Why dolphin kick is faster on your back

by SwimSwam 27

November 01st, 2017 Masters, Training

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)

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

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27 Comments on "Why dolphin kick is faster on your back"

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Hmm… I think it’s awesome that you’re taking a scientific approach to understanding swimming mechanics. It can only help. It’s also just plain interesting. Some of the assertions here don’t seem physically sound though. I DO think you’re correct that swimmers are generally faster while dolphin kicking on their backs. It’s pretty plain to see if you’ve coached or paid any attention to the times your fellow swimmers are going in 25’s underwater. I would like to investigate this phenomenon as well, but I want to do so in a way that is physically consistent. Admittedly, I am not super strong on fluid mechanics. However, I think I can still provide a little insight. Let’s simplify the scenario: ignore gravity… Read more »
i have a theory that just came to my mind; gravity may affect the swimmer in the water very little, but it affects he whole water and “pulls” it down. now what we seem to know is that the “forward” motion with the legs, (so while swimming on your stomach the downward motion and while on the back the upward motion) is the stronger one. now my theory why one is faster on their back is that gravity is pulling down the whole water mass and therefore provides more resistence against the stronger upward motion (i hope you can understand what i am trying to say, english is not my first language).so what im trying to say is that gravity… Read more »
I think that when you’re on your back, the up-kick is trying to “lift” that volume of water against gravity, rather than trying to push it down and around. Imagine if you could take the volume of water above the swimmers feet as being a discreet bucket of water and place it on the pool deck. Lifting this bucket up requires working against gravity. If you try to push that bucket of water back into the pool, it’s difficult, but you aren’t working against gravity so much as you are working to push the water out and around to accommodate the volume of water. I think you way actually find the results of these tests to differ slightly depending on… Read more »
I think you are on to something here. So the effect you describe in which gravity “pulls” the water down is water pressure. Gravity gives weight to water which results in water “pressing” down on anything below the surface. However, I’m not sure about water pressure providing more resistance to the upward kick while on one’s back. It should provide less resistance. What if we take your intuition about water pressure and apply it to the “weak” kick? On their front, a swimmer has to resist water pressure to generate propulsion out of this half of the kick. On their back, the swimmer is no longer resisting water pressure and may even be aided by it a bit. This would… Read more »

Once the swimmer is submersed, what matters is the weight of the legs in the water…which is very small. Water is an incompressible material with effectively the same density at all depths of the pool. It is the flow dynamics of the wake or vortex in relation to the two motions of the kick that really change in going from front to back sides in dolphin kick.

We actually did this study to determine the difference between greater and lesser hip undulation in the dolphin kick. It is almost impossible to not undulate some with a good dolphin kick. In order to kick dolphin fast, one needs to have a lot of plantar flexibility, and then the right amount of knee bend for the extension (down kick on stomach) and hip undulation. Too much of the latter two components will cause more harm than good. It turns out that with Junya, less undulation (about 6 inches of hand movement vs 8-10 inches) was slightly faster than with more undulation. Part of the reason for this was that the cycle time went from around .66 seconds to .6… Read more »

Does buoyancy have any effect here? I’ve observed that many of these swimmers who have great underwaters tend to have a deeper trajectory off the wall. Are they able to translate the bouyant force linearly as they rise to the surface?

The best example I can recall now is by Stanislov Donets. Watch his 100m back swims from the 2011 short course worlds. I believe there were some underwear water shots there… watch how deep he goes and how much separation he’s able to get from the others on the second half of his underwaters.

From my own experience, when I dolphin kick on my back, my upper body is more stable which enables me to exert greater strength and thus a more powerful kick. It feels as though the weight of the water on my chest keeps it from moving around, thus giving me a better platform from which to kick.

We don’t think the body or kicking motion changes appreciably from front to back…with Junya or any other good kicker. However, if you look at the orange curves above which measure acceleration and deceleration, you will see two instances where Junya’s deceleration exceeded -10 m/sec2, which led to slower velocities. In both cases, he bent the knees too much (to about 110 degrees). It seems that around 120 degrees is the threshold point for maximum knee bend (flexion). Even great swimmers need to be consistent.

For me it seems simple. When I’m on my back my natural kick more or less keeps me submerged. When on my stomach I need to waste energy to stop myself from floating to the surface.

To me it feels like the weight of the water pushes down on my chest and, though it sounds stupid, as if I can slip underneath it. I feel like the weight pushing down on me almost aids in propulsion

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