The 'Fifth Stroke': The Physics and Evolution of Swimming's Fastest Technique

The fastest way to win a swimming race is to spend as little time as possible actually swimming on the surface. In the modern era of elite aquatics, the margins between gold and missing the podium entirely are measured in hundredths of a second, and those fractions are won beneath the waves. The secret lies in the underwater dolphin kick, a full-body undulatory motion so powerful and essential that coaches and athletes universally refer to it as the "fifth stroke."[4][5]

Unlike the traditional four strokes—freestyle, backstroke, breaststroke, and butterfly—the fifth stroke is not an event of its own. Instead, it is the critical connective tissue of a race, utilized off the starting block and after every turn. When executed perfectly, an elite swimmer can reach velocities between 2.2 and 3.0 meters per second during their underwater glide and kick sequence, a speed that is physically impossible to sustain once they break the surface and begin their regular arm pulls.[1][3][5]

The reason for this dramatic speed differential comes down to the unforgiving physics of water. When an athlete swims on the surface, they must constantly break the plane between water and air. This displacement creates a bow wave in front of the swimmer, resulting in a phenomenon known as wave drag. Wave drag acts like an invisible, heavy blanket, exponentially increasing resistance the faster the athlete tries to move.[1][5]

Submerging the body entirely changes the hydrodynamic equation. By gliding at least half a meter below the surface, a swimmer completely eliminates wave drag, leaving only form drag and skin friction to overcome. Biomechanical studies utilizing computational fluid dynamics have demonstrated that swimming at a depth of roughly one meter can reduce total hydrodynamic resistance by up to 50 percent compared to surface swimming. The deeper the glide, the cleaner the water, and the faster the human body can travel.[1][2]

However, simply gliding underwater is not enough; the swimmer must generate propulsion without breaking their streamlined profile. This is where the biomechanics of the dolphin kick come into play. It is a common misconception that the kick is driven merely by bending and straightening the knees. In reality, a world-class dolphin kick is a complex kinematic synergy—a whiplash-like wave of energy that originates in the chest and sternum, travels down through the core and hips, and violently snaps through the ankles and toes.[2][5]

This undulatory motion mimics the tail action of marine mammals, allowing the swimmer to press against the water with the maximum possible surface area of their lower body. The core acts as the engine, while the legs function as a flexible, propulsive fin. The flexibility of the ankle joint is particularly crucial; swimmers with extreme plantar flexion can point their toes so far backward that the tops of their feet act like paddles, pushing water directly backward to drive the body forward.[1][2][5]

The greatest biomechanical challenge of the fifth stroke is human anatomy itself. Because humans evolved for upright, forward locomotion like walking and running, our anterior muscles—specifically the quadriceps—are naturally dominant. This makes the downward phase of the dolphin kick relatively easy to execute with power, as the body relies on its most developed muscle groups.[1][5]

The upward phase of the kick, however, requires immense strength from the posterior chain, including the glutes, hamstrings, and lower back. Most amateur swimmers simply let their legs float back up after a down-kick, wasting half of the propulsive cycle. Elite sprinters train their posterior chains relentlessly, turning the up-kick into an active, force-generating movement that maintains continuous velocity and prevents the body from stalling in the water.[1][5]

The realization that underwater kicking was vastly superior to surface swimming did not happen in a laboratory; it was discovered through athletic experimentation. In the 1970s and early 1980s, pioneers like Jesse Vassallo and Daichi Suzuki began extending their underwater glides to avoid the turbulent wake of larger competitors. But it was American backstroker David Berkoff who pushed the concept to its absolute limit, forever changing the sport.[4][5]

At the 1988 Olympic Games, Berkoff unveiled a technique that the media quickly dubbed the "Berkoff Blastoff." Off the start of the 100-meter backstroke, Berkoff stayed submerged on his back, furiously executing dolphin kicks for an astonishing 33 meters before finally surfacing to swim the remaining 17 meters of the lap. He shattered the world record, proving definitively that surface swimming was obsolete if a swimmer had the lung capacity to stay under.[4][5]

Berkoff's dominance sparked an immediate crisis within the sport's governing body, World Aquatics. Regulators feared that if left unchecked, backstroke and butterfly races would devolve into entirely underwater breath-holding contests, rendering the traditional arm strokes extinct. Furthermore, there was a severe safety concern: pushing athletes to swim 30 or 40 meters without oxygen at maximum exertion dramatically increased the risk of shallow water blackout, a potentially fatal condition caused by hypoxia.[3][4][5]

In response, the governing body instituted the 15-meter rule. The regulation mandates that a swimmer's head must break the surface of the water at or before the 15-meter mark following every start and turn. Originally applied to backstroke in 1991, the rule was eventually expanded to govern freestyle and butterfly events as well, creating a standardized boundary between underwater innovation and traditional surface racing.[3][4]

Today, the 15-meter mark is the most intensely scrutinized line in a swimming pool. Elite athletes aim to surface at exactly 14.9 meters, maximizing their underwater speed advantage without triggering a disqualification. This requires extraordinary spatial awareness and stroke counting, as the swimmer must perfectly time their ascent while their brain is screaming for oxygen.[3][5]

The physiological cost of the fifth stroke is immense. Engaging the massive muscle groups of the core and legs simultaneously burns through oxygen reserves at a ferocious rate. Swimmers must balance the hydrodynamic benefits of staying underwater against the aerobic debt they will carry into the final lengths of a race. If an athlete pushes their underwater kicks too far and accumulates too much lactic acid, their surface stroke will inevitably fall apart.[1][5]

The most critical fraction of a second in any race is the breakout—the exact moment the swimmer transitions from the underwater dolphin kick to their first surface arm pull. If the breakout is timed too early, the swimmer crashes into wave drag before they reach their top surface speed. If it is timed too late, they lose their underwater momentum and stall. The perfect breakout is a seamless transfer of kinetic energy from the fifth stroke into the first stroke.[5]

As the sport looks to the future, the evolution of the underwater dolphin kick is increasingly driven by data. Sports biomechanists are using 3D motion capture, electromyography, and kinematic synergy analysis to map the exact muscle firing sequences of Olympic champions. While the 15-meter rule places a hard physical limit on distance, the quest to optimize the speed, efficiency, and power of the human body's undulatory wave remains the most vital frontier in competitive swimming.[1][2][5]