The Science of the 'Fifth Stroke': Why Swimming's Fastest Phase Happens Completely Underwater

The paradox of competitive swimming is that the fastest way to move through the water is to avoid the surface entirely. When elite swimmers dive off the blocks or push off the wall, they do not immediately begin their designated stroke. Instead, they transition into a streamlined, undulating motion known as the underwater dolphin kick. Often referred to as swimming's "fifth stroke," this technique has become the single most critical differentiator in modern short-course and long-course racing. By staying submerged, athletes can achieve velocities that are physically impossible to maintain once they break the surface and begin pulling with their arms.[6]

The fundamental reason the underwater dolphin kick is so effective comes down to fluid dynamics and the concept of wave drag. When a swimmer moves along the surface of the pool, they collide with the water, pushing it forward and creating a bow wave. The energy required to push this water out of the way increases exponentially with speed. According to sports biomechanists, wave drag can account for up to sixty percent of the total resistance a swimmer faces. By diving just a few feet below the surface, a swimmer completely eliminates wave drag, moving through a state of laminar flow where the water glides smoothly over their streamlined body.[1][5]

To capitalize on this hydrodynamic advantage, the swimmer must maintain a rigid, torpedo-like posture known as a streamline. The arms are locked tightly behind the ears, one hand stacked over the other, while the head is tucked neutrally. From this rigid upper-body position, the propulsion is generated entirely by the core and the legs. Unlike a traditional flutter kick, which relies heavily on the quadriceps and hamstrings, the dolphin kick is a full-body whip that originates in the chest and sternum, ripples down through the abdominal muscles and hips, and finally snaps at the ankles.[2]

The mechanics of this whip-like motion are highly complex and require immense flexibility, particularly in the ankles. Sports science research indicates that elite underwater kickers possess a degree of plantar flexion—the ability to point the toes downward—that far exceeds the average human range of motion. This flexibility allows the tops of the feet to act like a pair of flippers, catching a massive volume of water and thrusting it backward. The kick consists of two phases: the powerful down-kick, which generates the primary forward thrust, and the up-kick, which repositions the legs while maintaining momentum.[5]

However, the immense speed of the fifth stroke comes with a severe physiological cost. Because the swimmer is entirely submerged, they are operating in a state of apnea, or breath-holding, while simultaneously executing one of the most metabolically demanding movements in sports. The large muscle groups of the core and legs consume oxygen at a rapid rate. As the swimmer undulates down the pool, carbon dioxide builds up in their bloodstream, triggering an agonizing physiological urge to breathe. Managing this oxygen debt is the primary limiting factor for how long an athlete can sustain the underwater kick.[3]

The trade-off between speed and oxygen depletion is a constant mathematical calculation for elite coaches and athletes. Staying underwater longer means moving faster and avoiding surface drag, but it also means arriving at the surface with heavily fatigued muscles and a desperate need for oxygen. If a swimmer pushes their underwater phase too far, they risk "tying up"—a sudden, catastrophic accumulation of lactic acid that destroys their stroke technique for the remainder of the lap. Consequently, training the fifth stroke is as much about expanding lung capacity and carbon dioxide tolerance as it is about building leg strength.[2][3]

The strategic application of the underwater kick varies significantly depending on the format of the race. In a long-course pool, which measures fifty meters in length, swimmers only have one wall to push off per lap. In a short-course pool, which measures twenty-five meters or yards, the number of turns is doubled. This makes the short-course format heavily reliant on the fifth stroke. In a twenty-five-yard race, an elite swimmer might spend more than sixty percent of the total race distance completely submerged, turning what is nominally a freestyle or backstroke event into an underwater kicking contest.[6]

To prevent the sport from losing its traditional identity, the global governing body for aquatic sports, World Aquatics, instituted strict regulations on how far a swimmer can travel underwater. Following a period in the late 1980s and 1990s where athletes began swimming nearly entire lengths of the pool submerged, officials implemented the fifteen-meter rule. This regulation mandates that a swimmer's head must break the surface of the water at or before the fifteen-meter mark off every start and turn. The rule applies to the freestyle, backstroke, and butterfly events, fundamentally shaping modern race strategy.[4]

The fifteen-meter mark is vividly indicated by a brightly colored marker on the lane ropes, serving as a visual cue for both the athletes and the officials. Pushing the absolute limit of this boundary is a high-stakes gamble. Swimmers meticulously count their kicks off every wall—often executing exactly six or seven undulations—to ensure they maximize their underwater advantage without crossing the fifteen-meter line. A miscalculation of even a few inches can result in immediate disqualification, a heartbreaking outcome that frequently occurs even at the highest levels of international competition.[4][6]

The transition from the underwater kick to surface swimming, known as the breakout, is arguably the most technically demanding fraction of a second in a race. As the swimmer approaches the surface, they must perfectly time their first arm pull to coincide with their head breaking the water. If they pull too early, their arms meet the massive resistance of the water above them, killing their momentum. If they pull too late, they lose the speed they generated off the wall and stall on the surface. A flawless breakout seamlessly transfers the high velocity of the underwater phase into the rhythm of the surface stroke.[2]

The evolution of the fifth stroke has driven significant advancements in sports technology and biomechanical analysis. Today, elite swimming programs utilize high-speed underwater cameras, wearable accelerometers, and computational fluid dynamics software to analyze an athlete's kick down to the millisecond. Coaches look for microscopic inefficiencies in the amplitude of the kick, the angle of the hips, and the tightness of the streamline. By quantifying these variables, sports scientists can help swimmers shave hundredths of a second off their turns, a margin that frequently determines the difference between a gold medal and missing the podium entirely.[5][6]

Looking ahead, the mastery of the underwater dolphin kick will only become more critical as surface swimming techniques approach their theoretical biomechanical limits. While athletes continue to refine their arm pulls and body rotation, the most significant remaining frontier for time drops lies beneath the waves. As training methods for hypoxia and core strength continue to advance, the next generation of swimmers will likely push the fifteen-meter boundary with even greater power and efficiency, cementing the fifth stroke as the ultimate weapon in aquatic sports.[3][6]