It’s called dolphin kick, fish kick, butterfly kick, and other names, but what everybody can now agree upon is that underwater dolphin kicking is the fastest form of swimming. In this post I cover the history of underwater kicking, why its so fast, and which body orientation is best.
If you’re not aware of how fast underwater kicking is, here’s a great video. Hill Taylor does a 23.10 for 50m LC with a backstroke start. Give him a dive and this would be faster than the 50 Free world record.
Fastest 50m Underwater Dolphin Kick Hill Taylor
But how did we get to this point of discovering how fast underwater kick is? Well, it turns out that underwater kicking has a long and fascinating history, probably much longer than you thought.
There is some controversy surrounding the inventor of the dolphin kick. One legend has it that Canadian George Corsan, an instructor and pool designer for the YMCA was teaching “fishtail” kicking in swimming way back in 1911. Another legend has Jack Sieg of the University of Iowa developing it much later in 1935. They both could be true. But both had a common problem. They had no place to use this kick!
Dolphin kick was much slower than flutter kick for swimming freestyle or backstroke, and there was no such thing as butterfly yet. The only other stroke, breaststroke, required whip kicks. So that wonderful insight into swimming (perhaps invented twice!) was lost to the world for until the official recognition of butterfly in 1952.
The next big innovation came about in the early 1950s, when it was realized that breaking the surface increased friction and actually slowed down the swimmer. This spawned the idea of swimming breaststroke underwater for as long as possible, surfacing, then going under again. This reached its peak in the 1956 Olympics when Masaru Furukawa of Japan swam the first 3 lengths of the 200 breaststroke by going 45m underwater and coming up to turn. On the last length he could only go 25m underwater and had to actually swim breaststroke for the last 25m, winning by a body length.
1956 Melbourne Olympics – Men’s 200m BR Click on picture in 8th row from top, second from left.
Within a year the rule was changed to restrict breaststrokers to only one arm pull and one kick underwater after the start and each turn.
The next major impact on underwater swimming then occurred in the late 1970s by American Jesse Vassallo (pictured above. I once raced him – he crushed me!) Vassallo started doing a few dolphin kicks off his starts and turns, but not for speed. He said he was using them to avoid waves from bigger swimmers, and to help stabilize his body prior to starting his arm strokes. Since he never pushed the underwater aspect too far, nobody at the time caught on to its potential.
Nobody, that is, except for Daichi Suzuki of Japan (pictured above). Suzuki started exploring Vassallo’s idea, but pushed for underwater dolphin kick for far longer distances. By the 1984 Olympics, Suzuki was going roughly 25m underwater quickly, but didn’t make100 backstroke finals and so the world didn’t really notice. Some swimmers took notice though.
Shortly after those Olympics, American freshman David Berkoff and his Harvard coach started using fluid dynamics experts to help develop an underwater kick. Berkoff met with quick success, and the media, ignoring Vassallo and Suzuki, dubbed the technique the Berkoff Blastoff. Unfortunately, that name stuck.
Once the world found out about this, many more backstrokers started practicing underwater dolphin kicking, and by the time the 1988 OIympics rolled around, 5 of the eight finalists went at least 25m underwater off the start. Check out this video for a look at what must be one of the most unorthodox and chaotic beginnings of a 100 backstroke race ever. Suzuki ended up out touching Berkoff for the gold.
1988 Seoul Olympics – Men’s 100 BK
The world swim organization, FINA, quickly met and within a few weeks moved to restrict backstrokers to a maximum of 10m underwater for “health and safety reasons”. In 1991 they moved this distance out to 15 m. Interestingly, at that time there was little interest in underwater for freestyle or butterfly, and so no restriction was put on underwater distances for those stroke. It wasn’t until the late 1990s that face-down dolphin kick started catching on, and in 1998 the 15m limit was extended to these strokes as well.
The next underwater controversy arose in 2004 at the Athens Olympics, but this time the stroke was breaststroke. At that time, no dolphin kicks were allowed during the breaststroke pullout following starts and turns. However, Kosuke Kitajima was caught on video doing at least one dolphin kick during the start and turn pullouts in his 100 breaststroke victory. The world clearly saw these infractions, but being in the middle of the pool, the officials could not see them. (Officials are also not allowed to use video, so blatant cheating was witnessed by the world, with no repercussions on the swimmer.)
2004 Athens Olympics – Men’s 100 BR video clip
Following this controversy, FINA met and this time changed the rules to allow one dolphin kick during each pull out, as well as to deny officials the chance to view video as part of their judging. This issue continues to rage on even now, as the 2012 Olympic 100 Breaststroke winner Cameron van der Burgh admitted to cheating in taking more than one dolphin kick during his race (video shows him taking three), while pointing out that all of his competitors did the same (they did).
Enough of history. Let’s explore why the underwater this amazing dolphin kick is so fast.
How It Works
So how does this simple kick let us go so fast?
The key, surprisingly, is that the kick doesn’t actually cause the high speed, but rather it helps carry the high speed you have from the dive or the push off the wall. Let’s look at these numbers (see here and here):
Max speed in air after a dive: ~6 m/s
Max speed during glide phase after a dive: ~4 m/s
Max speed during glide phase after a turn: ~3 m/s
Max underwater kick speed: ~2.2 m/s
Max swim speed: ~2.1 m/s
So let’s work through the numbers.
If you did a 50m race with a dive, and you could hold your glide speed the whole way without slow down, you could finish the 50m in about 12 seconds.
If you did that 50m with a push off the wall and you could hold your glide speed the whole way without slowing down, it would take under 16 seconds.
This means that the purpose of underwater kicking is NOT to generate speed (you’re already much moving faster than you can swim!) but rather to delay the slowing down process as long as possible. And to do this you need to provide a propulsive force to offset the resistive force of the water. This concept is very important, and it leads to three key elements:
- you want a tight streamline in order to minimize the water resistive force
- you want a powerful kick to help offset the water resistive force
- larger kicks tend to be more powerful and create more water resistance
Here’s an explanation of that last point. A streamlined body moving through water will have a smooth laminar layer of water flow around it, producing relatively little drag (see above). However, if you break through that laminar layer you create turbulence, and the drag is increased significantly (lower 3 pictures on the right). This means that a small, fast kick that keeps the body within the laminar layer will minimize the water resistive forces, and a large kick will tend to disturb the laminar flow and cause turbulence and much higher resistive forces.
Studies done by Ryan Atkinson, biomechanics expert at the Toronto National Training Centre, have come up with a few other tidbits for us also (see here):
- symmetry between the backward kick (feet moving from in front of us to behind us) and the forward kick (feet moving from behind us to in front of us) is very important, but this is difficult as the quadriceps powering the forward kick are 30-60% more powerful than the ham strings which power the backward kick
- high toe velocity correlates to fast underwater speed
Studies show that up to 90% of the thrust is coming from the feet during the kick. But this has often caused people to misunderstand the fluid dynamics at work. The feet are only the last element at the end of a chain of muscular contractions.
Think of kicking a ball on land. The key to a hard kick is to generate maximum toe speed, just as we want with underwater kicking. The kick on land starts with a tightening of the core, then the hips drive hard and stop, upper legs take over the drive then stop, and then the lower legs, and finally the feet whip through. But it all starts with the core and the hips. If you use the core and snap the hips, you can get higher toe speeds. We need to apply exactly the same idea with underwater dolphin kicking, except that we are repeatedly kicking balls forward and backwards
Front, Back or Side?
There has long been a debate about which is the best body orientation for underwater kicking: on your front, back or side. The answer, as it turns out, depends a lot on how deep you are.
We’ve known for a while that there is less drag when underwater than on the surface. But the difference in drag forces is surprising. One study determined that there is 5 times the drag on the surface compared to ~1m below the surface. This is largely because of turbulence from surface waves, as well as energy lost to the air in the water – air boundary.
When kicking near the surface, kicking on your front or back produce surface water disturbances which also causes small air disturbances. And kicking air really doesn’t help us. However, kicking on your side doesn’t produce the same disturbances as the kicking forces are parallel to the surface. So for shallow depths, kicking on your side is better. Although caution should be used as it is not uncommon to kick off course when you can’t see the line on the bottom of the pool)
Some have suggest that kicking on your side evens out the forward and backward kicks, as gravity isn’t affecting one direction more than the other. But as Atkinson pointed out, the disparity in forward and backward kicks isn’t caused by gravity, but by the muscles powering the two kicks.
So if kick symmetry is important, then kicking on your back should be the fastest (if we ignore depth) as gravity helps to balance out the muscle strength disparity. The stronger forward kick works against gravity, while the weaker backward kicks works with gravity.
There is one other phenomena contributing to faster kicking on your back. Our spines form the structural basis for the torso, and when in the water our torso basically hangs off the spine. So when we’re on our back, the soft tissues in our torso are resting on the spine. This results in little sagging caused by gravity. However, when we’re on our fronts, the soft tissues in our torso now hang down off our spine. Training, and especially core training, can help minimize that sag, but some amount will still be there. So kicking on our backs should naturally provide a better streamline, resulting in faster speed.
So which is best? Practically speaking, for free and fly since you push off on your side, you should maintain the side position until you get to a lower depth (of 1 metre or deeper), when a smooth transition to your stomach can take place. For backstroke it makes sense to just stay on your back the whole time.
Our knowledge of how underwater kicking can be incorporated into swimming is still very young. I think we can expect more surprises as we continue to learn. But what all of us should do is continue to experiment. We need more pioneers ready to try new things. We need someone to discover the next major innovation in swimming.
21 thoughts on “More Than You Want to Know About Underwater Kicking”
still not sure how to email you about discussing your article at a class on campus in Huntsville
As a fellow nerd (I use that term respectfully), I must say this is another great article. Thanks for your dedication. Very informative.
You say that turbulent flow has MORE drag than laminar flow? This is opposite how it works in air (which is why golf balls have dimples and race cars have spoilers – both trip turbulent flow earlier and the air hugs the boundary layer instead of diverging). So unless hydrodynamics are fundamentally different than aerodynamics (I’m sure they are in some ways) I think you are wrong about this point.
Hi Craig. Thanks for your comments. I should have provided a more complete explanation.
Your assertion that turbulent flow has less drag than laminar flow isn’t true all of the time. Aerodynamics and hydrodynamics would be a lot simpler if that were the case.
Let me explain. Laminar flow actually has less drag than turbulent flow, but only as long as the laminar layers haven’t separated. And these layers want to separate whenever the flow changes directions. When that happens, the drag goes up very high. You are very right in pointing out that this is why golf balls have dimples. The dimples, or surface lines on a shark, or the patterns on a racing suit, are sometimes called turbulators. Their purpose is to created small amounts of turbulence as a buffer zone next to the boundary layers, and thereby smooth out the laminar layers. This smoothing creates less separation in the laminar layers, and therefore less drag.
Spoilers on cars, besides creating a strong downward force to keep the tires on the road, perform the same function a little differently. The back half of a car usually has a fairly steep slope, and this sudden change in air flow direction has the laminar layers separating. The spoiler acts to lessen this slope, causing less disruption to the laminar layers, and therefore reducing drag.
Now for a swimmer doing the dolphin kick we are talking about something completely different. An overly large dolphin kick will cause significant turbulence that interferes directly with the laminar flow, causing significant distortion to, and separation of the laminar layers. This shoots the amount of drag very high. It’s for this reason that we want smaller kicks, and ones that are generally trailing the body, and not deviating far above or below the body.
The anatomy of the leg makes the forward and backward movements fundamentally different. The foot works more efficient when moved forward. Although symmetry of movement is essential why is symmetry of force needed?
This is a pretty complex area, as witnessed by the many studies, models and theories on the subject. In simplest terms, I look at the kick as having two separate components. An upkick and a downkick. For maximum propulsion, we want both kicks to be powerful. The problem, as you point out, is that one kick is far more natural, while the other is far less natural. However, that doesn’t change the fact that maximizing the propulsive force of both kicks will increase the overall effectiveness.
You mention symmetry of force as being ideal, and that is true. Think of it this way. Your natural kick will have your foot travel a certain distance and provide a certain amount of force. The distance your foot travels on the reverse (unatural) path has to be the same distance in order to get back to the same starting point. So ideally you want to provide as close to that natural force as you can. This maximizes the overall force you produce.
Amazingly, there is another sport that has almost an identical system at play. Cycling. When clip-in shoes are used, the foot is locked into the pedal. The downstroke with each leg is a far more natural, gravity-aided, push, and provides a significant amount of force. However, by being locked in to the pedal, the cyclist can also provide an upforce on the pedal. Together the upstroke and downstroke forces combine to provide higher force than just downstrokes alone.
I have heard it stated often that equal, or similar, power/emphasis/force has to be used for both the upkick and the downkick. If you ignore the foot and look at the force vectors that are produced during the kick it seems to me that there is only a very small backward component (thus producing a forward force directed to where you want to go). Most of the force vectors will be directed either upward or downward. If you add the foot then the “natural” kick will produce a bigger positive force vector. However, the force vector of the “unnatural” kick will be affected much less. In the original article by Atkinson et al (Human Movement Science
Volume 33, February 2014, Pages 298–311) they show a swimmer with his legs at the maximum “natural” extension. (I could not understand their explanations otherwise I would not be discussing this) At this moment the foot is in a relative relaxed position. Moving the leg in the “unnatural” direction is accompanied by plantar flexing the foot. It seems to me that his is the only moment in which a decent positive force vector can be generated. If this is true than shouldn’t coaches recommend that during the kick the legs travel the same distanced up as down. But that the emphasis has to be on kicking hard in the natural way. Continue kicking hard when the direction of the movement changes (into the “unnatural” way).Then backing off as most of the muscle power will be used for up and downward forces, places that we don’t want to go.
In cycling the downstroke and the upstroke will produce the same forward directed force vector (although as you explained of smaller magnitude for the upstroke).
I believe I can see your point. That so little useful force can be generated in the unnatural foot direction that we should only emphasize the moments when the feet can provide a useful force.
There are 2 aspects of the kick that we haven’t really talked about.
1. The kick is to maintain speed, and not generate speed. This has enormous implications. First off, we should be trying to generate useful force in order to offset water resistance at all times. Any point when we are not generating a useful force, we are slowing down more quickly than we want to. So this means we need to use the relaxed and floppy feet to generate the force in both directions.
2. We can think of the hips, legs, feet as a whip system. If we are using a whip, we don’t really focus on the tip (analogous to the feet), but rather we focus on the handle (analogous to our hips). By moving our hips up and down quickly, even violently, we are initiating the start of the whip motion. Our legs are really just there to transfer the momentum to our feet. And the role of our feet are to be relaxed to provide maximum travel distance. The analogy breaks down somewhat at this point, because we can aid this travel distance by forcing our feet up or down to generate the useful force. (I believe you call this the positive force vector).
When we put this together, we get a kick that starts at the hip, and travels down the legs to our fast moving feet. Relaxed feet, aided by muscular effort in speeding up the foot movement, end up providing the useful force. The fact that the legs are not as efficient in the backward motion is a factor, but not nearly as much as you’d think as the legs are not initiating the movement. By emphasizing both directions, we have a much better opportunity to offset the continuous water resistance that our body encounters.,
Your are painting a very nice picture! I would like to add that the arms and shoulders provide a stabilizing force to the undulating movement of the rest of the body. They also might add some propulsive force through the reverse body wave described by Maglischo. Through an intricate series of muscle tension and relaxation events the movement starts in the core, propagates through the hips and legs and terminates in the feet. The forward/natural movement is accompanied by substantial leg muscle action whereas this is less pronounced for the backward/unnatural movement. This later action is mainly driven by the core and the hips. Its purpose is not to actively displace water but rather to bring the legs and feet back into the starting position for the forward/natural kicking action. Thus it has to be a fast but not forceful leg movement.
You raised a great point. The arms, shoulders, and I would add, upper torso provide a stabilizing force. My only comment in slight contradiction with Maglischo, is that those components should be rock solid, and so should not be providing any propulsive force (a stable position cannot create any propulsive force). The issue is that the upper body and especially the arms define the initial envelope of the newly created laminar layers. We don’t want any movement in our body here, as the laminar layers can easily separate, causing drag to drastically increase. You can easily see this in young swimmers who undulate their arms, head and shoulders when they dolphin kick underwater. Their speed is drastically slower than those who hold a more stable position.
Oh, one more thing.
Ryan Atkinson provides a great, scientific explanation of the forces in this SwimmingScience interview. You can find it here.
Hello, coach Rick Madge.
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Glad you liked the article. Yes, you can translate it. Please include a link to my post, and a statement saying “Original Post by CoachRickSwimming.com”
Thank you, sir. And I’ll surely include the link and appreciating words.
Thank you for the article.
Can I translate your article, and share my blog?
I will surely include the your link.