The fastest you will ever travel in a pool is the instant your feet leave the wall. Not during your best stroke, not on your last 25 of a set. Off the wall. No swimmer alive can generate that speed by swimming, so every metre after toe-off is a question of how slowly you give it back.
That is the whole argument for streamline. It is the cheapest speed in the sport, and most swimmers give it back inside half a second. The stakes are not small either: the paper on gliding and underwater kicking in the swim turn opens by noting that swimming turns "have been reported as comprising over one-third of the total race time in all events of 200 m and longer". In a 25 m pool the walls come twice as often, and you rehearse whatever you do at them.
What is a streamline position in swimming?
One continuous line from fingertips to toes, with nothing sticking out of it.
The computational fluid dynamics study on the effect of depth on drag during the streamlined glide describes the position it modelled plainly: "hands overlapping, head between the extended arms, feet together and plantar flexed". Read that as a checklist, because every item is a place swimmers leak speed. Hands overlapping, not side by side with a gap between the thumbs. Head between the arms, biceps squeezing the ears. Feet together, toes pointed, because a flexed foot is a brake hanging off the end of a good line.
The one I correct most on deck is the head. The arms lock, the chin lifts to see where the swimmer is going, and water that ran over the hands hits the forehead instead.
How deep should you push off the wall?
Deeper than the surface, and how much deeper depends on how fast you are leaving.
That turn study towed forty experienced swimmers at four depths and six speeds: "Results demonstrated significantly higher drag at the surface than at 0.2, 0.4 and 0.6 m underwater for all velocities tested."
Then comes the part coaching advice skips. Their conclusion splits by speed: "a depth of 0.4 m will minimise the drag for velocities above 1.9 ms-1, and a depth of 0.2 m for slower velocities." Depth is not one number for everybody.
Here is my own arithmetic on their figure, not a finding of theirs. At 1.9 metres per second you would cover 25 m in a little over 13 seconds, quicker than most adult swimmers push off. If that is you, their numbers point to the shallower end, around 0.2 m, rather than the deep push-off you see on television. Depth you are not fast enough to use only costs you the climb back up, a trade I wrote about in the post on how to do a flip turn.
How far should you glide before you come up?
About a metre at depth, then a gradual ascent over the next metre, for a fast push-off.
The authors modelled an optimal glide path and stated it as instructions: swimmers "should push-off the wall at approximately 0.4 m deep and maintain this glide depth for approximately 1 m for maximum drag reduction benefits at higher velocities. Following this, swimmers should begin to ascend over the following 1 m to reach the surface and resume stroking at race pace."
Two metres in total: not the long glide held until you have stopped, and not a scramble for the surface either. Picture a shallow ramp, not a dive and a climb.
When should you start kicking off the wall?
When you have slowed to roughly race speed. The study puts a number on it: "swimmers should initiate underwater kicking using either kicking technique at between 2.2 and 1.9 ms-1."
Both errors cost you. On the early one the authors are blunt: "By initiating the underwater kick too early in the glide, there will be an increase in active drag which will act to slow the swimmer down." The late one is just as real, since "gliding too long underwater before kicking will decelerate the swimmer to less than race pace and mean an increased energy demand having to accelerate the swimmer back to free swimming speed."
One line from that paper should change how you picture the underwater: "at no velocity did a positive net force occur, which would have indicated that the swimmer was accelerating as a result of underwater kicking." The kick does not speed you up off the wall. It slows the rate at which you lose the wall's speed, a more honest job description, and what makes it worth doing sits in the post on the underwater dolphin kick. Push off below 1.9 metres per second, as most adult swimmers do, and that threshold is already behind you at toe-off, so kick almost straight away.
Does it matter whether you streamline on your front or your side?
Less than people argue about. The same study compared a prone streamline glide against a lateral one and found that "no significant difference was found between the prone and lateral streamline glides at any velocity, although the lateral streamline position tended to consistently record higher negative net forces at each velocity."
Pick whichever lets you finish the turn without fighting yourself. A tighter shape beats a cleverer orientation.
How does SwimRead read your body line?
It reads the body line in your freestyle, and it is explicit that the wall is out of scope for now.
SwimRead's home page states that "Backstroke, breaststroke, butterfly, underwater footage, starts, and turns are not supported analysis modes in this beta", and that "The current beta analyzes freestyle filmed from the side of the pool." I would rather say so than have you film a push-off and wait for a reading that is not coming.
What it does read is the same long flat line this post is about, in the part of the length where you hold it yourself. So film the swimming, not the wall. The capture guide asks you to "Aim for 10 seconds of continuous freestyle. Film in landscape from the side, with the whole body visible, a stable camera, standard lens, and normal playback speed." Its own advice is the habit that makes any of it useful: "Keep the filming comparable."
How do you practise streamline and push-offs?
Separately from swimming, and with something to measure. A streamline you only perform into a set never improves.
- 10 push-offs, no kick, no stroke, marking where you stop with a tile. The number to beat is your own from last week.
- 5 push-offs with a partner watching one thing only: whether your head stays between your arms the whole way out.
- 8 x 25 counting strokes, shallow push-off for four and deeper for four. Fewer strokes at the same time tells you which depth is yours.
- 6 push-offs holding streamline until you drop below swimming speed, then one kick, then breakout. You are hunting that handover, not a long glide.
- 6 x 50 from a push on fixed rest, times through the pace calculator. Walls move a 50 time before your stroke does.
Watch for the gaps: between the hands, between the feet, between the arms and the ears. Three gaps, three places the water gets in.
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Streamline is the one piece of technique where the physics does the work and you only have to stop interfering. I built SwimRead because most swimmers never see their own body line, and the gap between what it feels like and what it looks like is where the years go. Its home page states that "SwimRead is free while in beta." If you want to see that line instead of guessing at it, join the beta.