Sometimes coaches are resting swimmers by having them not swim morning relays (completely valid). But also, NCAA rules require swimmers who only qualified for the meet in a relay to swim that relay in the morning (at least) so they can swim their other “B” cuts as well. For example, Caroline wilson actually did not swim the 400 MR in the morning because Williams’ Becca Maher had qualified on the breaststroke leg of that relay and needed to swim that to compete in her mile. Caroline went on at night because she was the faster choice.
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