RIO DE JANEIRO — A little smile appeared to form on Elaine Thompson’s face as she took her final steps across the finish line. Soon, it expanded into a look of full incredulity, her mouth agape. And seconds later, as she began to process that she had won the gold medal in the women’s 100-meter dash, she began hopping up and down on both feet, arms out at her sides.

By then, she looked a bit dazed.