Should you swim your second lap faster? In fly, usually not — how to work with the typical positive split instead of against it.
The honest answer to "should I try to swim my second lap faster than my first?": realistically in fly: no — and that's okay. Unlike long open-water distances, the 100m butterfly is a sprint race, and sprint races come down to a positive split for practically every athlete: the second half is slower than the first.
A typical elite profile looks like this: the first 50 meters driven by the dive start and raw explosive power, often around 23 seconds, the second 50 meters carried purely by speed endurance, often around 25 seconds — a difference of roughly two seconds. To put that in perspective: it's like sprinting the first 50 meters flat out and then dragging the second 50 through in whatever gear you have left — the start advantage and later fatigue add up to this pattern, not an even pace.
The decisive point for your training: precisely because the race itself runs positively split, it pays to practice the opposite in preparation. Negative splits in training — deliberately swimming the second half of a practice distance faster than the first — build the pacing discipline that stops you from exploding too early in a race and collapsing completely. You're training deliberately against the natural race pattern so you can use it in a controlled way when it counts, instead of being at its mercy.
Swim a control time over 100 meters and note both 50-meter split times. Most recreational and club swimmers are far more positively split here than the elite pattern — often by more than the typical two-second difference.
The first instinct is to max out the first lap because it feels easy right after the dive. But that's exactly what usually leads to an excessive collapse afterward. Deliberately hold 5–10% below your maximum pace to keep reserves for the second half.
Build training sets with 50-meter segments at your planned race pace, instead of only swimming long, easy distances. That gets your body used to exactly the intensity the race demands.
Regularly swim training distances where the second half deliberately has to be faster than the first — even if that feels "unnaturally" slow at the start in training. This trains exactly the pacing control you're missing in the race.
Count your arm strokes per lap half across several repeats. If the stroke count rises sharply in the second half, that's an objective sign your technique is collapsing under fatigue (see the error-chain guide) — before the clock even shows it.
A too-aggressive start without reserve doesn't just lead to a slower second half — it often triggers exactly the error chain from the corresponding guide, and technique collapse under fatigue additionally raises the risk of shoulder and back load (see the prevention guide). Plan race-pace training sets with enough recovery between repeats instead of repeating them in a fully exhausted state.
Set yourself a target stroke count per lap half for your race pace, not just a target time. Time alone doesn't tell you whether you swam clean or with collapsing technique — stroke count does. Whoever needs fewer strokes for the same time was more efficient, not just faster.
Realistically, usually not — the 100m fly is a sprint race, and sprint races come down to a positive split for most athletes, often around a two-second difference between halves. The goal isn't a negative split in the race but a controlled rather than uncontrolled positive split.
Because it's carried by the dive start and raw explosive power, while the second half relies purely on speed endurance once the start advantage has worn off. That's a normal, physiological pattern for sprint distances, not a sign of poor preparation.
Because negative splits in training build the pacing discipline that stops you from exploding too early and completely in the race. You train against the natural pattern so it plays out in a controlled way when it counts, not chaotically.
Count your arm strokes per lap half. If stroke count rises sharply in the second half, your technique is collapsing under fatigue (see the error-chain guide) — if it stays stable but the time still drops, fitness is more likely the limiting factor.
Butterfly: Getting Started and Overview