What does a smooth running stride need from your body?

The second runner is leaking energy. Some goes up, into bouncing, some sideways, into the swinging shoulders, and some into the foot that lands in front and slows him down. None of it carries him forward.

The best runners in any big race bounce very little, and their trunk stays still while their legs work.

Researchers at Loughborough University in England filmed 97 runners of every standard on a treadmill and measured how much oxygen each one used. Some runners bounced more than twice as much as others, and the more they bounced, the more oxygen they used at the same speed. Three things in the stride stood out: how far the hips rose and fell while the foot was on the ground, how deeply the knee bent as it took the landing, and how much the runner slowed down on each step. Together those three explained 39 percent of the differences in how much energy the runners spent.

Now look at who wins a local race. Sometimes it is the runner with the ugliest stride in the field. A big engine and years of training can carry a lot of wasted movement.

The Loughborough researchers also set each runner's stride against their best race time that season. There, the stride explained about a third of the differences. The rest came from the heart, the lungs and the training.

A stride copied whole from a picture can even cost you. In the United States, eight triathletes spent 12 weeks being taught a complete new running style, with its own rules for posture and for how the foot lands. Afterwards they bounced less and took shorter strides, and they used nearly 8 percent more oxygen at the same speed. They looked more like the picture and ran less economically. Remaking your stride to match a picture has not been shown to make anyone faster.

Still, at any level of fitness, a smoother stride helps. If you waste less effort at a given speed, you can hold that speed for longer. And if you put less stress through your legs on every step, you get hurt less.

In Hong Kong, 320 people who had just taken up running joined a trial. Half of them spent two weeks on a treadmill learning to land more softly, watching a screen that showed how hard each foot hit the belt. The other half ran without the screen. Over the next year, 16 percent of the soft landers were injured, against 38 percent of the others.

A stride that costs less and hurts less is one you can keep running on for decades, into old age.

A smooth stride uses all four of the qualities in the earlier chapters at once. Every step is a landing on one leg, and balance lets the ankle and knee settle straight under you instead of wobbling. Each push sends the leg out behind you, and range lets it get there without your lower back arching to make room. Power makes the push quick and springy, so each foot spends a short moment on the ground rather than a long, heavy one. And structure holds your trunk firm, so your legs have something solid to push from, an hour into the run as much as at the start.

Look at the floating runner from the side. Ankle, knee, hip, shoulder and ear sit in one line, each on top of the one below, and that line stays upright from the first kilometre to the last.

Each foot lands under that line rather than out in front of it, and catches your weight. Then it pushes the ground away behind you, and that push is what sends you forward.

In the Loughborough study, the runners who ran with a more upright trunk raced faster, and those whose trunk rocked forward and back on each step used more energy.

Your feet have to stay steady under the push, and your trunk has to stay quiet above it, without twisting from side to side. Then your energy goes into the ground behind you, and from there into moving forward.

Endurance is a game of efficiency. A long run is thousands of steps, and a small waste on each one adds up to a large cost.

You find some of this just by running. In England, ten women new to running followed a ten-week programme. By the end they used 8 percent less oxygen at the same speed. What changed was the push. As each foot pushed off, the force now ran straight along the leg instead of across it, and it pointed further back behind them. More of each push went into moving them forward.

Uphill, you push harder. Your steps get shorter, you stay tall, and your eyes look up the hill rather than at your feet.

Downhill is flow. On a descent you can run, let your legs roll and let gravity carry you down. Take quick, light steps, each foot landing under you and leaving again before it has time to stop you. Never brake. The runners who come down a mountain fastest look loose, their feet turning over quickly underneath them.

A footballer does not only play matches. She also practises parts of the game, slowly and on purpose. Most runners only ever run.

To become a better runner you need mobility and strength, and you also need skill. Skill comes from practising the movement, and that is what running drills are for. Each one takes a piece of the stride and slows it down or shrinks it, so you can feel which muscles work and when.

In an A-skip, you skip forward lifting one knee to hip height, then drive the foot straight down under your hip. It practises a tall body and a foot that lands beneath you, not out in front. A B-skip adds a reach at the top: the lower leg swings forward, then sweeps back so the foot lands already moving backwards, the opposite of the slapping step of the second runner. In ankling, you take tiny, quick steps off the balls of your feet with the knees almost straight, and practise a quick, light touch on the ground. Pogo hops and bounds work on the spring and the push.

It is no accident that professional runners do drills like these before they run, season after season.

Training the spring in your legs pays off too. In Sydney, 17 male runners were split into two groups, and for six weeks one group added jump training to its usual running. The jumpers got 2.7 percent faster over 3 kilometres and used less oxygen at every speed tested. Their lower legs gave less as each foot landed and sprang back faster. Their aerobic capacity had not changed.

Some of the reasons to build a smooth stride are ones you feel: your feet landing under you and pushing away, your trunk quiet, your hips riding level. Running feels like moving well rather than getting through it, and it gets more fun. You trust your legs on any ground: a rocky trail, a city road, the hard sand of a beach at low tide.

A smooth stride is built in two layers. Underneath are balance, range, power and structure, which kettlebells and yoga build. On top is the skill of the stride itself, practised in drills and refined over many kilometres at base effort. With both, you spend less effort at the same speed, put less stress through your legs, and keep running for many more years.