At the International Team Trials in Greensboro, North Carolina, Armstrong ran a mark of 23.71 seconds in the non-Olympic event.
The competition is determining who will represent the United States at the global championships in Budapest, Hungary, this summer.
After the 2021 World Championships in Fukuoka, Japan, were postponed twice due to the coronavirus epidemic, the event was added to the calendar.
That tournament has been rescheduled for 2023.
Armstrong, a 21-year-old Ohio State student, earned a gold medal in the backstroke leg of the 4x100 medley relay prelims in the Tokyo Olympics last summer.
Nonetheless, he was astounded by his time in the 50 back, which beat Russia's Kliment Kolesnikov's mark of 23.80 established in 2021.
In the morning prelims, Armstrong set an American record, then ran even faster in the evening.
"It was completely unexpected," he remarked. "My best time before today was a 25.50, and I was able to run a 24.00 in prelims, which gave me optimism."
However, you are aware that Kolesnikov is one of the world's quickest backstrokers and freestylers. It's fantastic to be up there with him." He Said
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