Bill Iffrig, 78, of Lake Stevens, Washington, even crossed the finish line and completed the Boston Marathon after he was helped to his feet by race workers.
His story is a single pleasant surprise in a day that has been bleak for the people of Boston.
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Dramatic: Bill Iffrig, 78, became an icon of the Boston Marathon bombings with this photo, which was seen around the world
Horrific: Mr Iffrig can be seen falling to the ground as the explosion rocked the sidelines near the finish line of the marathon
Lucky: Mr Iffrig suffered little more than a skinned knee after the blast and counts himself lucky to be alive
'My legs just gave out,' he told CNN on Monday night.
Everyone who has seen the heart-rending footage of the first bomb blast near the finish line of the marathon has watched Mr Iffrig, wearing an orange tank top, tumble to the hard ground.
Boston Globe photographer John Tlumacki took a photograph of Mr Iffrig after he fell that became an instant icon. Mr Iffrig is shielding his face on his back while several Boston police officers brace themselves and jump into action.
According to numerous reports, the bomb that detonated was packed with ball bearings. Three people died in the twin blasts and at least 144 were injured - many of them losing legs, calves or feet in the explosions.
Mr Iffrig, a retired mason, said he didn't even think about not crossing the finish line at the time because his head was set on the race.
Video: The moment a bomb blast knocked Bill Iffrig to the ground
'I ended up second in my division,' he said. 'After you've run 26 miles you're not going to stop there.'
He said he finished in 4 hours, 3 minutes, 47 seconds.
It wasn't until Mr Iffrig walked six blocks back to his hotel room on flipped on his television that he realized the magnitude of what had happened.
'It was a close one though, boy. I'll tell you, that scared me,' he told his hometown newspaper, the Everett Herald.
This was Mr Iffrig's third Boston Marathon. He has run a total of 45 marathons in his life.
A race volunteer helped Mr Iffrig to his feet after the first blast, as a second explosion can be seen in the background. Mr Iffrig continue on and finished the marathon