed obsession, illegal in a number of U.S. states, to a multibillion-dollar industry.

But even here there are limits to the harm allowed. Referees, often former fighters or trainers themselves, can stop a fight if they think a fighter is too injured to defend him- or herself. So can ringside physicians, who determine whether fighters are fit to step into the ring and to stay there. In combat sports, physicians have had to reckon with the precarious ethics of their role.

“I’m clearing someone to fight today, 20 years from now he walks into my office and has C.T.E., he has Parkinson’s,” said Dr. Nitin Sethi, a neurologist at Weill Cornell Medicine and board member of the Association of Ringside Physicians, or A.R.P., which formed in 1997. “Every doctor who works ringside should feel conflicted.”

In 2019, Dr. Sethi stopped a fight at Madison Square Garden between two U.F.C. fighters, Nate Diaz and Jorge Masvidal. With the fourth round about to start, a deep cut above Mr. Diaz’s eye opened up; he seemed heavily concussed, and the skin on his forehead was drooping over his eye. When Dr. Sethi intervened, the crowd booed and both fighters protested; afterward, his office phones rang off the hook with abusive messages.

“But how can you let a fighter who is getting injured on your watch go on?” said Dr. Sethi, who has worked ringside for a decade. He quickly noted the paradox of this statement; every moment he sits beside the ring is a moment he lets fighters get injured. “It’s impossible to make this sport safe,” he said.

Dr. Fabry, who started her private practice in 2021, has been doing ringside work for a little over a year. When the opening bell rang in Chattanooga, she leaned forward in her seat and watched the two fighters move toward each other. It wasn’t Madison Square Garden, but the medical stakes — for her and for the combatants — were just as high.

“You can never tell how it’ll go,” she said. In her previous event, a fighter had taken three minutes to revive after being knocked out cold by an uppercut.

“That scares me,” Dr. Fabry said. “That’s where you start to say, ‘OK, this is serious.’” She added: “At the same time, they’re all adults. They know what they’re getting into.”

The Check-In

Dr. Fabry drove down from Nashville on Friday, the day before the fight, with her boyfriend and a friend. By 4 p.m. on Saturday, she was in a makeshift locker room, working through pre-fight physicals for more than a dozen jittery men.

“You see the adrenaline from the second they walk into the room,” Dr. Fabry said as she waited for one man’s blood pressure reading and studied the quivering pupils of another.

“Push me away,” she instructed the second man — a test of his mobility and ability to follow basic directions. “Pull me toward you.” Then: “Can you feel when I rub down your arm?” He obeyed as the other man looked on. “Hopefully you’re not fighting each other,” she joked. They were not.

Growing up in Cincinnati, Dr. Fabry had attended a couple of combat events, but her interest blossomed in medical school, when she picked up boxing to relieve stress. “I feel like I always look at it as a doctor,” she said. “I’m like, ‘Oh, that’s going to be a problem.’ But I love boxing, and I love M.M.A. It’s something that I want to be a part of.”

In 2021, shortly after moving to Nashville, she heard that fight promoters were looking for physicians to sit ringside in Kentucky and Tennessee. She quickly had six job offers. A gig typically paid a couple hundred dollars, plus travel and lodging — a free weekend trip, a free fight. She decided to try it.

Professional combat sports are overseen by state agencies, and the standards for medical screenings vary. New York requires fighters to undergo a neurological exam, electrocardiogram, dilated eye exam and an M.R.I. before each fight. Most other states just ask for blood work, to check for blood-borne diseases, and a physical. The ringside physician interprets the results and decides who can or cannot fight.

“The commission doesn’t give you anything,” Dr. Fabry said of Tennessee’s medical guidelines for amateur fights, which are overseen by the International Sport Karate Association, or I.S.K.A. “They just give you a short thing” — a vague, quarter-page checklist of body parts and organ systems. Eyes? Check. Abdomen? Check. Neurological? Check.

To fill in her knowledge, Dr. Fabry said, she spent a few days looking over sports-physical checklists online: “I wanted to know, ‘What else should I be looking for?’” After a couple of fights, she had the hang of it. “It’s a lot like the physicals I do as a primary care physician, just a lot faster,” she said.

In Chattanooga, a blood pressure monitor on one of the fighter’s arms beeped ready: 210 over 185. Dr. Fabry shook her head. The number was way too high; if correct, it could indicate an underlying heart condition. But the man was nervous and chattering, and, like most fighters, he had probably dehydrated himself to make his weight class; most have elevated blood pressure before a fight. Dr. Fabry was also thinking about the crowd, the promotion and the man’s opponent, who had come from Knoxville for the event.

“You feel bad, because it’s your call, and you’re, like, ‘I just messed the whole card up for this guy,’” she said.

To the fighter she said: “That’s too high. Tough weight cut?” He shrugged. “OK, stop talking and relax,” she said. She took his blood pressure again: 161 over 86. “Much better,” she said, and cleared him to fight.

‘Why We Do What We Do’

After check-in, the fighters gathered awkwardly in the locker room as officials laid the ground rules: No kneeing a downed opponent. No elbows to the face. No eye pokes, crotch shots, glove-grabbing. “The number one thing for us is fighter safety,” said Brandon Higdon, a B2 promoter.

Bobby Wombacher, the night’s referee, added: “It’s all about fighter safety.” Todd Murray, who was overseeing the event for the I.S.K.A., chimed in: “We don’t want any of y’all getting hurt.”

As the meeting ended, Mr. Higdon hinted that he might give a $100 “locker-room bonus” to fighters who could pull off special finishes — something more dramatic than a judge’s decision. Amateur fighters are otherwise unpaid. In contrast, the U.F.C. pays its top fighters for each bout, plus as much as $50,000 for a particularly spectacular knockout or submission.

The regulation of combat sports is inherently contradictory: A good fight is violent and unsafe — but not too violent or unsafe. (The U.F.C. has fired officials who have allowed fights to go on too long.) From a medical standpoint, each time a fighter is hit in the head, he or she risks a brain bleed that can kill within minutes. And repeated trauma can result years later in chronic traumatic encephalopathy, or C.T.E., which can cause aggressive behavior, depression and eventually dementia.

Many physicians, as well as the American Medical Association and the World Medical Association, have called for the elimination of sanctioned combat sports. “We need to spread the word that brain-bashing is not a socially acceptable spectator sport,” Dr. Stephen Hauser, a neurologist at the University of California, San Francisco, wrote in 2012 in the medical journal Annals of Neurology.

For those who opt to be involved, the A.R.P. has created a standardized set of instructions and recommendations to remove some of the ambiguity of ringside medicine. The group has certified more than 100 doctors across 34 states and 11 countries since its founding.

But once the bell sounds, every ringside physician is alone, charting a calculus of risk, harm and entertainment. “You cannot become a fan,” Dr. Sethi said. “You stop it too late, and the damage is already done.”

A week earlier, Dr. Sethi and several dozen physicians had attended a virtual seminar hosted by the A.R.P. — a new course on the basics of ringside medicine. This was “Round 8,” dedicated to ethics, and it was led by Dr. Ed Amores, an emergency medicine specialist at NewYork-Presbyterian Hospital and an association board member.

Dr. Amores began by showing a video of a South African boxer who had died from a subdural hematoma a couple of days earlier. The video was from the end of boxer’s tenth round, and the fight had been called; the boxer was clearly injured, punching the air above him. “This is why we do what we do,” Dr. Amores said to the attendees.

At the seminar, Dr. Amores, sporting a neat goatee onscreen, seemed to be struggling with his role as a ringside arbiter. He read from an article in the Western Journal of Medicine by Dr. Suzanne Leclerc of McGill University and Christopher Herrera, a bioethicist at Montclair State University. “The mere presence of a sport physician at a boxing match lends an air of legitimacy to behavior that is medically and ethically unacceptable,” the authors had written.

But, Dr. Amores countered aloud, fighters would fight with or without physician involvement. “There are people who live dangerous lives,” he said. “Do I agree with what risk they’re putting themselves in? No. But at the end of the day I just try to do whatever I can to help them.”

Dr. Louis Durkin, an emergency medicine specialist at Mercy Medical Center in Massachusetts and vice president of the A.R.P., jumped in: Ringside physicians were like pulmonologists who take care of smokers, even though they disapprove of smoking. “We’re E.R. docs,” Dr. Durkin said with a laugh. “We would have nothing to do all day if it wasn’t for bad behavior.”

Dr. Amores nodded, noting that the American Academy of Neurology recommends the presence of a doctor at combat events. Then he added, “Sometimes I feel very enthusiastic about making this unsafe sport safer, and sometimes I really question myself and wonder whether I really should be doing this.”

Dr. Sethi spoke up: “Ed, if you’re not feeling conflicted, I think there’s something majorly wrong.”

Boxers in their twenties come to Dr. Sethi all the time asking to be cleared to fight despite M.R.I.s brimming with small “white” scars that form after traumatic brain injuries. “On our watch, we probably have a bunch of athletes that are going to develop C.T.E.,” he said. “When you and I hang up our gloves, would you be comfortable going to bed and saying, ‘I did the right thing?’”

After the Bell

On that Saturday night in Chattanooga, Tyler Britt entered the cage wearing a cape of animal pelts and a demon mask; it was the penultimate fight of the night, and the crowd was buzzing. He glared at his opponent, Antonio Holt, and drew a finger across his throat.

Mr. Wombacher, standing in the middle of the cage, checked in with the fighters one last time. Ready? Ready. Ready? Ready. Ringside, Dr. Fabry rubbed her legs in anticipation. “This is going to be good,” she said.

In front of her were the forms she had filled out during check-in; she would use the flip side and the margins to note any injuries during the fight. “There needs to be an organization to this for everyone’s safety,” she said. She had heard of the A.R.P. only recently; she felt she could figure things out pretty well on her own, she said.

At one point in the bout Mr. Britt twisted underneath Mr. Holt and grabbed his right arm, pulling it back like a chicken wing — a kimura lock. “Break his arm!” yelled fans in the crowd. “Break his arm!”

Mr. Holt, stuck in the lock, did not tap to concede the fight, but he did not move. The bones in his forearm looked as though they might burst through the skin. “I’m gonna break your arm,” Mr. Britt said through clenched teeth, tightening the hold.

Mr. Holt reached back, trying to relieve pressure by grabbing his right hand with his left. He swiped at the air once or twice. “I think he’s trying to tap,” Dr. Fabry said aloud to herself; she was poised to rise from her seat. A broken arm could mean the end of Mr. Holt’s fighting career and thousands of dollars in medical bills.

“He’s tapping! He’s tapping!” came voices from the crowd. The referee let the fight continue.

Later, when the excitement had died down and the hall was emptying — after Mr. Holt managed to escape the kimura and went on to win in a technical knockout — Mr. Wombacher and Dr. Fabry stood in the locker room. There was a brief conversation about the fights, and then the doctor headed off to a bar with her companions. Mr. Wombacher lingered. He acknowledged that he could have stopped the Britt-Holt fight during the arm lock.

“It was really deep,” he said, squinting. “Look — the guy kept saying ‘I’ll break your arm’ while on the ground. Well, don’t just say it. Do it.”