American Manteo Mitchell competed with a broken leg in the 4x400m relay event at the Olympic Stadium.
Mitchell, 25, says he tripped on some stairs earlier this week, got some treatment and felt fine.
He says that 100m into his opening relay leg on Thursday it did start to feel weird and then after 200m he heard it break.
But he did not want to let his teammates down, so he kept going and managed to break 45 seconds, helping the US to the joint fastest time with the Bahamas.
After the race, Mitchell was taken to the Athlete village medical area, where his leg was X-rayed and diagnosed as a break of his left fibula bone.