MATINA, Costa Rica — Just keep moving. Even if only an inch forward. That’s the key.

How else would cyclists riding in La Ruta de Los Conquistadores — a mountain bike race marketed, quite reasonably, as the world’s toughest — make it to the finish line in one piece? How else would they conquer a merciless route that includes steep climbs, choking humidity, muddy jungle trails and swift-moving rivers that may or may not contain the occasional hungry crocodile?

But first things first: If they did not press forward, how would they get off this bridge?

The course on this, the third and final day of La Ruta, was flat, and the finish line beckoned from a soft-sand beach less than 20 miles down the road. But first there was a cruel twist. Before they reached the beach, competitors had to traverse railroad tracks that crossed several high bridges like this one, with murky rivers swirling ominously below their unevenly spaced wooden ties.

The ties could be rickety, or slick with oil, or set wide enough apart for a human body to slip through — or all three. To cross, hundreds of cyclists had to carry their bikes, or roll them, as they stepped gingerly, silently and in single file, as if in a church procession.

Not everyone could handle the stress. One racer near the front of the pack dared to look down between the wooden ties and froze, immediately creating a backup of riders who wanted nothing more than to keep moving, if only an inch, because momentum helped their balance. The rider, a Costa Rican, needed to be rescued by a wooden cart that was kept handy to ferry “chicken people who don’t want to walk,” said the race’s founder, Román Urbina.