Most self-taught butterfly has one usable kick. The swimmer kicks the hands into the water, feels the front of the stroke lift, then nothing happens at the back end. The hands come out with the hips already sinking, the recovery turns into a shrug, and the next entry starts from a body pointing downhill. The missing piece is the second kick.
Butterfly is the second leg of the 200 individual medley, the event I swam at two Olympic Games for Brazil and won at the 2015 Pan American Games in Toronto in 1:57.06. In a medley the fly has to be fast and cheap at once, and a fly running on one kick is never cheap.
What is the second kick in butterfly?
It is the downbeat that lands as your hands finish the pull and leave the water. Butterfly runs two dolphin kicks per arm cycle, and the two have different jobs. US Masters Swimming compresses the structure into three words in its master butterfly article: "The limb coordination for butterfly is two kicks to every arm cycle: kick-pull-kick." One kick sits at each end of the pull, and the one at the back end is the one almost nobody trains.
Where exactly should the second kick land?
As your hands exit the water at the end of the pull. The US Masters Swimming guide to butterfly timing is exact about it. It tells you to "Do two kicks per pull at very specific times", then gives the instruction "Finish one kick at the completion of your pull." In its list of faults it repeats the fix: "The second kick should occur as your hands exit."
The other kick you probably already have. The same guide says "Your other kick should finish right after your hands enter the water following your recovery", and the master butterfly article agrees: "The first kick is synched to the entry of the hands, and the second to their exit at the beginning of the recovery."
Both reference points are your hands, not a feeling in the water. Hands in, kick. Hands out, kick. Hold the second thought, because the first one tends to happen on its own.
Why does the second kick matter more than the first?
Because it arrives while your hands are at their fastest, and it is the kick holding your hips up for the recovery. The master butterfly article separates the jobs: the entry kick "is more of a balancing motion, arising mainly from the upward lift of the hips", while "The second kick, when timed with the finishing forces of the hands, combines to create the most propulsive phase."
The timing guide covers the body position side. That kick "allows your hips to elevate in the water when you begin to recover your arms", and its blunt version: "The most effective way to get your hips back to the surface is to kick."
A 2008 pilot study on predicting velocity variation from segmental velocities in butterfly says the same from the data side: "The second downbeat has to be as strong as possible to keep the hip near to surface."
What you buy is a smoother cycle. A chapter on the biomechanics of competitive swimming strokes states that "a high segmental velocity of the legs during the downbeats, specially the second one, will decrease the speed fluctuation." Every slowdown inside a cycle has to be paid for again with acceleration.
What goes wrong when the second kick is early, late, or missing?
Three faults, and the timing guide names them all. The first is "Not kicking at all upon exit", the one-kick butterfly. The second is "Kicking twice after your hands enter", where both downbeats fire at the front and the recovery gets no support. The third is a kick that is there but weak, listed as "A weak kick upon entry" with the instruction "Keep it strong."
Being off by a fraction of a second costs you twice. Too soon and you waste the kick: "If you try to kick early, while you're still recovering your arms, your kick isn't likely to do much of anything." Too late and you waste the stroke around it: "If you're late for your kick, you're going to spend more time in a bad body position, which you want to avoid."
Breathing is usually why the timing drifts. If your head is still up when your hands land, the whole cycle shifts back. I covered that sequence in butterfly breathing timing, and a 2025 Frontiers in Sports and Active Living study on muscle activity and coordination in butterfly found that "different breathing frequencies cause slight changes in the phase structure of leg movement".
Why does swimming butterfly slowly hide the problem?
Because the gap between the two kicks only opens at speed. The 2008 pilot study measured foot velocities at different speeds and found that at high speed "the Vy-2dwn presented a higher mean value than Vy-1dwn", the second downbeat being the faster of the two, while at slow speed "Vy-1dwn and Vy-2dwn mean values were close one to the other." Eleven swimmers across two parts of the experiment make that a hint, not a law.
It matches the deck: slow butterfly gives two small, equal, polite kicks and feels fine. Race pace asks for a bigger second one, and that is where a one-kick stroke falls apart. I would rather watch four good 25s than a laboured 100.
How does SwimRead read this part of your swimming?
It does not read butterfly, and you should know that before you film. The SwimRead home page states that "The current beta supports freestyle filmed from the side of the pool", and that "Backstroke, breaststroke, butterfly, underwater footage, starts, and turns are not supported analysis modes in this beta."
What it does today is read a short freestyle clip. The page asks you to "Aim for 10 seconds of continuous freestyle", filmed "in landscape from the side, with the whole body visible, a stable camera, standard lens, and normal playback speed", and from that "On-device pose detection estimates up to 33 body landmarks in sampled video frames." It stops short of a verdict, since "These estimates are not validated technique grades."
On butterfly you are your own analyst, and this fault does not need software. Phone on the deck in landscape, film one 25m from the side, and scrub to the frame where your hands break the surface on the way out. Your feet should be coming down in that frame. For the freestyle reading the app does give you, see freestyle kick.
What should you drill this week?
- One arm butterfly, other arm at your side. Kick the working hand in, kick it out. There is nowhere to hide a missing kick.
- Three kicks, one stroke, over 25m. Pulls the two kicks apart so you feel them as separate events.
- Two strokes without a breath, then one with. Keeps the head down so the timing does not drift, which is the rhythm work behind the butterfly hub.
- Butterfly kick on your back, hands at your sides. Shows whether the wave starts at your chest or your knees, the check I use in the underwater dolphin kick work.
Time those 25s in my free swim pace calculator to check whether the second kick made you faster rather than just making the stroke feel tidier.
SwimRead is free while in beta, and paid plans will come later with beta testers told before anything changes. For a reading of your freestyle, join the beta and leave your email for an invitation as beta spots open.