At a certain point Monday, it was no longer the aching in his forearms that made each step a chore, nor the growing fatigue in his right leg. What was making the 118th Boston Marathon difficult for Larry Chloupek were the tears in his eyes. Every impromptu standing ovation from the spectators along the route and every word of encouragement from every awestruck able-bodied runner who passed him would start the waterworks again.

“I was blown away,” said Chloupek, 53, from Potomac. “The crowd support, from mile one until the end, the clapping that went on — it was absolutely incredible.”

The reactions of the spectators and his fellow racers were understandable. It isn’t every day you see a man completing a marathon on crutches. It isn’t every Boston Marathon that sees one either. It is rare enough that race organizers don’t even keep precise records on it, but a race spokesman said Chloupek’s feat was not unprecedented.

In any case, as he lined up for the 8:50 start to the mobility-impaired portion of the marathon Monday morning in Hopkinton, Chloupek looked around and saw wheelchair racers, hand-cycle racers and several amputees with prosthetic legs — but no one else but him about to race 26.2 miles on crutches.

Chloupek, a management liaison director for the National Institutes of Health, lost his left leg to bone cancer at the age of 7. The amputation was near his hip, which has made prosthetics difficult for him to use. So he gets around primarily with forearm crutches. Despite his disability, he has had a remarkably full athletic career, having completed five half-marathons, played seated volleyball in the 1996 Atlanta Paralympics and coached baseball at Churchill High.

A look at today's arce and the factors that may have hlped the winner. (Bonnie Berkowitz and Richard Johnson/Boston Athletic Association, Science of Sport)

Over the past 12 months, of course, Boston — and all of America — had gotten to know more about amputees than it ever cared to learn. Sixteen people lost limbs in last April’s Boston Marathon bombing that also left three people dead. Some of them were escorted to the course Monday near the end to cross the finish line along with the competitors running in their honor.

Of all the years for Chloupek to qualify for Boston — based on his time in the 2013 D.C. Rock ’n’ Roll Marathon — it seemed somehow fitting, even fateful, that it would be this year. And once he got in, he made it his mission to use his platform to send a message to those injured by the bombing.

“I wanted to give them some strength,” Chloupek said. “They’re new amputees. I wanted to prove a point to them — that despite what happened last year, they can overcome it.”

With his wife, Jenn, running alongside as his guide, Chloupek completed the storied course in 5 hours 20 minutes, stopping only for water and energy bars. His motto for attacking Boston’s notorious hills was “Slow and steady” — which applied both to the rises and the descents, the latter being equally harrowing to a runner on crutches.

All along, the crowds cheered wildly as soon as they spotted Chloupek, at many points rising to their feet as one. At the infamous Heartbreak Hill, near Mile 21, it was the crowd support that propelled him.

“Normally, when you hit the 21-mile mark you’re struggling — especially here,” Chloupek said. “But [the applause] was a shot of adrenaline. I’d never experienced that before.”

He crossed the finish line to one last raucous ovation. “Such a special moment,” he said. “I’ll never, ever forget it. Definitely one of my top three life experiences.”

Larry Chloupek of Potomac is the only participant to race the Boston Marathon on crutches. (STEVEN BOGNAR/STEVEN BOGNAR/WBZ-TV)

Afterward, his entire body ached. Most runners don’t have to worry too much about pain and numbness in their arms and hands as Chloupek does. He and Jenn had agreed beforehand — this was going to be their final marathon.

“But after today,” he said, “I am seriously considering a return visit.”