The shoelace is worn and frayed, with loops tied at each end. David Brown curls his fingers tightly around one loop, then Jerome Avery grabs the other.
"Four inches," David says. "That's all we have between us."
The sprinters keep hold of the cord as they burst from the starting blocks, charging side-by-side down the track.
"Pick it up," Jerome growls.
Their arms pump in unison and their legs churn in identical strides so that, on a hushed autumn morning, they sound like one person running. David moans with exertion.
"C'mon," Jerome tells him. "Drive."
No blind athlete has ever run 100 meters as fast...