It's being called the "best turn ever done." Halfway through his leg of the 4x100-meter freestyle swimming relay, Michael Phelps and the United States trailed France's Fabien Gilot. Phelps pushed off the wall and started kicking, staying underwater much longer than his competitors. When he finally surfaced, the US was ahead for good.
"In executing the turn, Phelps surfaced an astonishing 2.5 seconds after the Frenchman who touched the wall in the lead," Timothy Burke of Deadspin wrote.
In the swimming world, it's conventional knowledge that following the turn, dolphin kicks under the water are indeed faster than swimming on the surface. While on the surface, swimmers create a "bow wave" that slows down forward progress. There is also surface resistance—the friction of the top of the water pushing against the body of the athlete—that can be avoided by swimming underwater. Wave resistance increases as the swimmer's velocity increases, so simply powering through it on the surface isn't ever going to be the most efficient. In fact, swimming underwater became such a popular technique that it's now only legal to swim underwater for just 15 meters after diving into the pool or doing the turn.
But if underwater swimming is so undeniably better, then why was Phelps the only one who stayed underwater for so long? Sure, he's a freakish athlete, the best Olympian of all time. But his opponents are no slouches either—why did they decide to surface so quickly?
"If you're going underwater for a long period of time, you're putting yourself in oxygen debt and stressing your respiratory muscles"
I was surprised to learn that there's a slowly brewing controversy in the world of swimming science about underwater swimming strategy. Phelps's competitors surfaced earlier not because they are dumb, but because it was likely faster for their particular skill sets (it's worth noting that Phelps's time was only fourth best in the race).
Though we've known that dolphin kicking has had the potential to be the fastest method of swimming for about 20 years, it's an incredibly complex stroke, and few people, even today, do it with maximum efficiency for the entire 15 meters allowed in today's rulebook.
"It also has some drawbacks—if you're going underwater for a long period of time, you're putting yourself in oxygen debt and stressing your respiratory muscles," Mullen said, adding that unless it's executed perfectly, staying underwater for longer is not necessarily faster for every swimmer. "A few studies have shown it's the fastest, but there's still a generation of Olympians who maybe weren't getting underwater training when they were really young, really just learning how to swim."
That humans are bad at swimming (compared to like, a fish), leaving lots of room for both strategic and mechanical improvement is part of what makes Olympic swimming so inherently thrilling. There is now serious scientific research that's teaching us more about how humans can best move through the water, but unless you're a freak like Phelps, what makes the most sense in a vacuum doesn't always make the most sense in actual competitions.
"The 100 m faster swimmers at the World Championships did not spend longer underwater than slower swimmers"
A paper published in March in the European Journal of Sport Science titled "Do faster swimmers spend longer underwater than slower swimmers at World Championships" found that, though general underwater swim times have been increasing over the last few decades (by an average of between 2 and 6 meters more than in the past), the world's best swimmers spend slightly less time underwater in the 100 meter freestyle than their slower counterparts.
"The 100 m faster swimmers at the World Championships did not spend longer underwater than slower swimmers," Santiago Veiga, who studied underwater versus surface swim times at the 2013 World Championships, wrote. "These results were unexpected … elite 100 m swimmers may prioritize emerging to surface swimming instead of traveling long distances underwater in order to maximize forward speed."
Veiga, who studies swimming science for the Madrid Swimming Federation, noted that, basically, not everyone can pull off underwater dolphin kicks as well as Phelps can. This is likely because underwater kicking has long been a part of the swimming game of backstroke and butterfly specialists, because those are slower stroked than the forward crawl. Phelps is an excellent butterfly swimmer and honed his technique underwater in that event.
Veiga told me in an email that freestyle competitors have been slowest to embrace underwater swimming because a forward crawl is nearly as fast as swimming underwater—freestyle specialists don't see the same sort of speed advantage swimming underwater as someone swimming backstroke or butterfly would.
"Freestyle is a bit different because the surface swimming velocity is faster that in the remaining events (around 2 m/s in freestyle but 1.7-1.8 m/s in backstroke and butterfly)," he said. "Therefore, only if their underwater velocity is high (or they have high underwater kicking proficiency), they can take advantage of extending the underwater kicking. This usually occurs with butterfly/backstroke specialists who compete in freestyle events (Phelps, Jeremy Stravius of France or Chad Le Clos of South Africa. It is rare to observe great kicking in freestyle specialists."
Top-level swimmers should go underwater for as long as it's faster, and then they surface and continue once it's no longer faster to swim underwater; this of course makes a ton of sense, but it wasn't until recently that we began to see a noted preference for longer underwater swims in short freestyle events.
"When swimmers travel longer underwater they also obtain a positive effect on surface swimming, which is something we have just observed in our latest research," Veiga said. "Backstroke and butterfly swimmers who were better underwater obtained a positive transfer of momentum when recommencing surface swimming. Therefore, by doing the dolphin kick longer, Phelps reached faster underwater velocities but also reached faster velocity when recommencing surface swimming."
It's not as simple as "underwater is always better," especially when swimming techniques and swimmers' bodies all vary.
"Present research should be taken with caution as the underwater swimming strategies should be individualized according to the swimmer's drag characteristics or their kicking proficiency," Veiga wrote in his paper.
Mullen told me that, because humans are so inefficient at swimming, the sport is constantly changing. Sometimes, it takes decades for a particular technique to make its way through the ranks.
"Phelps's turn was important, especially for someone like him who is such a good underwater kicker. His underwaters are such an asset, and we're just now starting to see 100 meter specialists go a little further underwater," he said. "The swimmers who are in the 26-32 range, they weren't really getting underwater training when they were young. The 16-20 year olds who are at their first Olympics, they've spent their whole lives developing the ability to maintain speeds underwater."