Scientific discoveries can be serendipitous, and so it was when Jay L. Alberts, then a Parkinson’s disease researcher at Emory University in Atlanta, mounted a tandem bike with Cathy Frazier, a Parkinson’s patient. The two were riding the 2003 RAGBRAI bicycle tour across Iowa, hoping to raise awareness of the neurodegenerative disease and “show people with Parkinson’s that you don’t have to sit back and let the disease take over your life,” Dr. Alberts said.

But something unexpected happened after the first day’s riding. One of Ms. Frazier’s symptoms was micrographia, a condition in which her handwriting, legible at first, would quickly become smaller, more spidery and unreadable as she continued to write. After a day of pedaling, though, she signed a birthday card with no difficulty, her signature “beautifully written,” Dr. Alberts said. She also told him that she felt as if she didn’t have Parkinson’s....