Watch an adult swimmer from the deck and count the breaths. On most strokes the legs stay inside the body line, a busy little flutter behind the hips. Then the head turns, and for half a second the legs open like scissors before snapping shut.
That is the most common kick fault I see, and almost nobody knows they do it. It shows up only on the breathing stroke, the one stroke a swimmer cannot feel properly, because the attention is all on air.
What is a scissor kick in freestyle?
A scissor kick is when your legs separate sideways instead of fluttering in line behind you. One leg sweeps forward across the body, the other sweeps back, forming a wide V from the front.
In breaststroke that split is propulsion. In freestyle it is a brake. The flutter should be narrow and fast, driven from the hip with a loose ankle, living inside the shadow your shoulders cut through the water. A leg that leaves that shadow becomes a flat plate pushed sideways through water.
The rhythm tells you which fault you have. A scissor on every kick is a kicking habit, usually kicking from the knee. A scissor once per cycle, always on the same side, always as the head turns, is a breathing problem wearing a kicking costume.
Why do your legs split when you breathe?
Your legs split because they are stopping you from sinking or rolling too far, and a wide leg is the fastest stabiliser the body has.
Look at what the breath asks of you. Lift the head instead of rolling it and the hips drop at the other end of that lever. Roll past the point where your mouth clears and you keep going until the body panics about rolling onto its back. Either way the brain gets the same message, that balance is going right now, and solves it the way anyone on ice does, by widening the base.
This is why coaching the legs rarely works. Tell a swimmer to hold their legs together and they manage one length and lose it on the breath, because the split is not a decision. It is a reflex covering for something upstream, almost always a lifting head or a rotation running too far. I covered the second in how much freestyle body rotation is too much and the sinking half in why your legs sink when swimming.
A crossover at hand entry does it from the other end, twisting the body sideways so the legs answer the same way, which is why a crossover on one side so often pairs with a scissor on the other.
Is the scissor kick slowing you down?
Yes, twice over. First in drag, because a leg outside your body line meets water with its whole flat surface, when you are least streamlined anyway. Second in lost drive, because for the length of the scissor the legs correct instead of propelling.
Does the breath change more than your legs?
It does, and the research measured it in the arms rather than the legs, worth knowing before you blame the kick.
A study on underwater stroke kinematics during breathing and breath-holding front crawl swimming reports that "breathing caused significantly increases in the stroke duration" and that "the displacement of the hand during the push phase" was significantly reduced during breathing. The authors summarise the overall effect as the "breathing action in front crawl swimming caused significant modifications in both the basic stroke parameters and the overall motor pattern were, possibly due to body roll during breathing".
The breathing stroke is slower with a weaker finish even in trained swimmers, and the paper points at body roll as the likely reason. It does not measure the legs, and I will not pretend otherwise. It tells you where your stroke is most fragile.
Why does it happen on only one side?
Because most swimmers breathe to one side, and that side rolls further.
A study on the effect of breathing laterality on hip roll kinematics in submaximal front crawl swimming states that "Unilateral breathing is common and inherently asymmetric, as most swimmers have a preferred breathing side", so that "a swimmer rolls more to one side (the breathing side) than the other". In their measurements "Peak hip roll to the preferred side was significantly greater in the unilateral condition", while "bilateral breathing...produced significant reductions in preferred side hip roll".
That last line is the practical one. If the roll to your breathing side is what the legs correct for, breathing to both sides shrinks the thing being corrected and stops you building the fault into one side. More on that habit in how to breathe in freestyle swimming.
How does SwimRead read this fault?
SwimRead gives a frame by frame technique breakdown, and four of its scored categories sit on this problem. The home page lists them as Entry, Catch, Pull, Recovery, Rotation, Kick, Head position and Breathing, so the kick, the roll, the head and the breath are read separately rather than bundled into one verdict. That separation is the useful part here, because a scissor kick is a kick symptom with a head or rotation cause.
On filming, the page says to "Record 8 to 20 seconds per lap clip from up to 4 angles: side, front, underwater." I would not skip the front angle, since a split leg is most obvious straight down the lane, while a side clip of the same lap shows whether the head is lifting. Film laps where you breathe, not a held breath sprint. The page also states that "Freestyle is the calibrated stroke today" and that "All analysis runs on your phone. No video upload. No cloud processing."
Which drills fix a scissor kick?
Work the cause, not the legs.
- Six kicks per side with one arm stroke between them. The breath happens in the side position, so the body learns to breathe with the roll, not separately from it.
- Vertical kicking in deep water, arms crossed on the chest. A scissor gets you nowhere here, so feedback is immediate.
- Single arm freestyle, idle arm at your side, breathing to the working side. It isolates the roll and shows how little rotation the breath needs.
- One set per session with a snorkel. The paper above found snorkel breathing reduced roll asymmetry, so you feel a symmetrical roll and a narrow kick together.
The cue through all of them is to breathe in the trough your head makes rather than above the water, and to stop the roll the moment your mouth clears. The target is not a smaller kick. It is the kick you already had, surviving the breath.
What do you check when it comes back under fatigue?
Count strokes per length at the start and the end of a long set. When the roll gets lazy and the head lifts, the kick widens to compensate, so the count climbs while the splits slow. The swim pace calculator shows what those seconds cost you.
The difficulty is that you cannot feel this one. The breathing stroke feels fine because you got your air, and air is what your brain was grading. From the deck it takes one length to spot, and most adult swimmers are never watched.
That is the gap SwimRead is built for. The home page states that it is free while in beta, that paid plans come later, and that beta testers will be told before anything changes. If you would rather see your breathing stroke than guess at it, join the beta.