The crack of two skulls colliding echoed around the sumo arena. One of the wrestlers returned to his starting position in the ring, apparently unfazed. But his opponent, Shonannoumi, was clearly in distress.

“Is he OK?” asked the TV commentator after Shonannoumi’s leg buckled as he attempted to raise his 164kg (25st 11lb) frame from the ground. The referee and judges exchanged looks but with no ringside doctor to consult none seemed sure how to react.

After second failed attempt, the 22-year-old finally got to his feet and staggered to the centre of the ring, indicating he wanted to continue. He went on to win the bout at the new year tournament last month.

For some sumo fans, Shonannoumi demonstrated the indomitable fighting spirit needed to thrive in a brutal sport whose traditions stretch back centuries.

For others, his plight should be the trigger for sumo to protect its athletes amid evidence of a possible link between concussion in sport and serious health problems including dementia later in life.

“It absolutely was horrifying,” wrote one commenter below a video clip of the bout posted on Twitter. Another asked: “What can fans do to make the sumo association sit up and take note that it’s not acceptable?”

Football and rugby are responding to evidence revealed in 2019 by Queen Elizabeth university hospital in Glasgow that players in those sports are at a higher risk of suffering brain injury and chronic traumatic encephalopathy (CTE), a degenerative brain condition first identified in American footballers.

After decades of inaction and ignorance, they are making long overdue adjustments to prevent head injuries sustained from direct contact with opponents or from repeatedly heading a ball.

But sumo, a highly physical contact sport, is unlikely to follow suit, despite the frequency with which rikishi clash heads during tournaments and in daily training sessions.

“Banging heads is very common,” says John Gunning, a former amateur sumo wrestler who now commentates and writes about the sport in Japan. “It’s still very much old school, going in hard and head first. The sound of two heads cracking together like coconuts is a common one in a sumo training session. What you see in a tournament is only a fraction of what goes on.”

Shonannoumi is not the only wrestler who has felt compelled to continue fighting after receiving a heavy blow to the head, aware that retiring would mean ceding victory to his opponent.

In 2018, Hokutofuji fought on after a blow to the head left him unsteady on his feet. A year earlier, two wrestlers at one tournament were left prone on the dirt after being knocked out at the same tournament.

Gunning, who represented Ireland three times at the sumo world championships, recalled an incident in 2006 in which he witnessed a wrestler get knocked out, only to be dragged unconscious from the ring by his heels and left to recover unattended. When the wrestler came to, he was mocked by his fellow stablemates for being floored by a rival from a lower rank.

Gunning has called on the sumo authorities to ditch a rule that requires wrestlers to touch the dirt of the dohyo (ring) with both hands before they engage – a stance that increases the likelihood of clashing heads. Instead, he says, they should be allowed to adopt a standing stance used in the 1970s and 80s.

“It would significantly lessen the number of blows they are taking, and it would be very easy to implement,” he says, adding that the absence of ringside doctors proves sumo is failing to provide “a level of care you would expect at the top of a professional sport”.

Tomoyuki Shimakawa, an orthopaedic surgeon at Harue hospital in Fukui prefecture who has conducted research into head injuries in sport, says sumo elders should consider introducing penalties for actions that risk seriously injuring an opponent’s head.

Shimakawa believes memory loss and other cognitive issues are common among retired wrestlers, some of whom display symptoms often associated with punch-drunk former boxers.

In a co-authored report on sumo injuries, he blamed the strict ranking system for encouraging wrestlers to take unnecessary risks. “Once a wrestler stops competing and is absent from a tournament due to injury, his rank steadily decreases,” the report said. “For this reason, wrestlers often continue to compete, regardless of symptoms.”

Days after Shonannoumi struggled to his feet, the Japanese sumo association said that from next month it would allow judges to withdraw a concussed athlete from the ring, with his opponent declared the default winner – a move some fear will only encourage wrestlers to play down their symptoms rather than forfeit the match.

“Your whole life and lifestyle can depend on a single win or loss,” Gunning says. “And even if there is a cultural shift in sumo, what do you change? They can change tackling rules in rugby or introduce position-specific helmets in American football, but sumo is all about crashing into your opponent. It’s power versus power.”

Gunning believes sumo authorities are aware of the risks brain injuries pose to wrestlers, but says the momentum for change has yet to build in a sport run by men who have been immersed in sumo’s conservative culture since their mid-teens.

“It’s a sport – and a lifestyle – that hasn’t changed much in a couple of hundred years. So how do you change anything without fundamentally changing what sumo is?”