The US superstar, 24, blamed “the twisties” for her dramatic exit from the team event at the Tokyo Games, where she was aiming to equal or even surpass the all-time women’s gymnastics record of nine Olympic golds.
Biles exploded down the runway for her opening vault in the team final on Tuesday, launching her Tokyo odyssey with an Amanar at the Ariake Gymnastics Centre.
The skill — a back handspring onto the vault then two-and-a-half twists in mid-air with a blind landing facing away from the apparatus — is not for the faint-hearted.
It is a fiendishly daring and dangerous move but one she normally executes time and again with pinpoint perfection.
On Tuesday, with her diminutive frame shouldering the crushing weight of expectation that comes with being one of the faces of the Games, the “twisties” struck.
Losing her sense of spatial awareness, with the real danger of doing herself serious harm, Biles bailed out of the Amanar in mid-air, turning just one-and-a-half times and stumbling on landing.
Fellow gymnasts have described the condition that interrupts normal communications between brain and body.
“You have absolutely no control over your body and what it does,” US gymnast Aleah Finnegan explained on Twitter.
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The loss of form is complex, difficult to treat, and can be compounded by pressure, a French gymnastics coach told AFP.
Any gymnast who falls victim to the twisties is “paralysed by the fear of losing” and serious injury, he said.
Biles, with the razor-sharp mental vision gymnasts require to perform their gravity-defying routines clouded by doubt and fear, bravely took the decision to stand down from the remaining three elements to focus on her mental health.
Instead, she assumed the role of cheerleader-in-chief as her teammates soldiered on to claim silver behind the Russians.
Biles, who has four Olympic gold medals and 19 world titles, then pulled out of Thursday’s defence of her all-around crown, with her participation in the four apparatus finals remaining uncertain.
The gymnast has received support from across the world, including from former US first lady Michelle Obama and swimming great Michael Phelps.
She tried to explain what had happened in the immediate aftermath of Tuesday’s competition
“I did not choose to do a one-and-a-half, this (pointing to her head) chose to do a one-and-a-half,” she said. “I was trying to do a two-and-a-half, and that just was not clicking.”
She said her teammates first noticed something was amiss in the run-up to Tuesday’s final.
“They (her teammates) saw it a little bit in practice. I was having a little bit of the twisties.”
“She was giving us a little heart attack,” teammate Jordan Chiles interjected.
Biles continued: “It’s very uncharacteristic of me. It just sucks that happens here at the Olympic Games.”
Christina Myers, former athlete and now coach, told the BBC that the twisties can occur “when your brain and body disconnect”.
British gymnast Claudia Fragapane knows just how perilous the sport can be, falling off the beam at the Rio Olympics and again in training in April, sustaining a head injury.
“Everyone thinks that she is just going to be absolutely out of this world and perfect and she’s not human,” she told the BBC. “But actually she is human, and I think the pressure just got too much.”