What do long hours on the bike need from your body?

When you run, your whole body moves forward on every step. When you swim, it rolls from side to side. On a bike, there is almost no movement above your hips. Your legs turn the pedals over and over, and the rest of you stays where it is, for hours.

The movement is small and it never stops. The skill of riding is keeping your body steady on top while your legs turn the pedals at an even rate underneath.

An even pedal stroke looks like the simplest thing in sport, and it is learned. In Japan, researchers had trained cyclists and athletes from other sports, with the same fitness, pedal at the same power. At the hardest point of each push, as the pedal went down, the cyclists pressed less hard. They used the muscles at the back of the thigh more, so the big muscles at the front had less to do. Years of riding had spread the work across more muscles. Each push was smaller, so they could repeat it for longer.

You learn this by riding a great deal at base effort. That is about three out of ten, where ten is the hardest you could go. At base, you can feel each leg turning the pedals over at an even rate.

In Pittsburgh, 15 competitive cyclists rode bikes on a treadmill, a moving belt wide enough for a bike, which can tilt into a hill. It got a little steeper every few minutes until they could not go on. They did it twice, once fresh and once after a hard workout for their trunk muscles.

With the trunk tired, they pushed on the pedals just as hard. What changed was the line of the legs. Their knees swung further from side to side, and their knees and ankles bent through bigger angles on every stroke.

So a steady trunk keeps your legs on a steady line. Riders fold as they tire. A firm, relaxed middle that keeps its shape while a load tries to bend it is what kettlebells build.

On a smooth road, keeping still is easy. On gravel, roots, rocks or broken tarmac, the bike jumps, slides and tips beneath you. So you lift off the saddle and stand on both pedals, with your hands on the bars. You are no longer sitting on the bike. You are balanced on top of it.

Your knees and elbows bend and straighten, so the bike can rise, drop and twist under you while your hips and head stay level. The front wheel drops into a hole, and your arms let it go down and come back. A root kicks the back wheel up, and your legs take it in. Lock your arms and legs, or sit heavy on the saddle, and every jolt goes straight through you.

That is balance: staying in control while what is under you keeps changing. Yoga builds it, in shapes where you hold yourself steady as your support shifts.

Over hours in the saddle, it is easy to let go. The back rounds, the chest drops towards the belly, the ribs have less room to move, and every breath takes more work.

To ride low without rounding, you fold at the hips with your back long, and that takes room at the hips. Yoga builds that room. Keep your head in line with your back, eyes up the road, and your hands light on the bars.

Not every round back is a stiff body, though. Bars set too far away or too low will pull anyone into one. And if you have rarely sat on a bike for three hours, you have not yet learned to. Before you blame your hips, check the bike and give yourself the hours.

Holding still does not mean holding one position all day. Good riders move on purpose. They stand on a short rise. They move their hands to a new place on the bars and back. They sit up on a long descent and let their back open.

Standing up does not waste energy. In France, eight trained riders rode hard on the flat and up a hill, some trials seated and some standing. The share of their energy that reached the pedals was the same either way. So a rise is a chance to stand and stretch your back, then sit down again.

The push starts in the big muscles of your hips and thighs. It reaches the pedal through your ankle.

In California, engineers built a computer model of a leg sprinting a bike from a standstill, and checked it against real riders. Of the energy the hip and thigh muscles made on the way down, more than half travelled through the calf before it reached the pedal.

On a long ride the pushes are far smaller, and there are thousands of them. Your ankle has to stay firm under every one, so the push from your hips and thighs reaches the pedal instead of bending your foot. You saw it in Pittsburgh: once the trunk tired, the ankles moved more.

Holding an ankle firm all day is not about speed. It is holding a joint's shape against a load, stroke after stroke, which is structure. Power makes the push hard and quick when you need it. Kettlebells build both. Your feet are where it all meets the pedal.

Riding is quiet. What you have is your breath and your legs, turning over together.

At base, your breath is slow and even, and the pedalling can settle into time with it. Breathing in and out through your nose keeps you there. When your breath gets quick and loud, you have drifted above base. Ease off, and the rhythm comes back.

Holding that rhythm for hours is the work. When your attention drifts, your position drifts with it. When you notice, you lengthen your back, let your shoulders soften, keep your elbows bent, and come back to the breath. Noticing that when you are tired is part of the training.

Look at your shadow again in the third hour, later in the season. The back is long and the shoulders are soft. Your breathing is the same as it was in the first hour, and your legs are still turning. Little of that was made in the third hour itself. The firm trunk and ankles, the room at the hips and the balance on rough ground were built off the bike. The even stroke, the steady breath and the habit of noticing were built on it, at base.

After every ride, easy or hard, there is something to give back. For hours your hips stayed closed and the front of your body stayed folded. Afterwards you move the other way, with your hips wide and your chest open, in the shapes the saddle never allowed. Then you still move freely the next morning, and you can ride again.