WINSTON-SALEM, N.C. — When a pack of whip-thin girls zipped across the finish of the 1,600-meter race at a recent track meet here, the smallest runner’s legs wobbled like rubber, and she flopped into her waiting coach’s arms. She collapses every time she races.

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.

Because M.S. blocks nerve signals from Montgomery’s legs to her brain, particularly as her body temperature increases, she can move at steady speeds that cause other runners pain she cannot sense, creating the peculiar circumstance in which the symptoms of a disease might confer an athletic advantage.

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.