CHULA VISTA, Calif. — Wesley Williams placed his arms on the shoulders of Lex Gillette, a blind long jumper, to line him up on the runway, then guided him down the track to let him feel the landing pit. Gillette made a couple of small hops into the sand.

Williams then steered Gillette back to the start of the runway and put him as far to the left in the lane as possible, because Gillette usually veers right when he runs. After positioning him, Williams stood down the runway, in the middle of the lane, at the lip of the sand pit. He raised his arms in the air and, with the steady precision of a drummer, began to clap.

“Lai! Lai! Lai! Lai! Lai! Lai!” Williams shouted, using the Chinese word for “come” because he finds that it carries better in the din of stadium noise at competitions.

Relying heavily on muscle memory, Gillette sprinted down the track. He knew to jump on his 16th step, after more than 108 feet. He listened to Williams’s call as confirmation that he was running straight. As he approached the sand, Williams stepped to the side.