Physics for Swimmers, Coaches and Parents – Propulsion
Outside of the starts and turns, the propulsion forces of a swimmer are derived purely from the kick and the pull. More specifically, except for the up kick, where the entire lower leg and foot can create propulsion, nearly all of the other propulsive forces (down kick and pull) occur at the feet and hands.
In addition, the propulsion from the kick and pull can be influenced by other motions of our body that produce no propulsion at all. These are called coupling motions. Two examples of coupling motions in freestyle are the rotation of the body and the recovery of the arm over the water. Neither motion produces any propulsion by itself, but when timed or coupled with a propulsive pull or kick, either motion can make either force greater. High energy coupling motions can significantly increase the propulsion of a swimmer in all four strokes, as well as on the start.
The propulsion of a swimmer that is derived from the hands and feet differ in that the hands are moving through water that is essentially still (static fluid), while the feet are moving through water that is flowing (dynamic fluid). Some understanding of fluid mechanics is therefore necessary to understand how propulsion is achieved within these two different environments.
Since water is liquid, not solid, in order to create propulsion, the hand or foot needs to be moving backward relative to the water. In shoulder-driven freestyle technique, with a relatively higher stroke rate, if one were to map the pathway of the pulling hand from the side, relative to a fixed point in the pool, one would find that the hand moves in nearly a perfect circle of around 2 feet in diameter.
If we consider the circle as a clock, the hand would enter the water at 12 o’clock. Since the swimmer’s body is moving forward, as the hand enters the water, the hand will move forward also. The swimmer begins the pulling motion by pushing the hand downward in order to reverse its direction and push it backward. The result is that the hand follows the clock to the 3 o’clock position, moving both downward and forward. We call this the lift phase, since most of the forces are downward, creating lift.
From 3 o’clock, when the hand is just in front of the swimmer’s shoulder, it begins moving backward, creating propulsion. The hand continues going deeper in the water as it follows the clock from 3 o’clock to 6 o’clock on its way backward. In an effort to continue pushing the hand backward past 6 o’clock with the maximum hand surface area, the arm needs to elevate and the wrist dorsiflex, resulting in the hand cutting a quarter of the clock off in moving from 6 o’clock to 9 o’clock. The backward hand motion from 3 to 9 o’clock is called the propulsion phase.
Once the hand reaches 9 o’clock, the arm runs out of length, so the hand cannot move backward any further. Instea, it quickly slides forward with the least resistance possible to leave the water nearly exactly where it began the circuitous route, at 12 o’clock. This last phase is called the release phase. The net distance that the hand travels from entry to exit is zero.
Unlike the hand, in order to create propulsion, the feet rely on the vortices caused by both the swimmer’s body and the motion of the foot and leg itself. In both freestyle and dolphin kick, the motion of the kicking foot is nearly straight up and straight down, relative to a fixed object in the pool. However, the water is not still in the path of the foot. Because the human body is a non-streamlined shape, there is a forward flow of water following the swimmer caused by the body’s vortex or wake (slipstream). There is also a second vortex caused by the motion of the feet and leg which creates a smaller stream that follows the path of the feet. Even though the feet are not moving backward relative to a fixed object in the pool, they are moving backward relative to the water, which is moving forward. Therefore, the feet can create propulsion without actually moving backward.
In dolphin kick, for example, there are four potential places where the feet can create propulsion. The first is at the beginning of the down kick. The propulsion here is achieved by quickly reversing the direction of the feet and pushing down against the vortex that was created by drawing the feet and leg upward and forward. The second is achieved as the feet traverse the body’s vortex (slipstream) on the way down. The third is achieved at the initiation of the up kick, as the feet and leg quickly reverse direction and push upward against the vortex they created on the way down. The fourth is achieved as the feet and leg move upward and traverse the body’s vortex (slipstream) on the way up. Only the fastest dolphin kickers will achieve propulsion in all four of these locations. Most swimmers derive propulsion in only one or two of them.
In flutter kick, there are two potential points of propulsion. Since the down kick and up kick occur simultaneously, one point is at the initiation of each, utilizing the vortices of the feet and legs. The second occurs as both feet pass through the slipstream on the way up or down.
With breaststroke kick, nearly all of the propulsion occurs from the instep of the feet pushing backward. The peak force occurs when the feet are about half way back toward complete extension of the legs. The narrower the kick, the more advantage the breaststroke kicker will derive from the body’s slipstream and large vortex resulting from drawing the legs and feet forward. With a wide breaststroke kick, the feet may be pushing backward in relatively still water, rather than against a stream of water. That can significantly affect propulsion. A small amount of propulsion is also possible from the up kick that occurs at the end of the breaststroke kick. Not every breaststroker will get that second propulsion.
In summary, the propulsion of the pull is determined by the surface area of the hand pushing backward and the rate at which that effective hand surface area accelerates through the propulsion phase. The propulsion from the kick is determined by the surface area of the feet (and legs), the rate at which the feet accelerate through the vortices and the strength of the vortices (slipstream) that the feet move through. Further, the propulsive forces of either the pull or kick can be augmented by the amount of kinetic energy within the properly-timed coupling motions, such as the body rotating, the head snapping down, or the arms recovering.
This week in Lanes 2, 3 and 4, we will feature a classroom discussion on the four different types of pulling motions. We hope you will hop on in! https://theraceclub.com/membership/raceclub-subscriptions/
Yours in swimming,