LONDON - LaShawn Merritt, the American sprinter who was favourite to retain his 400m title at the London Olympics, pulled up in his heat on Saturday and limped off the track.
Merritt, who has struggled back from a doping ban disgrace, stopped running after 250 metres and crossed the line at walking pace.
The 26-year-old American won the chance to compete at London by taking the title at last month's US Olympic trials in 44.12 seconds, the fastest time in the world this year.
But he pulled up in the Monaco Diamond League meet last month, similarly coming to a halt after 300 metres.
Merritt tested positive for steroids three times in late 2009 and served a 21-month doping ban before winning an appeal to an arbitrator and successfully fighting an Olympic rule that would have banned him from the London Games.
That victory from the Court of Arbitration for Sport came with a price, however. Merritt revealed that he tested positive because he took a US$6 (S$7.50) male enhancement product, ExtenZe, that contained the banned substance DHEA.
Merritt came back last year from the ban to take second at the world championships in Daegu.