Story highlights U.S. runner initially hurt leg on a staircase 
 
 At 200 meters in Thurday's race it snapped 
 
 He still ran 46.1 for one lap 
 
 The U.S. rallied to finish second in its heat 
 
 Just after the U.S. team finished second in its 4x400-meter relay heat, a limping Manteo Mitchell told reporters his injury was nothing serious. 
 
 "It might be a simple cramp," he said. 
 
 Turns out it was a lot more than that. 
 
 Mitchell broke a bone in his left leg, USA Track and Field said in a news release. The 25-year-old, running the leadoff leg, felt his fibula snap with 200 meters to go. He had to finish or the United States would have been out of the event. 
 
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 "I wanted to just lie down. It felt like somebody literally just snapped my leg in half," he said. 
 
 For a half-lap he ran in excruciating pain until he handed off to Josh Mance. 
 
 Mitchell's split was 46.1 and the Americans went on to finish in 2:58.87 to qualify for Friday's final. 
 
 Mitchell said on Monday he was going up a set of stairs, missed one and landed poorly. 
 
 "I got treatment and I was fine," he said, adding that he felt good warming up for Thursday's race. 
 
 An X-ray taken Thursday after the relay showed the break. 
 
 He seemed to have good spirits about it. 
 
 "Never thought that the phrase 'break a leg' would reach reality for me lol," he tweeted later. 
 
 USA Track and Field has not said who will take Mitchell's place on the team. Two other members of the relay team pool -- Jeremy Wariner and LaShawn Merritt -- have injured hamstrings and cannot run. 
 
 The Americans will likely turn to one of their 400-meter hurdlers. Angelo Taylor, who was fifth in the 400 hurdles, ran in the 4x400-meter relay final at the world championships last year. Kerron Clement ran for the U.S. at the Beijing Games. But that was in a qualifying race while Taylor ran in the final, winning gold. 
 
 Bolt makes history ||||| Photo by Feng Li/Getty Images. 
 
 "I heard it and I felt it," American sprinter Manteo Mitchell told the Associated Press after breaking his leg on Thursday. Mitchell, who was halfway through his segment of the men’s 4-by-400 relay when he felt something snap, did what he said “almost any person would've done in that situation”—he finished the remaining 200 meters. His endurance wasn’t for naught: After Mitchell limped off, his three teammates completed the relay and qualified for the 4-by-400 final. After the race, doctors diagnosed Mitchell with a complete break of the left fibula and said the bone would heal in four to six weeks. How difficult is it to run 200 meters on a broken fibula? 
 
 Very difficult, but not impossible. The fibula is the smaller of the two bones that make up the lower leg, the other being the tibia. Unlike the tibia, the fibula is non-weight-bearing. As such, it is possible to run on a broken fibula. The pain, though, would be intense and would likely debilitate many athletes. The fact that Mitchell made it around the track in just more than 46 seconds—a time that placed him in a tie for fifth out of eight runners—is extremely impressive. 
 
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– The US made it to the men's 4x400-meter relay finals with the help of a runner who deserves a gold medal for pain endurance. Manteo Mitchell was 200 meters into the race's first leg when a bone in his leg snapped. He managed to finish the rest of the lap in excruciating pain, and keep the US in the competition, CNN reports. After he limped off, an X-ray confirmed that the 25-year-old athlete had a broken left fibula, the smaller of the two leg bones. It is possible to run with a broken fibula, but most people would find the pain unbearable, Slate notes. Mitchell originally hurt himself missing a step as he went up some stairs at the Olympic Village earlier in the week, but he says he felt fine after treatment. But "as soon as I took the first step past the 200-meter mark, I felt it break. I heard it," he says. "I even put out a little war cry, but the crowd was so loud you couldn't hear it. I wanted to just lie down. It felt like somebody literally just snapped my leg in half." The US—which has won gold at the eight Olympic 4x400-meter relays it entered—now advances to tonight's final, although the team has not yet announced its replacement for Mitchell.