Is sailing the hardest sport in the world?

. . . matter of opinion. I vote for gymnastics - requires near total lifelong dedication and peak physical conditioning to even attempt some of the moves.

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I agree -gymnastics ages the body faster than any other sport.

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In my opinion I would say that swimming is right up there with gymnastics. Like gymnastics, it is one of the few sports that actually require you to use all of your muscles not just a certain group of muscles. it may not be high impact but training for swimming includes running, weights, biking, and everything else you can think of.