The opening semi-final was panning out as expected until Werro suddenly stumbled and fell face first onto the track with just under 140m left to go.
“It was a hard hit on the floor. These tracks are incredibly hard, especially when they’ve been sun-baked,” former Olympic long jump champion Greg Rutherford said on BBC TV.
“It really was a difficult thing to watch, and you’re just hoping at that point [that] there’s no serious injury.”
Werro appeared to be in some discomfort, rolling onto her side and clutching her chest before getting back up and finishing the race at a slow jog.
Paula Radcliffe, a former women’s marathon world record holder, said: “Athletes know if that happens and you go down, you have to get up and finish.
“That is the only criteria that you’re going to make it through. If you don’t do that, you have no chance of being advanced through to that final.
“You could see she was not comfortable.”
However, there was considerable confusion over whether Werro had tripped herself or been impeded by another athlete.
Rutherford believed Werro had “kicked her own leg”, while former men’s 1500m world champion Steve Cram said she had “tripped herself”.
Werro herself did not know, telling BBC TV: “I was feeling great but unfortunately I don’t know, something happened and then I was on the ground.”
A closer look at the footage showed that Bourgoin, who had been on Werro’s shoulder for much of the race, disrupted the Swiss athlete’s rhythm with a slight touch to her foot with her left hand.
“The question is whether Werro is OK – it was a really hard fall,” Cram said.
“But we are getting the final we wanted. That’s one way to look at it.”
