'I was dreading every step, every stride, and always thinking the worst'

It was something she had done thousands of times before – yet on the biggest stage of all, at this year's Tokyo Olympics, it suddenly could not have felt more alien.

Simone Biles is the greatest gymnast of her generation and among the most decorated in the sport's history. As she set off down the runway to tackle the vault in pursuit of her fifth Olympic gold in the team final, she was widely expected to get the USA off to a flyer in what is one of her strongest events, as individual gold at Rio in 2016 testifies.

Instead, though, Biles looked dazed and confused, just about managing to land on her feet after a vault that came nowhere close to her usual standard. She later explained that she had been suffering with what she called "the twisties" – a kind of mental block causing a gymnast to lose their sense of space in the air and, in the worst cases, leaving them unable to land safely.