The Dynamics of Dolphin Kick Part II: Why Dolphin is Faster on the Back

The Dynamics of Dolphin Kick Part II: Why Dolphin is Faster on the Back

Using The Race Club’s Velocity Meter technology, I analyze 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.

Read The Dynamics of Dolphin Kick Part I: Using the Vortex

Come to The Race Club and get Velocity Meter test done for yourself.

23 Responses to The Dynamics of Dolphin Kick Part II: Why Dolphin is Faster on the Back

1. Joe Rainero

When you took these measurements, did you measure the distance of the kick? In other words, how far did his feet go up and down when he was on his stomach versus on his back or side? If there is a difference in this, it may explain why one way produces faster results than the other.

• Gary Hall Sr.

The power of the kick depends on ankle (plantar) flexibility, knee bend and hip undulation. Now there appears to be a fourth component, given a swimmers leg strength, and that is the vortices or flow of water in the kicking path. Roland Schoeman measured a pretty consistent 120 degree maximum knee bend before the strong kick, while Junya is around 110 degrees in this photo. More knee bend than that and the brakes go on as you can see from the orange deceleration curves. With the one kick where his deceleration exceeds -10 m/sec2, his knees bent too much. overall, we did not find much differences in the mechanics of the kick whether on front, side or stomach.

I’ve long wondered why the dolphin kick seems to be faster on your back ‘no matter what’?? The only reason I thought made any (practical) sense was, that swimmers are generally kicking ‘upward’ toward the surface in an arc, and gained an advantage from that. BUT….Now Caeleb Dressel (40.46!!) and Ryan Hoffer (41.23!!) are swimming the 100 free with ‘big’ underwaters, and seem to be going JUST as fast on their stomach?? So, isn’t it possible to swim just as fast either way? What say you?

• Gary Hall Sr.

Fast dolphin kickers are fast on their stomachs, sides or backs…it doesn’t matter. In fact, the two you mentioned and Phelps, Lochte, Le Clos, Natalie Coughlin etc are faster underwater doing dolphin kick than they are on the surface doing freestyle. I am not sure that all dolphin kickers are faster on their backs, but those that do a lot of backstroke seem to be..probably for the reason I mention in the article.

3. Larry Walker

First, I agree that I’m faster with dolphin kicks on my back than on my stomach. However, it would help to understand the analysis if you had time ticks on the horizontal axis…
If I assume that each kick sequence shown (on-stomach and on-back) is over the same time frame, then the number of kick cycles on the back is a lot faster than on the stomach – don’t know if this is the case or not as there is no initial time value shown. So, how long was each kick cycle? Or kick cycles per second?

Could you label the vertical axes also?

• Gary Hall Sr.

In this study we were looking at the speed of undulating vs non-undulating speed. The cycle times were very similar for the undulating kicks, whether on stomach, side or front..about average of .6 seconds. Without undulation, whether on stomach or back, the cycle time reduced to .5 seconds. The speed was slightly faster with the non-undulating kick…although he was undulating…just less. The vertical axis is labeled on the far right side in m/sec for the velocity (green curve) and m/sec2 for acc/dec (orange curve) The speed is shown at each point and each click of our cursor adds .02 seconds to the position. The time shown is irrelevant as it denotes from when we started the program, not from the push off the wall.

• Larry Walker

Your graphic presentation is misleading because there are 12 peaks of acceleration on the stomach over the span of the graph versus 18 peaks of acceleration while on the back. This implies that the kicking on the back was more rapid. So, with your reply that the cycle time was the same, the graphic had to be stretched to show fewer peaks while on the stomach.
None-the-less, it’s an interesting analysis. Thanks for your work on this.

4. J

Thanks for the article Gary. I really enjoy reading these.

5. Steve Dueball

The only rule in freestyle is that you can’t walk on the bottom and you must surface before the markers on lane lines. I realize that turning from front to back would take time but I don’t know of a regulation that you could not stay on your back underwater in freestyle until the mandatory surface requirement. Am I wrong?

• Gary Hall Sr.

Apparently, as I understand Ryan Lochte was called for kicking on his back off the wall too long on a freestyle race.

6. Henry Fincher

Gary, could it not also be the case that the air held in the chest is aiding the initial “pop” in the dolphin movement from the chest when facing upwards? I’ve heard a number of swimmers talk about the motion beginning at the chest which in turn rolls the water down the body to the feet, creating a bigger kick. With the chest facing upwards you would be given more power from natural physics. Chest down and you have the opposite problem, you fight against the same force. Could the movement in the chest be measured facing both upwards and downwards?

• Gary Hall Sr.

Henry, I have never given much thought to the position of the air in the lungs on the back or stomach. The lungs should be pretty full of air, either way, and I doubt there is much room inside our pleural cavity for the lungs to move. The upper body stays pretty much in the same position during the dolphin kick, articulating at the hip. In other words… undulating in different directions. I really believe that the up kick on the back creates such a strong vortex adding to the existing body’s vortex, that the subsequent down kick is like pushing against a wall. Once the backstrokers feel that, they work the down kick even harder to gain more speed.

7. trk

Hi Gary , Firts , i want to thank you for all this knowledge and tips that you share with us , it really helped me and i m sure many other people too , My question is if the dolphin kick in the back is faster so why coaches dont teach their swimmers to do dolphin kick in back in flip turns like in freestyle maybe in butterfly too . thank you

• Gary Hall Sr.

Actually, it is legal to remain on the back during the first fifteen meters in any freestyle or backstroke event. It is not legal to do that in any other event in butterfly, breaststroke or IM. Some, like Ryan Lochte, do just that in freestyle events.

Hi Gary, i just was referred to this today. Thanks very much for the great explanation on why it isn’t the effect of gravity that evens out the up and down kicks. I’m more than happy to be shown why my too-simple explanation was incorrect.

When you say that the feet move against the stream, are you saying they move outside of the laminar flow? From the pictures it would seem so. This would mean that the resulting force of the kick overcomes the increased drag caused by the feet. Does it take a lot of power to overcome that additional drag? And have you tested to see the resulting velocities with a smaller faster kick staying within the laminar flow?

I ask this because I’ve noticed a significant difference in kick amplitudes from elite men and women. It looks like women have those smaller faster kicks, while men have the larger amplitude kicks.

Once again, thanks. I’m always happy to learn.

• garyhallsr

Hi Rick!

When I think of the vortex behind the swimmer, I think of a relatively narrow (width of the body) swirl of water starting from the middle of the body, funneling out to the sides, then circling back to the midline of the swimmer, extending a few feet behind the swimmer. The vortex thus creates a stream that follows the swimmer, strongest in the midline of the swimmer and closer to the swimmer. The feet are then creating propulsion in the strongest part of the stream. The feet can also contribute more power to this stream by creating their own vortex that adds to this. The addition is greater to the vortex when the swimmer is on their back and the stronger kick is upward. Therefore, the weaker down kick on the swimmer’s back becomes much more propulsive with this stronger stream to push against.
I do notice there are significant differences in kick amplitudes among swimmers in dolphin kick. However, I have not noticed that particularly gender dependent. Misty Hyman and Natalie Coughlin, for example, had huge undulations on dolphin kick. Getting the right articulation of ankle, knee and hip is one of the big challenges of perfecting the dolphin kick.

Thank Gary, I think you’re referring to the laminar layer. When you see car commercials of a car in a wind tunnel, they inject a coloured gas, and that gas collects at the laminar layer. My comment was that disrupting that laminar layer creates turbulence (the vortices) that greatly increase drag. Normally this would be a bad thing as it slows the swimmer. But if I understand it, you’re saying that this turbulence can be beneficial if the kick can then use it to create additional force. That makes sense in that drag can certainly be used to improve power, but I just figured out in my head how turbulence-induced drag caused by disrupting the laminar flow can being used beneficially. That’s why I keep reverting to small fast kicks.

And you’re right in that I shouldn’t have differentiated on gender. Many elite swimmers use small fast kicks, and most of these are women. Many men use larger amplitudes, but not all. I’m speculating that in order to overcome the drag caused by the laminar flow disturbance, the kick has to be powerful. Otherwise it’s better to have a smaller kick.

that should be

“…but I just HAVEN’T figured out in my head how turbulence-induced drag caused …”

• garyhallsr

Yes, the separation of the boundary layer of water from the skin/suit, resulting in the formation of the body’s vortex, is the greatest single contributor to frontal drag (compared to friction and surface drag). We can only change the body’s shape a certain amount in order to reduce that form or pressure drag, but we can learn to use the vortices (from the body and our feet) to our advantage on the propulsion side by using the right kicking motions. That is what great kickers do.

9. OFIR VEKSLER

Hi Mr. Hall,
My name is Ofir and I am a student for physical education, competitive swimmer and also taking a course for competitive swimming coaches in Israel. I read the two parts of “The Dynamics” and yet few detais are not fully clear to me 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 on back and stomach?
2. In the previous part of “The Dynamics of Dolphin”, you claim that: ‘With the swimmer on his stomach, the down kick, which involves extension of the knee and flexion of the hip, is a biomechanically stronger motion than the motion of the up kick, which involves extension of the hip and flexion of the knee’. does this also correct on the back?
3.If we comparing the upkick on the back to downkick on the stomach according to the last fact and the law of inertia, how can we explain that the stomach is faster?

• garyhallsr

The peak acceleration and trough deceleration, as well as velocity, are measured with a Velocity Meter from AP Labs in Italy. Software used to synchronize is Biomovie.
Yes, the upkick on the swimmer’s back is biomechanically stronger than the down kick…yet the peak velocities achieved on the back are similar due to the differences in flow dynamics.
The dolphin kick on the back is faster because we are able to maintain a higher speed on the down kick (weaker) from the vortex.

• OFIR VEKSLER