The 100m Freestyle Lane: How the First 50m Split Decides Everything

In Paris, on the night of the men's 100m freestyle final, I sat in front of the screen with a notebook split into three columns. The first was the finish time. The second was the opening 50m split. The third I left blank, reserved for the thing the broadcast scoreboard never shows. When Pan Zhanle touched the wall at the halfway mark, a number appeared: 22.28 seconds. Beside me, a friend who works as a commentator blurted out: “Finish like that and nobody can catch you.” He was right, but for a reason he himself did not realise. In short-course racing, people love to savour the finish. The broadcast switches to slow motion, the commentator raises his voice, the crowd holds its breath over the final twenty-five metres. Based on my experience of tracking thousands of races, the finish almost always only confirms something decided earlier. The Saigon summer of that year taught me that data also needs watering — meaning it must be placed in the right context, the right split, the right moment. And the thing decided earlier lies mostly in the first fifteen metres, underwater, where no camera reaches. The 100m freestyle is the event nicknamed the “queen” of the short sprints. It compresses everything into less than fifty seconds: reaction off the blocks, the number of dolphin kicks underwater, stroke rate, and the ability to hold speed as lactic acid begins to bite in the shoulders. In Paris, organisers and media built a compelling story: a gathering of stars, a race billed as the most balanced in decades, and, as ever, one that “would be decided in the final metres”. That story sounded wonderful. It also sold easily. In the market, the odds for this race were priced extremely tight, reflecting the crowd's psychology perfectly: everyone believed it was a balanced race, so everyone was ready to back the “strong finishers”. But crowd belief and data reality are two different things. To test it, I opened my spreadsheet, broke the race into segments, and let the data speak. I still remember the first lesson about breaking data into pieces. In 2026, as a rookie employee, I once lost money by following an emotional opinion that a certain football team was playing well. From then on, I promised myself never to reach a conclusion without concrete numbers attached, whether in football or in the pool. That principle has followed me for years: numbers do not lie, but they do know how to hide something. In Vietnam, short-course swimming has a similar perception problem. Fans, and sometimes even coaches, fixate on the visible parts — stroke speed, the finishing kick — while the parts that create the difference, namely the start and the breakout, are underrated. Over years of watching domestic meets, I noticed that most young Vietnamese swimmers lose several metres right at the breakout, and they have no tool to measure it. No data, no concept, and therefore no incentive to improve. This is a gap that data can fill, but it must be done correctly. The first thing I did was compare the opening 50m split of all eight finalists. Pan Zhanle swam 22.28 for the first half. The gap he created over the rest was not large, but just enough that he never had to swim in anyone else's turbulence. In the 100m event, there is a rule I derived after cross-checking hundreds of finals: the leader at the 50m mark wins most races. Not because they swim the second half faster, but because they control the rhythm and the position. When you are in front, you do not have to adjust to avoid an opponent's wake; you swim your own rhythm. When you are behind with a final quarter to claw back, you are forced to raise your stroke rate, and raising stroke rate excessively is the shortest path to losing technique over the last twenty-five metres. From a physiological standpoint, this comes down to energy distribution: crossing the threshold too early prevents the muscles from sustaining output, and every extra stroke taken is traded for steadily diminishing propulsion. Kyle Chalmers is the classic example of an athlete who lives by the second half. He typically starts conservatively, saves energy, then unleashes his sprint in the final twenty-five metres. This distribution has won him many races, but it also puts him on the back foot against rivals with a dominant first half. Pan Zhanle, by contrast, represents the type who starts extremely fast and holds rhythm all the way to the wall. The contrast between these two models is the axis of every final analysis. But the 50m split is only the outer layer. The deeper layer lies in the first fifteen metres, after the dive. During that stretch, the swimmer stays underwater, dolphin kicking. This is where true peak speed is generated, because underwater there is no wave drag as there is on the surface. A strong swimmer kicks many times with a stable amplitude, keeps the lane line straight, and surfaces at the right moment while speed is still high. A weaker swimmer usually surfaces too early, before using up the momentum, then begins stroking after the glide has died. Under current World Aquatics rules, freestyle swimmers may stay underwater for up to fifteen metres after each start and each turn. This is the legal zone that many overlook, and it is also where the most valuable seconds are made. The broadcast scoreboard does not break things down that far. It only shows the total time and sometimes the 50m split. As a result, the entire decisive portion — the start, the underwater phase, the dolphin-kick rhythm — becomes a blind spot. I have measured footage myself and found that among world-class swimmers the underwater segment can account for a substantial share of the total time of a 100m race, whereas among young Vietnamese swimmers that share is usually markedly lower. The difference never appears on the electronic board, but it exists in every hundredth of a second. Turning to the second half, I analysed each swimmer's closing speed. Here a clear trap emerges. Closing speed is the most impressive concept, but also the most misleading. A swimmer with a fast second half is often fast because they swam the first half slower than their optimal pace. In other words, if effort is distributed properly, the second split should be stable and roughly equal to the first; if someone's second half spikes dramatically, it is usually a sign they miscalculated the first. The exception — a fast second half that still wins — exists, but far more rarely than the media claims. In Paris, the very swimmers famous for a “devastating finish” faded at the decisive moment. The reason is structural: when you rely on closing speed to win, you place the entire race into a single moment, under pressure to raise stroke rate while the muscles are already tired. The leader at the 50m mark, by contrast, holds absolute control: they can choose when to accelerate, and no one forces them to swim off rhythm. To verify further, I cross-checked the group with the fastest first halves against the final results across a sample of recent major finals. The outcome showed that the correlation between 50m rank and finishing rank is far stronger than the correlation between closing speed and finishing rank. In other words, if I had to pick a single indicator to predict with, I would pick the first split, not the last. This is where I want to push back against what most viewers believe. The media teaches us that short-course races are decided in the final metres. My data says the opposite: they are decided in the opening metres, and in many cases before the swimmer even surfaces. Closing speed is an attractive indicator because it is easy to see, easy to remember, easy to turn into legend. But it is like judging a football match only by the goals scored in the second half. It correlates with the result, yet correlation is not causation. There is a line I still use when training staff: the split index is not a number. It is a confession. When a swimmer has an extremely fast first half, that is a confession that they know the race will be won or lost in the first half. When a swimmer spikes in the second half, that is a confession that they lost time they could have controlled. Reading splits is not reading results; it is reading how a swimmer understands their own body. The interesting paradox is this: the very people labelled “finishers” carry the highest risk. They live by a script that only succeeds when everything goes perfectly — the opponent fades, the body still has reserves, and the water stays calm. Those three things rarely happen at once. The leader, with a controlled first half, only needs not to blunder. The difference between proactive and reactive is, after all, the difference between the person who owns the race and the person chasing it. Of course, I do not deny the value of closing speed. In some cases it is a decisive weapon, especially when a race is slow and tactical. But when it becomes the sole standard for judging a swimmer, we are misreading the nature of the sport. Swimming is not a contest of who sprints best; it is a contest of who distributes energy and controls technique best over the whole journey. There is one more layer few mention: the talent supply chain. In recent years, swimming powers have frequently recruited young athletes from developing nations, placing them into structured training systems very early. When these talents mature, they compete under a new flag. In data terms this is long-term investment: low initial cost, high return, and risk spread across many cohorts. For smaller nations, every talent lured away is a double loss — losing the athlete and losing the coaching experience they would have passed to the next generation. Vietnamese swimming is not outside that rule. For Vietnamese swimming, the lesson is not to celebrate a beautiful sprint. It is to ask how many hours we have spent training the part the naked eye cannot see: the start, the breakout, the kick rhythm in the opening fifteen metres. This is where I want to track the signal of the next cycle — not on the medal table, but in the split numbers nobody bothers to publish. Because if data needs watering, then the space beneath the surface is the most fertile ground still waiting to be cultivated.