Grubb said the biggest danger from fainting was in hitting the ground. That is why, since the day she received the diagnosis, Sam’s father has waited at the finish at nearly all her races, inspired by a video sent to him of people catching Kayla Montgomery, who similarly collapses at the finish line because of multiple sclerosis.

“If he’s not there,” Sam said, laughing, “the track hurts a lot.”

At practice, when the elder Peterman is not there, Lombardo, the coach, catches Sam. He also notifies other coaches, teams and race directors of Sam’s condition.

After races, Dale Peterman has mastered the routine: Lay Sam down and elevate her legs to return blood flow to normal and help her regain consciousness. Sam said she did not usually remember the ends of races; last week, running an 800-meter race, she blacked out for the final 60 meters.

She tries not to think about collapsing at the end. Instead, she tells herself to drive her knees and focus on her form, which breaks down as the dizziness takes hold: Her right arm juts out, and her head tilts back.

The biggest question Dale Peterman still has is why she does not pass out until she finishes the race. The doctors do not know, he said. He said he thought that it was intrinsic, that her desire powered her.

Sam, whose Twitter page reads, “Everybody falls, but we get back up because the ground is no place for a champion,” won the sectional title in both the 800 meters and the 1,500 meters this season. But she does not run because she wants attention, Lombardo said. She runs because she loves it, and because she has the opportunity to raise awareness of NCS, which accounts for 3.5 percent of emergency room visits in the United States every year, according to Grubb’s article.

Sam, who loves to go turkey hunting and play with children, who is quiet and unassuming and smiles frequently, wants to go to college to run and become a doctor to study NCS. She uses the condition not as an excuse but as an impetus.