What does a long swim stroke need from your body?

A runner can catch her reflection in a shop window. A rider can glance down at his knees. A swimmer has no mirror. Your face is in the water, your eyes look at the floor, and all you can see are your own hands arriving in front of you. Unless someone coached you as a child, it is hard even to know what good swimming looks like.

Water is about 800 times denser than air, so any part of you that sits out of line slows you down on every stroke. The usual culprit is the legs. Your lungs float and your legs are heavier, so your feet tend to sink.

Physiologists tied a tube round elite swimmers' waists and filled it with air one time and two kilograms of lead another. With the lead pulling their legs down, each metre cost them about 20 percent more energy, most of it spent pushing against extra drag. Even these swimmers could not fix sinking legs with their stroke. They just used more energy.

A Dutch physiologist compared competitive swimmers with triathletes, all of them highly trained. Using energy at the same rate, both groups took strokes at the same rate and did the same work on each one. The swimmers still went faster, because each stroke carried them 1.23 metres against the triathletes' 0.92. The triathletes spent much of their work pushing water backwards. The swimmers spent more of theirs moving themselves forward.

At that rate, the swimmers covered 1.17 metres a second and the triathletes 0.95. Over a kilometre, that is about 14 minutes against about 17 and a half. The energy going in was the same. The difference was the stroke, and the author concluded that the triathletes should work on their technique rather than on how much work they could do.

So the thing that decides what every length costs is the one thing you cannot watch. You learn it the other way, by feel, and every tool for learning to swim is a way of feeling what you cannot see.

That takes attention. There is no way to learn to swim except by swimming, but laps alone will not change your stroke. Swim without paying attention and you repeat the stroke you arrived with, a little more fixed each time. That does not mean stopping your swims until your technique is right. It means paying attention in them.

Attention needs time, and speed takes it away. Swim at base effort, about three out of ten, where ten is the hardest you could go. Holding back takes discipline. Float, and feel the stroke. Notice where your hips are, where your hand enters, and whether you are reaching long or snatching at the water.

It also takes a calm breath. You breathe out steadily with your face down, and breathe in deeply in the moment your mouth clears. On land you can choose how you breathe. In the water you fit your breath to the stroke, and you learn to do it without panic, because a rushed breath leaves no attention for anything else.

We find yoga helps with this. Holding a shape, breathing slowly and noticing where each part of you is, is practice for the same kind of attention.

A whole stroke is too much to feel at once. Each drill takes a single piece of it and lets you feel that piece alone. In the streamline kick, you lie face down with your arms long overhead and kick small from the hips, and you feel what a flat body is. In kick on side, you learn to balance on your side, where you spend part of every stroke. In catch-up, one hand waits out front until the other arrives to touch it, so each pull starts from full reach.

Drills are hard at first. They ask for coordination you have never needed, and they deserve your full attention.

A pull buoy is a foam float you hold between your thighs. When swimmers were towed through a pool with one, their legs rode higher, and the higher their legs, the less the water held them back.

A wetsuit does the same for your whole body. Swimmers and triathletes swam 400 metres with and without one. The swimmers' times did not change. The triathletes were 19 seconds faster. The researchers concluded that a wetsuit helps most the swimmers who float poorly and swim least efficiently. In a wetsuit you feel, perhaps for the first time, what level legs are like. Then you can go looking for that feeling without it.

A kickboard is a different tool. It takes your arms away so you can feel the kick on its own. Holding it lifts your head and lets your hips sink, though, so it will not show you level legs. That is why the streamline kick is done with your arms long in front instead.

Feeling a long stroke is one thing. Making it needs room in your body. It needs range in your shoulders and upper back, so your arm can go past your ear without your lower back arching to help. Yoga builds that range. It needs your trunk to roll as one firm piece, so your hips rise with your shoulders instead of sagging between them. Kettlebells build that firmness. It needs your head in line, turning to breathe rather than lifting. And it needs relaxed hands that catch and hold the water.

With that room, you can choose a long stroke or a shorter one, instead of being stuck with the one your shoulders allow. Straining for length without it only adds tension.

Swimmers tend to have broad shoulders that move freely, a lean middle, and everything stacked. Part of that build is the stroke, repeated for years. You lie long and level, and you pull with the big muscles of your shoulders and back, hundreds of times a session, so those muscles grow.

Length after length, your arms go overhead and your shoulders rotate. Most modern work keeps your shoulders rolled forward over a keyboard or a wheel. Front crawl moves them the other way, through their full range.

In the water you weigh almost nothing, and no foot ever hits the ground. A review of 64 studies found that swimmers' bone density was much like that of people who do no sport, and lower than in athletes from high-impact sports. Swimming does not harm bone, and it does not build it either. Bone is built by impact and load, on land.

That same lack of impact is why you can swim the day after a hard run and still train your endurance. Your legs get a rest from landing, and your heart and lungs keep working.