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Is This New Swim Stroke the Fastest Yet?

The surprising performance and physics of the fish kick.


I tug my black swim cap over my hair, strap on my pink goggles, and keep a focused calm, like Michael Phelps before a race. It’s lap swim on a Monday afternoon at my local YMCA, and I’m going to attempt the fish kick. Most fish move through the water with a horizontal wiggle. The fish kick challenges you to copy this movement: You completely submerge yourself underwater, position yourself on your side, keep your arms tight above your head in a streamline, and propel yourself forward with symmetrical undulations. After decades of swimming, some of it at the competitive level, I think I might have a shot. Pushing off the wall, and after what I can only describe as a struggle, the water resists my forward motion and I float to the surface, not unlike a dead fish.

Humans are land animals, and not natural swimmers. We have to learn how to swim, and it is up to us to find the fastest way to do so. The search may finally be coming to an end. In the last few decades, stroke mechanic experts have discovered that swimming under the surface is faster than swimming on the surface. “It’s hard to fathom that this could happen in track and field,” says Rick Madge, a swim coach and blogger. “Nobody is going to come up with a new way of running that is going to be faster than anything else. Yet we just did that in swimming.” And the fish kick may be the fastest subsurface form yet.

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But it’s also difficult. After my failed attempt at the YMCA, I reach out to Misty Hyman, who won gold in the 2000 Olympics using the fish kick in the underwater portion of her race, and is now an assistant coach at Arizona State University. She agrees to give me a lesson at an indoor pool in New Jersey, where she is leading a swim clinic. I find her there wearing her gold medal, surrounded by young swimmers asking for her autograph. She waves me over with a smile, which puts me at ease. I almost forget that my mediocre skills are about to be scrutinized by a world-class swimmer.

It is both natural and disconcerting to see a human form move this way.

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We head over to a swim lane, but before we jump in, she offers some tips. To do the fish kick, she says, I need to hinge at four places: shoulders, ribs, hips, and knees. I pull my arms together over my head to make a streamline, and tilt my head and arms back and forth in a single motion. She tells me to tighten my shoulders. I do. “Tighter still,” she says. I pull them in so hard it almost hurts. “That’s right,” she says. She tells me I’m good at the rib hinge, but it still requires my full concentration, and it feels like I’m collapsing my organs with each bend. I try the hip and knee hinges, which come more naturally.

We hop into the lane and do a couple warm-up laps. Then she shows me her fish kick. She pushes off the wall and glides away into the blue like a minnow. It is both natural and disconcerting to see a human form move this way.

Until recently, competitive swimming has focused almost entirely on what happens at the surface of the water. In early 19th-century England—which many consider to be the birthplace of the modern sport—swimmers raced using the breaststroke. A few decades later, Europeans learned a faster stroke when two Native Americans visiting London demonstrated a way of swimming they had learned growing up: the front crawl. One observer wrote, “they lash the water violently with their arms, like the sails of a windmill, and beat downwards with their feet, blowing with force, and forming grotesque antics.” The Brits eventually got over their shock. The backstroke came next, followed in the early 20th century by the butterfly stroke, which overcame the drag of the underwater recovery required by the breaststroke. The butterfly became the second fastest stroke after the front crawl.

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All swimming at the surface shares the same speed restriction. “You’re always limited by your hull speed,” says Ryan Atkison, a sport biomechanist at the Canadian Sport Institute Ontario. It’s a nautical principle that also applies to swimmers. The theory goes that a swimmer on the surface cannot go faster than the bow wave that he or she creates. The bow wave increases with swim speed until, in theory, it stretches along the whole length of the swimmer’s body. Atkison says that the maximum speed is one body-length per second, which is about 1.9 to 2.6 meters per second for a swimmer about 2 meters (6 feet, 5 inches) tall.

“You can’t go any faster than that unless you climb up over top of that wave,” says Atkison. “Some animals can, like dolphins can porpoise and jump over top of that bow wave, but humans can’t physically climb out of that trough,” he says. “The only real way to get faster is to be better under water, where we don’t really have those upper limits on speed.”

Coaches began to take advantage of this fact in the 1980s, when Harvard University coach Joe Bernal realized that some of his swimmers were faster if they stayed underwater and dolphin kicked. This is essentially identical to the fish kick, except that the swimmer is flat on his stomach, rather than turned on his side. Some especially strong underwater swimmers stayed submerged almost the entire length of the pool, since there was no rule against it. That all changed in 1998, when FINA, the world governing body of competitive swimming, ruled that swimmers performing the backstroke had to surface after 15 meters.

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Hyman came of age as a world-class swimmer during the underwater revolution. “I was 13 when I started staying under water longer than is typical,” she says, explaining she could go 30 meters without breathing. “I found I could be faster under water than at the surface.” Most swimmers were using the dolphin kick to propel themselves underwater, but Hyman’s coach, Bob Gillet, wanted to experiment. In 1995 he came across a study in Scientific American about how tuna were able to swim at almost 50 mph, where dolphins top out around 25 mph. The study found that the flick of a fish tail generated more efficient thrust than that of a marine mammal tail. Gillet wondered whether the dolphin kick might be more powerful on its side, so the undulations were horizontal, like those of a fish.

One cool December day in Phoenix in 1995, Gillet put it to the test. Hyman showed up for practice at Gillet’s outdoor pool, and he asked her to try it. “In the most respectful way, I called him a mad scientist,” she says. Her first attempts were awkward, and she ended up three lanes over from where she started. But she got better, and soon she was cutting through the water like an eel. She was going faster than she did with the dolphin kick. Faster than she had ever swum before. This gave Gillet another idea.

They went to the local country club pool, where the lighting was brighter and Gillet could walk out to the edge of a diving board to capture video. They took a long, thin rubber tube, fastened it to Hyman’s wrist, ran it down the length of one side of her body, and fastened the other end to her ankle. Then they filled the tube with store-bought food dye, and Hyman corked the tube with her thumb. She jumped into the pool, released her thumb, and took off as Gillet filmed. What they saw in the footage afterward astonished them. The dye swirled out to reveal huge vortices after each of her horizontal kicks. Gillet suspected that these miniature whirlpools, reaching 4 feet in diameter, propelled her forward. He also thought it was possible that when Hyman did the dolphin kick facedown, the bottom of the pool and the surface of the water interfered with these vortices and slowed her down.

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Once perfected, the fish kick may be hard to beat.

Although these ideas remain debated to this day, the fish kick has continued to gather fans. Luc Collard, a professor of sports science who has experimented with many different ways of swimming underwater—including swimming the fish kick with the arms down, along the swimmer’s side—says it may just be the fastest. “Its potential is impressive.” Raúl Arellano, a professor of physical education and sports at the University of Granada, agrees. “Lateral kicking is found to be quicker in some swimmers,” he says. “The hypothesis is that these vortices are not perturbed by the water surface and the pool bottom.”

Vortices are an inevitable consequence of moving through water. Some are counterproductive, impeding the swimmer’s motion. Others help propel the swimmer forward. “Vortices represent the transfer of momentum from a body into the water and vice versa,” says Atkison, “and thus can both propel swimmers forward and also slow them down.” The dolphin kick is more likely to send helpful vortices up to the pool surface, where they dissipate into waves, and down to the pool floor, where they create turbulence. The fish kick, by comparison, is more likely to send these vortices sideways, parallel to the water’s surface, where there are no obstructions. “Anything that makes waves on the surface is detrimental to swimming efficiently,” says Rajat Mittal, a computational scientist at Johns Hopkins University. “If you are swimming within a foot of the surface, there is a bigger chance that [you] will create waves on the surface than by kicking sideways.”

Atkison also believes that the fish kick is able to produce larger and more propulsive vortices when the swimmer finds herself in shallow waters. While the dolphin kicker draws in the water for her stroke from the volumes above and below her body, which are limited by the pool surface and floor, the fish kicker draws in water laterally and relatively without restriction.

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However, the dolphin kick does not always suffer from these disadvantages. Swimming at a depth below 1 meter greatly reduces any surface turbulence and any difference in the volumes of water drawn, and Olympic and World Championship pools now are at least 2 meters deep. The fish kick is also inherently more difficult. “The problem is that the swimmer cannot control its swimming direction and body position as compared with ventral underwater undulatory swimming,” says Arellano. All that being said, though, once perfected, the fish kick may be hard to beat.

Uncovering the fish kick’s true potential will require an event dedicated to underwater swimming. But many are skeptical that will happen. “Would that be a good sport on TV if everyone is swimming underwater?” says Mittal. “Part of the excitement of swimming is watching them, seeing that they see each other.” Hyman disagrees. She would love to see an underwater race incorporated into professional swimming, and believes that it will be intriguing in its own way. “You still don’t know where they will pop up. It actually adds a level of excitement.”

Back at the pool in New Jersey, I’m wondering where I will pop up.  I try four kicks, then quickly become disoriented and surface a grand total of 12 feet down the pool. Not great, but farther than what I did at the YMCA. Hyman tells me that when she swims the fish kick, she thinks of moving through invisible hula-hoops, undulating at the four points we had discussed. She also recommends kicking with equal strength on both sides, which should help me go straight. This seems more doable than the hula-hoops. I try it again. This time I focus on making both kicks strong, and I go straighter and farther than I had before. Almost 15 feet.

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“That’s great! Now try a dolphin kick and compare how different they feel,” she says.

This time when I’m under, I push off the wall face down and kick my legs up and down in sync. I imagine the whirling vortices, breaking apart as they smash into the pool floor below and break the surface of the water above. I can almost feel it slowing me down.

Regan Penaluna is a senior editor at Guernica magazine.

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