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Much of what makes Katie Ledecky and Michael Phelps the best and fastest swimmers in the world is up to them.
They're the ones who've done the training and developed the technique that propels them through the pool. It's their competitive spirit that pushes them to race against not just their competitors, but themselves.
But for them - and any other swimmer - to hit the fastest times ever, there are external factors that matter too. Like the pool.
Some pools are significantly faster than others, according to Dr. Michael Joyner, a physician and Mayo Clinic researcher who is one of the world's top experts on fitness and human performance.
Why the venue matters
One of the first factors to make a huge difference in freestyle times was the introduction of poolside gutters, according to a TED Talk by "The Sports Gene" author David Epstein. Those gutters allowed the water splashed up by swimmers to run off the sides of the pool instead of splashing back and forth, creating turbulent waves. A smoother surface is much faster for swimmers, as they don't have to battle through choppy water.
If the pool is empty, "you really want that pool to look like a lake in the Midwest in the summer at six o'clock in the morning," Scott Hester, president of a company that designs aquatic facilities, explained to Sports Illustrated.
Here are some of the other factors that can speed a pool up - or slow it down:
- Depth: You generally want a pool that's at least three meters deep; the deeper the better.
- Lane lines: The barriers between lanes help calm waves down before they travel into the next lane.
- Buffer lanes: In many important races, the lanes closest to the edge won't be used, so that a swimmer isn't creating waves against a wall.
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How the Rio pool compares
The pool used at the Beijing Olympics in 2008 - where 29 new records were set - is considered by some to be the fastest pool in the world, as Tom Taylor explains at Sports Illustrated
But the Rio pool is "a very good pool," US women's swimming head coach David Marsh told Taylor.
Taylor explained that a couple factors could still make those waters a little calmer and faster. There are flat walls at each end, which can create some waves. And the circulation system is cranked up a bit higher than it is at some other venues (though that still doesn't seem to have stopped the green water problem).
The Rio pool is three meters deep though, and as we've seen, athletes like Ledecky have set some pretty amazing records already.
In the end, the pool may not be that much of a limiting factor when you've got a person who might be the best athlete in the world swimming in it.