The Physics of the 'Fifth Stroke': Why the Fastest Way to Swim is Underwater

To watch a modern Olympic swimming race is to witness a sport that takes place largely out of sight. When the starting buzzer sounds, the athletes do not immediately begin churning the surface of the water with their arms. Instead, they disappear. For the first several seconds of the race, the pool surface remains eerily calm while a subterranean drag race occurs below. This submerged phase is driven by the "fifth stroke"—the underwater dolphin kick—a technique that has fundamentally rewritten the physics and strategy of competitive swimming.[3][7]

The counterintuitive truth of aquatic sports is that the surface of the water is the absolute slowest place to be. When a swimmer moves along the surface, they are fighting a brutal battle against physics. They must displace the water in front of them, pushing it up into the air and creating a bow wave, much like a speedboat. This phenomenon, known as wave drag, acts as an invisible brake on the athlete's forward momentum.[1][7]

Biomechanical research demonstrates just how punishing this surface tension can be. Depending on the athlete's velocity, wave drag can account for a staggering 20 to 50 percent of the total resistance a swimmer faces. The faster an athlete tries to swim on the surface, the exponentially larger the wave they create, and the harder the water pushes back. The only way to break this mathematical speed limit is to dive under it.[1]

By descending just a few feet below the chaotic boundary between air and water, a swimmer enters a realm of pure fluid dynamics. Studies indicate that traveling at least 0.5 meters (about 20 inches) below the surface allows an athlete to bypass the worst of the wave drag entirely. In this submerged corridor, the water flows smoothly over the back and shoulders, allowing the swimmer to maintain the explosive momentum generated from diving off the starting block or pushing off the wall.[1][7]

But gliding only lasts so long. To maintain that high-speed momentum, swimmers employ Undulatory Underwater Swimming (UUS), commonly known as the dolphin kick. Unlike the alternating flutter kick used in freestyle, the dolphin kick is not driven solely by the legs. It is a full-body wave that originates in the chest and core, snapping down through the hips, knees, and finally the ankles, mimicking the powerful tail propulsion of cetaceans.[2][5]

Advanced computational fluid dynamics (CFD) modeling has finally allowed sports scientists to visualize exactly why this undulating motion is so effective. As the swimmer's body snaps through the wave motion, it generates microscopic whirlpools, or vortices, in the water. The most efficient swimmers create a large, high-circulation vortex on the ventral (stomach) side of their trunk, and another powerful vortex trailing behind their feet.[2]

These vortices act as temporary, invisible walls of water pressure. By perfectly timing their undulations, elite swimmers literally push off these swirling pockets of water, generating massive forward thrust without the braking effect of surface drag. The result is a burst velocity that can reach between 2.2 and 3.0 meters per second—speeds that are physically impossible to sustain once the swimmer breaks the surface and begins a traditional stroke.[2][7]

The realization that underwater swimming was vastly superior to surface swimming did not happen in a laboratory; it happened in the pool, sparking one of the greatest controversies in the sport's history. In the late 1980s, an American backstroker named David Berkoff began experimenting with staying underwater for extended periods. He realized that if he simply held his breath and dolphin-kicked on his back, he could easily outpace competitors who were swimming traditional backstroke on the surface.[3][4][6]

Berkoff unleashed this technique, dubbed the "Berkoff Blastoff," at the 1988 Olympic Games in Seoul. He stunned the world by swimming more than 30 meters of the 100-meter race completely submerged, shattering the world record in the process. His dominance proved unequivocally that traditional surface strokes were obsolete compared to the raw hydrodynamic efficiency of the dolphin kick.[4][6]

The international governing body for swimming, FINA (now World Aquatics), panicked. Regulators feared that if left unchecked, swimming would devolve into a breath-holding contest where athletes simply dolphin-kicked underwater for the entire length of the pool, rendering the traditional disciplines of backstroke, butterfly, and freestyle unrecognizable. Furthermore, coaches raised severe safety concerns about athletes pushing themselves into hypoxia and risking shallow water blackout.[4][6]

In response, regulators instituted the now-famous 15-meter rule. The mandate is strict: after every start and every turn, a swimmer's head must break the surface of the water by the time they reach the 15-meter mark. Originally applied only to backstroke in 1988, the rule was eventually expanded to govern butterfly and freestyle events in the late 1990s, forever changing the geometry of a swimming race.[4][6]

Today, that 15-meter zone is the most heavily contested real estate in the pool. In a standard 50-meter Olympic pool, a swimmer can legally spend 30 percent of the race underwater. In a 25-yard short-course pool, where turns happen twice as often, an athlete can spend up to 60 percent of the race submerged. Races are no longer won purely on surface speed; they are won in the transitions.[7]

Coaches have spent the last two decades engineering new ways to maximize this underwater advantage. Legendary coach Bob Gillett pioneered the use of the monofin—a single, large flipper that binds the feet together—to teach swimmers the precise core-driven undulation required for a perfect dolphin kick. Gillett's protégé, Misty Hyman, used these techniques to perfect a lateral (side-lying) underwater kick, which further reduced wave drag and propelled her to a stunning gold medal in the 200-meter butterfly at the 2000 Sydney Olympics.[3][5]

Biomechanical research continues to refine the technique. Recent studies suggest that the lateral position Hyman used is indeed hydrodynamically superior when an athlete is forced to swim close to the surface, as it deflects water differently than a flat stomach-down or back-down posture. Swimmers are now trained to dynamically adjust their body rotation as they ascend toward their breakout point to shave off microscopic fractions of drag.[1][5]

The evolution of the fifth stroke represents a perfect marriage of athletic endurance and applied physics. As human physiology approaches its natural limits on the surface of the water, the quest for speed has driven the sport downward. The modern swimmer is no longer just an athlete pulling themselves across a pool; they are a hydrodynamic vessel, engineered to slip through the water with the silent, devastating efficiency of the marine life they emulate.[7]