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,