The Dynamics of Dolphin Kick Part I: Using the Vortex
The two sources of propulsion we use while swimming, the hands and the feet, work in different environments. Although the hands and feet are creating nearly all of the propulsion in the water, the flow dynamics where the forces are taking place for each one differ considerably.
In order to produce propulsion, the surface areas creating the forces (from the hands and feet) must move backwards relative to the water. The hands begin to create propulsion when they are about one foot in front of the shoulder. Since the hand is in front of and to the side of the body when the pulling propulsion begins, the vortex created by the body does not affect the water where the hand is pulling until the end of the pulling motion. In other words, the hand is pushing backward against relatively still water during the times it is producing most of the propulsion. Not so with the feet.
Behind every swimmer moving forward is a vortex of water (wake) created by the separation of the water moving along the body (as the body moves forward). This vortex creates a stream of water that follows the swimmer. It is this stream that enables a swimmer to swim right behind another swimmer and catch a ride. The bigger and the faster the swimmer, the bigger the vortex and the faster the stream is moving. The wake behind a swimmer extends for a few feet behind him and funnels out wider from the feet, but it does not extend very deep in the water. The width of the stream even enables a swimmer that is swimming in the next lane, yet close enough to the swimmer, to catch some of the ride. The most famous example of that was Jason Lezak riding Alain Bernard’s wake as long as he could in the 4 x100 free relay in Beijing, completing the fastest relay leg in history.
Since the propulsion from the feet is occurring, for the most part, within this stream, the stream affects the dynamics of the kicking motion. Relative to a stationary point in the pool, the feet in the dolphin kick (while the swimmer is on the stomach) are moving backward only during the beginning of the down kick. There is virtually no backward motion of the feet at all during the up kick. Yet the feet can create propulsion on both the down and the up kicks. The reason is that the feet need to be moving backward relative to the water only, and since the water is moving forward, propulsion can occur when the feet are moving upward or downward, in addition to backward.
The dynamics of the kick are made even more complicated by another smaller vortex that forms behind the feet as they move across the stream of the body’s vortex. Since most of the motion of the foot is upward, downward or forward, the vortex of the moving foot, somewhat like a tributary connecting to a river, contributes to the fluid dynamics of the body’s stream, generally adding more water moving forward behind the swimmer.
Using the velocity meter technology at The Race Club, from the changes of the body’s velocity and acceleration resulting from each of the down or up kicks, one can gain an appreciation for the amount of propulsive force generated by the foot moving in each direction. 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. Further, with the extraordinary plantar flexibility of the ankle of a fast kicker, the surface area of the foot moving backward relative to the water is greater on the down kick (top of the foot) than it is during the up kick (sole of the foot).
In the gym, for example, I can lift about 70 pounds 50 times before exhaustion on the down kick motion. However, using the up kick motion, I can lift 30 pounds for about 30 reps before exhaustion. The difference would suggest that the down kick motion is at least twice as powerful as the up kick motion. Therefore, when analyzing the velocity and acceleration of the swimmer’s body while dolphin kicking, one would expect to see the greatest acceleration and peak velocity occur from the down kick, rather than from the up kick. While a swimmer is on his stomach that is precisely what one sees. However, the difference in speed resulting from the down kick and the up kick on the stomach will surprise you.
In the next article, we will examine in detail the acceleration, velocity and differences between peak and trough velocities resulting from the dolphin kick performed on the stomach, side and on the back.
Yours in swimming,
Pictured is Olympian Junya Koga from Japan.