Kayla Montgomery, 18, was found to have multiple sclerosis three years ago. Defying most logic, she has gone on to become one of the fastest young distance runners in the country — one who cannot stay on her feet after crossing the finish line.

Her M.S. is blocking signals that cause pain in other runners, allows her to push through, extend her tolerances.

But intense exercise can also trigger weakness and instability; as Montgomery goes numb in races, she can continue moving forward as if on autopilot, but any disruption, like stopping, makes her lose control.

“When I finish, it feels like there’s nothing underneath me,” Montgomery said. “I start out feeling normal and then my legs gradually go numb. I’ve trained myself to think about other things while I race, to get through. But when I break the motion, I can’t control them and I fall.”

Fascinating and poignant story.