Fifteen Meters Underwater: Where Modern Swimming Speed Is Really Decided

In a track final, Justin Gatlin's reaction time was measured at 0.138 seconds and Christian Coleman's at 0.116 — a gap of just 22 thousandths of a second, yet enough to keep me up all night in Melbourne in 2026 writing my first blog about the "reaction equation." In a swimming final, the equivalent number is nearly five times longer: roughly 0.6 to 0.7 seconds from the starting beep to the moment the feet leave the block. The paradox of swimming does not lie in that number. It lies elsewhere. A 100-meter track race is decided by what happens on land — the feet, the Achilles tendon, the backward force from the surface. A 100-meter swim is decided by what happens underwater, in a zone the stands can barely see and television rarely replays. In the first fifteen meters after the beep, while most spectators are still waiting for the swimmers to surface, most of the race has already been written. That is why I chose to begin this championship season from underwater rather than from the results board. After years of covering swimming, I have noticed one thing: fans remember who touched the wall first, but almost no one remembers who traveled underwater the farthest and the fastest. That gap is exactly where modern swimming places the entire weight of its competition. The global swimming landscape at this moment is the landscape of a generation trained more systematically, measured more carefully, and specialized earlier than any generation before it. After the Paris 2026 Olympics — where Leon Marchand won four gold medals on home soil, where Canada's Summer McIntosh asserted herself in the medley events, where Pan Zhanle broke the men's 100-meter freestyle world record in 46.40 seconds — an old question returns in a new form: what actually produces speed? Swimming once went through a suit revolution in the late 2000s, when polyurethane bodysuits helped topple dozens of world records in a single season before being banned from 2026. Since then, speed has had to come from the body and from technique, not from fabric. And when speed is forced to come from technique, people begin to notice the least visible part of technique: the underwater phase. The current World Aquatics rules state that in freestyle, backstroke, and butterfly events, a swimmer may stay underwater for a maximum of fifteen meters after the start and after each turn. In breaststroke, only one dolphin kick is permitted immediately after the start and after each turn. This is a technical regulation, and also a boundary drawn after much dispute — a boundary that has reshaped the entire tactics of the sport. In the late 1980s, David Berkoff threw the backstroke world into a panic when he dove deep and swam underwater for nearly a full length of the pool in the 100-meter backstroke. The technique was so effective that a rule had to be created to limit it. The story repeats in a familiar circle: an athlete finds a loophole in the rules, the rules are patched, and a new loophole opens somewhere else. Fifteen meters is the product of that circle. To understand why those fifteen meters matter, you need a little physics. Water is about eight hundred times denser than air, and the drag it applies to a swimmer's body does not rise linearly but with the square of speed. That means: swim twice as fast and drag quadruples, and the body must produce many times more power to sustain the speed. This is why swimming is among the most mechanically demanding of all sports. There is a detail few outside the technical world notice: when the body is fully submerged, it produces no waves. The phenomenon of "wave-making drag" disappears when the body is deep enough. At the surface, every stroke pushes water into ripples, and those ripples pull back against the swimmer. Underwater, the swimmer faces only form drag and friction drag — considerably smaller. That is the purely physical reason the underwater phase, done correctly, is faster than surface swimming. The condition for that advantage is the "streamline" position — both arms extended overhead, pressed against the ears, head tucked between the arms, the body taut as a spear. Any small deviation creates turbulence and destroys the advantage. Here swimming meets track again: just as a sprinter must keep the body's axis stable so as not to lose force to the sides, a swimmer must keep the vertical axis perfect so as not to sell drag to the water. The dolphin kick is the main force-producing movement of the underwater phase. Both legs kick up and down together in a wave-like motion that begins at the hips, travels through the knees, and reaches the feet, exactly the way a dolphin swims. Mechanically, this is a wave-based propulsion system, quite different from the stroke, which produces force in discrete beats. A wave allows more continuous force, and that is why the top swimmers can sustain astonishing speed across fifteen meters. Some analysts call the underwater phase the "fifth stroke" — alongside the four official strokes of freestyle, backstroke, breaststroke, and butterfly. The label is a little exaggerated, but it reflects a truth: in many modern races, the skill that decides victory is a skill that belongs to none of those four strokes. Michael Phelps raised this technique to an art form. His height, long arm span, and core control allowed him to sustain the underwater phase longer and more effectively than his rivals, especially in butterfly and medley. In the next generation, Leon Marchand inherited and refined that legacy. When Marchand won four golds in Paris, what stood out was the way he turned the underwater phase into a strategic weapon in every event he entered. If you look at the splits — the segment times — of a 100-meter final, a striking pattern emerges. The winner is usually not the swimmer with the fastest surface segment, but the one who loses the least time in the start and the turn. In races where the gap between first and second is only a few hundredths of a second, that difference usually lies entirely within the fifteen-meter zone the spectators cannot clearly see. Even the start reaction, which seems purely instinctive, is measured to the thousandth of a second. Reaction time in swimming runs from the signal to the moment the feet leave the block, including the push-off and the airborne phase. A reaction a tenth of a second faster can be the difference between a gold medal and fourth place. But a fast reaction with a lost body position entering the water is meaningless — a crooked entry can destroy the entire reaction advantage and the underwater phase that follows. This is a trade-off analysts rarely discuss: between reaction speed and entry quality. Some swimmers react extremely fast but enter shallow, creating turbulence and losing force. Others react more slowly but enter cleanly, immediately shifting into the dolphin kick and recovering the lost time. In fifteen meters, the race is not only about who is faster, but about who errs less. After surfacing, the story shifts to another axis: stroke rate and distance per stroke. Two swimmers can swim the same speed in two entirely different ways — one with a fast stroke and short distance, one with a slow stroke and long distance. Over short distances, a high rate usually wins because there is little time for error. Over long distances, a long stroke saves more energy. But the underwater phase is where both axes are temporarily suspended, leaving only one variable: the quality of the body position. Underwater strategy also changes with distance. In the 50 meters, where everything is decided in just over twenty seconds, the start and the underwater phase are almost everything. In the 200 meters, where endurance speaks, swimmers must weigh: going deeper and longer underwater can create an early advantage, but the price paid at the end can be greater. This is an optimization problem each swimmer must solve alone, based on their own body and feel for the water. The turn is another underrated moment. A good turn not only saves time in itself, but also places the swimmer in a perfect streamline position for the next fifteen meters. This is why top national teams spend hours each week simply practicing turns and push-offs — movements spectators almost never remember by name. Here I find myself bridging the track and the pool again. In athletics, the first acceleration phase after the start — the drive phase — decides most of a sprint, even though it covers only a small part of the distance. In swimming, the underwater phase plays a similar role: a short, unseen segment that compresses the entire tactical advantage. The Gatlin–Coleman equation taught me that speed is never a single variable. When I worked with Dr. Emily Chen at the Australian Institute of Sport during the period when global sport was halted by the pandemic, I learned to read data the way you read a character. We analyzed the ground contact time of hurdlers and uncovered technical flaws that the results board concealed. The COVID laboratory taught me that data feels pain — if only we choose to listen. Swimming is the same: a beautiful splits sheet can conceal a weak underwater phase. At this point I want to turn to a different angle, one the sports media often avoids because it is not glamorous. When World Aquatics drew the fifteen-meter boundary, the original intent was to protect the purity of the strokes — to let victory be decided by the arm stroke, by butterfly technique, by the rhythm of breaststroke, rather than by a diving technique anyone could train equally. The result ran against that intent. Rather than reducing the importance of the underwater phase, the boundary concentrated it. With only fifteen meters left, those fifteen meters became a winner-take-all contest zone. There was no room for spreading effort; every hundredth of a second was compressed into a short, unseen stretch. The result is that national teams invest in the underwater phase more than ever, and a skill once seen as auxiliary has become the center of tactics. This leads to a consequence rarely discussed: the underwater phase favors certain body types. A long arm span, a flexible torso, a strong core, and especially good ankle flexibility are genetic factors that training can improve only to a point. A sport that calls itself democratic, where anyone who works hard can improve, is quietly screening by characteristics the athlete cannot choose. There is another, less-mentioned price: the dolphin kick, repeated at high intensity over many years, places great stress on the lumbar spine. Many butterfly and medley swimmers struggle with back injuries whose cause lies precisely in the technique that lifted them to the top. Performance and injury, here, are two sides of the same movement. At a deeper level, this story touches a paradox of modern sport: specialization and versatility. Leon Marchand or Summer McIntosh win multiple events not because they are average at everything, but because they reach the peak in one foundational skill — the underwater phase and feel for the water — and then apply it to different strokes. Their versatility, paradoxically, is built on an extremely deep specialization. That is what youth-development models built on "trying everything" often miss. Academies and satellite clubs have, in a sense, turned talent-hunting into an assembly line. Young swimmers with suitable physical profiles are promoted early, trained in the underwater phase from childhood, and sometimes pushed into specialized events before they have fully developed. This is the dark side of an efficient system: it optimizes for results, but sometimes forgets the person behind the result. What I want to stress is this: behind every record there is always a hidden structure — a network of rules, bodies, training technology, and health trade-offs. To look at a result without looking at that structure is to look at the tip of the iceberg. And in swimming, the submerged part lies exactly where the eye cannot reach: the first fifteen meters. Perhaps that is why swimming remains the strangest and most seductive sport to me. It is the common language of humanity — everyone has learned to swim, everyone has feared the water — yet it is also a language only a few speak fluently. Every record is a confirmed hypothesis; every failure is an equation waiting to be solved again. This championship season will produce new champions. When they stand on the podium, the cameras will point at their faces. But I will remember that somewhere underwater, in the first fifteen meters of a final, another race was decided long ago — by people who never appear in the frame.