Gretchen Walsh and the 100m Butterfly Record: Where the Data Is Written Underwater

55.18 seconds. The scoreboard in Indianapolis showed that number on the night of the U.S. Olympic Trials 2026 women's 100m butterfly final. The stands erupted. But when I rewound the slow-motion footage at the 15-meter mark, the notable thing was not the final number. It was the underwater segment after the start — where Gretchen Walsh's peak speed exceeded her surface racing speed by nearly 30%. In 21 years of watching swim meets from the stands and from the data room, I have drawn one simple rule: in the 100-meter events, medals are not decided in the final 50 meters. They are decided in the first 15. For more than a decade, the women's 100m butterfly belonged to Sarah Sjöström. The Swede held the world record of 55.48 seconds from 2026 to 2026 — a record that lasted eight years, rare in an event where records are usually erased within a few seasons. The interesting part is how Sjöström held it. She was not the swimmer with the highest stroke rate, nor the one with the longest jump. Sjöström won with steady rhythm and the ability to hold speed over the final 25 meters — a classic 'slow burn' model. Walsh took the opposite path. Born in 2026, the University of Virginia swimmer does not try to swim evenly. She attacks. She pours all her advantage into the first 35 meters, especially the underwater phase after the dive, then relies on her physical base to survive the rest. When I looked at her splits in the heats, the gap between Walsh and the chasers was created almost entirely before the 35-meter mark. After that, her speed dropped exactly as the model predicted — but the gap was too large to close. This is where the method matters. I do not argue with emotion; I present chains of data. For each of Walsh's swims, I recorded four metrics: the 15-meter time (including the start and underwater phase), the 25-meter time, the 50-meter split, and the average stroke rate per 25 meters. Those four metrics are enough to reconstruct almost the entire story of a 100m butterfly race — without any interpolation. The result is clear. Over the first 15 meters, Walsh recorded times among the fastest in the history of the event. She performed two to three more underwater dolphin kicks than her rivals before surfacing. In butterfly, each underwater dolphin kick generates more propulsion than a surface stroke while consuming less oxygen — at least in the early phase. This is why swimming data analysts increasingly treat the underwater phase as an 'overtime period' the rules have yet to fully define. In other words, the most important part of a 100m butterfly race is not the part the crowd sees most clearly. It lies in the part where the stands only see ripples. In short-course events, the rules limit the number of underwater meters after each dive, but over 100 meters there are two start/dive moments — the opening and after the turn — each permitting up to 15 legal underwater meters. Multiplied out, that is up to 30 meters, nearly a third of the total distance, that can be decided beneath the surface. One technical point deserves clarification. In butterfly, current rules allow swimmers to stay underwater for a maximum of 15 meters after the start and after each turn, before the head must surface. Unlike backstroke, the rules do not cap the number of underwater dolphin kicks — only the distance. This creates a tactical gray zone: swimmers can maximize kicks within those 15 meters as long as they surface in time. For years, coaches treated underwater work as a 'bonus' after surface training. Today, in short-course events, it is the foundation. Look at the 50-meter split structure of a typical record swim. An ordinary swimmer goes out in about 26 seconds and finishes in 29. Walsh goes out about half a second faster than the field, but what truly makes the difference is that she does not collapse too quickly over the second 50. This is the crux: the record does not come from swimming faster in the freshest phase, but from decelerating less in the most fatigued phase. I often liken this structure to budgeting. The first 50 meters is income; the second 50 is spending. Most swimmers borrow too much in the first half to cover a weak second half and go 'bankrupt' in the final 15 meters. Walsh builds a large buffer in the first 35 meters, then spends the rest under control. The buffer is thick enough that a slightly weaker second half does not matter. When I analyze the data over time, I notice an interesting pattern: records at 50 and 100 meters tend to fall in 'clusters' — one swimmer breaking a record pulls a wave of others to break records within 12 to 18 months. This happened in the men's 100m butterfly in the 2000s. The reason is not only the 'if they can do it, so can I' psychology, but also that teams begin copying training models, especially underwater volume. This is precisely where the counterintuitive angle appears. The strength of Walsh's style is also its weakness. A front-loaded race depends enormously on the quality of a few opening seconds: the dive, the entry angle, and the kick rhythm. Any small error in those three factors amplifies across the entire race. A 'slow burn' swimmer like Sjöström has room to correct mistakes; an 'attacking' swimmer like Walsh has almost no safety net. Moreover, correlation and causation must be distinguished. The fact that new-generation swimmers are setting records does not automatically prove that underwater technique is the sole cause. Many variables need controlling: the pool, water temperature, suit material, and the competition schedule. High-tech suits were restricted from 2026, but optimized versions for specific events still exist. A single record does not create a trend — a principle I set for myself after years of being 'tricked' by data. The psychological factor cannot be ignored either. Being right too early is also a form of rejection. When I predicted a shift in short-course records before an Olympics, several colleagues objected. Yet, looking at the age data and improvement trajectories, the model indicated the next wave would arrive sooner than expected. It unfolded exactly that way — it just arrived so fast that many were unprepared. And when the editor says no, I learn to listen to the data. Years ago, when I proposed analyzing a swimmer purely from underwater data, my editor refused, arguing readers cannot see what does not appear on television. But precisely because they cannot see it, it is the competitive edge. Sjöström held a record for eight years with rhythm; Walsh broke it by exploiting a gap the rules have yet to close. Two philosophies, two eras, one event. The race ends, but the data keeps playing stoppage time. After the scoreboard goes dark, the real story is only beginning. I follow the sport not to know who won, but to understand why. With Walsh, the answer lies where few look: beneath the surface, in the first meters the stands feel only as splashing. When you watch the 100m butterfly next season, pay more attention to the first 15 meters — that is where records are written, and where I will keep sitting with the numbers, amid the roaring stands.