From what I've heard, those sprinters are terrified of something blowing out, because during many of those lightning sprints they feel their ligaments about to go. And with many of them competing in multiple medal events a la Michael Phelps, they don't want one particular race to become an all-or-nothing.
In 1996, I remember Michael Johnson similarly "slowing down" during heat after heat. During his famous 200, when he dropped a 19.32, he had a slight stumble at the start, so then he gave it his all for the entire race because he was afraid of finishing below 20.00.
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