Why dolphin kick is faster on your back

November 01st, 2017

Courtesy of Gary Hall Sr., 10-time World Record Holder, 3-time Olympian, 1976 Olympic Games US Flagbearer and The Race Club co-founder.

Analyzing the dolphin kick of world champion backstroker, Junya Koga, while on his stomach, he generates acceleration of about .7 m/sec2 on the up kick and 14 m/sec2 on the down kick, a significantly greater difference than one would expect on the basis of strength alone.

The up kick results in a peak velocity of about 1.5 m/sec while the down kick results in a peak velocity of over 2 m/sec. However, like acceleration, a better representation of the power of the kicks is the difference between trough and peak velocities from both the up and down kicks (Delta PT). For the up kick, the delta PT is a trivial .1 m/sec and for the down kick, it is around .8 m/sec, also a significantly greater difference than one would expect based purely on strength.

When Junya dolphin kicks on his back, we find an extremely different velocity curve. Now, on the up kick, the stronger motion, we find a peak acceleration of around 3 m/sec2, while on the weaker down kick, we find an acceleration of around 5 m/sec2. The peak velocities of the down kick are also greater than the up kick, 2.1 m/sec compared to 1.9 m/sec. The delta PT is still greater on the up kick, but not by much, .4 m/sec compared to .35 m/sec. All of this suggests that the propulsion from the weaker down kick while dolphin kicking on the back is about the same or greater than the propulsion of the stronger up kick.

With the vast difference in biomechanical strength between these two motions, how can this be?

It cannot be explained by a difference frontal drag, since the body positions are very similar. One coach, Rick Madge has proposed that the differences in power comparing the up kick and down kick while kicking on the back versus the stomach can be attributed to gravitational force. I don’t agree.

While gravity still applies in water, the actual force in water, reflected by our body weight, is considerably different. While the legs have negative buoyancy, they probably weigh only a few pounds in the water. That is not enough to affect our ability to kick up or down in water. I believe the differences observed on the velocity meter studies from front to back can be attributed to the vortices formed behind the body and feet of the swimmer.

When Junya is on his stomach, the down kick begins with the knees bent and the feet pushing back against the stream of water moving forward behind the body. The result from this strong motion against a current of water results in an extraordinarily strong surge of power and speed forward; more than one would expect from just the biomechanics.

With the up kick, the feet begin the upward movement below the stream from the body’s vortex and do not produce any meaningful propulsion until they enter the stream. By that time, the amount of propulsion is significantly less than that provided by the down kick. However, a strong upward and forward movement of the feet will create another vortex that will contribute to the stream and result in a greater force with the following down kick.

While on his back, Junya’s up kick begins with the feet below the stream and consequently, the feet do not produce as much force as when they are pushing against the stream. Again, the up kick will add even more power to the stream from the stronger vortex following the feet. When he begins the weaker down kick, he is now pushing against a substantial forward movement of water, almost as if he were pushing against a wall. As a result, there is a greater surge of velocity after the down kick than one would expect from this motion.

While all of these differing vortices may change the fluid mechanics of the kick, the important question is, which way is faster? In this particular study, Junya’s average dolphin kick speed on his stomach was 1.76 m/sec. On his back, it was 1.81 m/sec. .05 m/sec difference may not seem like much, but on an underwater kick off a start or turn lasting five seconds, that is 10 inches further ahead or behind that the swimmer would be; enough to win or lose a race.

I suspect that the difference in a swimmer’s speed from stomach to back has more to do with the law of inertia than to any difference in biomechanical strength or frontal drag. The lower delta PT on the back simply means that the kick is more efficient than while kicking on the stomach, since the swimmer maintains a more constant speed.

For completeness sake, we also tested Junya on his side and found that the velocity curves are similar to the ones on his stomach. The average velocity was measured at 1.71 m/sec, slightly slower than on the stomach, so there does not appear to be any clear benefit to kicking on one’s side compared to the stomach. Since the rules preclude us from remaining on our backs dolphin kicking during the underwater portion of a freestyle or fly race, we cannot recommend using this technique on any race other than the backstroke.

Ryan Lochte and other great backstrokers have figured out that they can kick dolphin kick faster on their backs than on their stomachs or sides. Now we know why.

Yours in swimming,

Gary Sr.

Gary Hall, Sr., Technical Director and Head Coach of The Race Club (courtesy of TRC)

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OFIR_VEK
6 years ago

Hi,
My name is Ofir and I am a student for physical education, competitive swimmer and aslo taking a course for competitive swimming coaches. I read the article and yet few detais are not fully explained and I’d like to understand them better.
1. what does the peak acceleration stand for- what was measured?and how ? so does the avg. speed both back and stomach?
2. while on the back ,does the gravity has role in downkick comparing to stomach? this is the reason why downkick on the back is faster than upkick on the other side?
3. how can the difference between the speed of upkick on the stomach and the downkick on the back be explained?

igor kuksa
6 years ago

in honor of,
Gary Hall,

Greetings,

If I may express a different opinion on why the swimmer progresses faster with a dolphin kick on the back than on the stomach; the reason for faster progress on the back than on the stomach, why in the acceleration of a stronger kick – progress is lower, and with a weaker kick the progress is faster?

The answer lies not in inertia or in gravitation. These solutions do not give a true answer to this paradox.

Every movement has a general mechanical force of movement and is at a 90 degree angle to the movement’s support area. But in swimming – this power is not always parallel to our progress direction;

A… Read more »

OFIR_VEK
6 years ago

Hi,
My name is Ofir and I am a student for physical educationl, competitive swimmer and aslo taking a course for competitive swimming coaches. I read the article and yet few detais are not fully explained and I’d like to understand them better.
1. what does the peak acceleration stand for- what was measured? ? so does the avg. speed both back and stomach?
2. while on the back ,does the gravity has role in downkick comparing to stomach? this is the reason why downkich on the back us faster than upkick on the other side?

sven
6 years ago

Someone else alluded to this in another comment, but I think it’s due to the buoyant force pulling the body up. On your stomach, the stronger front-kick aids the buoyant force, increasing the magnitude of the vertical component of motion. This could cause the swimmer to either surface earlier or force them to angle down to stay underwater, which increases frontal drag (decreasing horizontal velocity) and complicates the breakout.

On the back, the stronger front-kick resists the buoyant force and decreases the magnitude of the vertical component of motion. If the overall magnitude of the swimmer’s velocity is the same in both scenarios, the swimmer with the smaller vertical component is going to have the larger horizontal component.

All that… Read more »

M L
6 years ago

I think several commenters are picking up on an explanation along these lines. Shifting the hands, arms, and shoulders toward’s one’s back/dorsal side during the weaker (sole-first) kick serves to counterweight and thus strengthen that kick, and it also enables the swimmer to “catch” the water with their arms and chest and use them as a lever during the stronger kick. You can do this on your stomach, too (Tom Shields has talked about Caeleb Dressel kicking on his stomach as if he’s on his back), but two other factors then come into play: (1) gravity doesn’t assist the setup of the arm-chest catch when you’re on your stomach, and (2) setting up the arm-chest catch on your stomach may… Read more »

6 years ago

Nobody explained to me dolphin kicking like an ex-Longhorn backstroker who made his living underwater: “You need to catch water with your body and release with your feet.” That is why you see the best of them with large undulation, such as Chad, Phelps, Lochte, catching and releasing more water. I do believe large kicks are faster then smaller and faster, for that reason. But, you have to have flexibility and mental toughness to do it. I tell my children to dolphin kick “from your heart” meaning from hear down undulation, from heart up perfect streamline, and with heart (tough).

Dude
6 years ago

Maby not for everybody ? My 25meters stomack kick is 10.7 and back kick is 10.6 so there is only 0.1second differece ?

Simon Thirsk
6 years ago

I think you should test Chad Le Clos on his stomach, as I am sure you will see a much greater strength than Junya, who trains predominately on his back. Chad is very powerful and you will also note that his motion is very long, versus many who do smaller “more powerful,fast” kicks.

Andrew Alberico
6 years ago

I always felt a little more urgency on my back underwater. Perhaps it was psychological because I knew I only had a limited amount of fly kicks before I had to surface – I needed to make the most of them! On my stomach I knew there was more ahead of me.
Also a natural aversion to water up my nose always intensified things 🙂
Btw- thank you for your absolute devotion to helping swimmers maximize their performances with applied science. So impressive.