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.
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.
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.)
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.