Depends on when in history you think the first athletes appeared. In ancient Greece nude and oiled up was the most common sports dress.
Only men were allowed to compete in the Ancient Olympics and they didn't wear anything while competing, they went naked.
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athletes would be naked.
Gymnasts could roll their ankles if wearing shoes while competing in gymnastics. Gymnasts could get really hurt. It isn't save.
nothing
What they wear depends on what events they are competing in. Most athletes wear modern athletic attire that can include warm up suits, shorts and tanks.
Some people that wear compression shorts are athletes. Athletes wear these because they are nicer to run or do whatever they are doing with them. Makes it a lot easier on them.
Alot I can not name them all.
because to get the grip
the olympics have allowed women to wear religious head gear while competing
On a general basis at lower level competing, Men are known to wear white jodphurs, and either a tweed or black jacket, shirt, hat and boots etc. However this is only at a low competing level; the requirements for dress can differentiate as the shows progress.
nothing (They competed in the nude.)