Dr. Bastiaan R. Bloem of the Radboud University Nijmegen Medical Center in the Netherlands thought he had seen it all in his years of caring for patients with Parkinson’s disease. But the 58-year-old man who came to see him recently was a total surprise.

The man had had Parkinson’s disease for 10 years, and it had progressed until he was severely affected. Parkinson’s, a neurological disorder in which some of the brain cells that control movement die, had made him unable to walk. He trembled and could walk only a few steps before falling. He froze in place, his feet feeling as if they were bolted to the floor.

But the man told Dr. Bloem something amazing: he said he was a regular exerciser — a cyclist, in fact — something that should not be possible for patients at his stage of the disease, Dr. Bloem thought.

“He said, ‘Just yesterday I rode my bicycle for 10 kilometers’ — six miles,” Dr. Bloem said. “He said he rides his bicycle for miles and miles every day.”