Strong core. Strong everything.

Stand with your pelvis level, neither arched nor tucked.

Breathe in through your nose and send the breath low, into your belly, your sides and the small of your back. Feel your waist widen all the way round, like a barrel filling.

Now brace. Don't suck your belly in. Press out in every direction at once, front, sides and back, and keep breathing against that pressure.

Notice how solid you feel. That is your core working the way it is built to.

You can feel the difference on any hill. Run uphill with your core strong and your chest up, and you can breathe deeply and push hard all the way to the top. Let your posture slump and your middle collapse, and your breath gets short and your legs lose their power. The hill did not change. Your core did.

Most people think the core is the abs. The abs are one wall of it. The core is a barrel. The diaphragm, the dome-shaped muscle under your lungs, is the lid. The pelvic floor is the base. The muscles of your belly and sides wrap the front, and the muscles along your spine close the back.

When the walls brace and the lid presses down, the pressure inside the barrel rises, and a barrel under pressure is stiff. When researchers switched on the diaphragm alone, the pressure rose and the lower spine got stiffer.

The diaphragm has two jobs, and it does both at the same time: it is your main breathing muscle, and it is the lid that holds the pressure. That is why the way you breathe changes how well your trunk holds you up. A breath that fills the belly, sides and back loads the barrel. A short breath high in the chest barely touches it.

The body is a chain. Your feet push against the ground, and your hips, shoulders and hands deliver the work. Everything between them goes through the barrel. A swing drives from the hips and finishes at the arms. A stride pushes from the ground and carries the whole body forward. A pedal stroke needs a steady middle to push against. A swim stroke pulls with the arms and moves the hips through the water. If the middle holds, the push goes all the way through. If it gives, part of it is lost in the middle, and your lower back takes the load.

The body knows this. In healthy people, the deep belly muscles switch on a split second before the leg moves, so the middle is already firm when the push arrives. In people with low back pain, that early switch comes late.

So a strong core is not one more muscle group. It is what lets every other muscle you train do its full job.

Bracing is for short, hard efforts: a heavy lift, a hard swing, the push off the wall. Take a big low breath and press out, the way you did standing up.

On a run, a ride or a hike you don't brace. You stay relaxed and tall: chest up, shoulders loose, breath going low and wide into the belly, sides and back. That posture keeps the barrel full without gripping it, and it is what kept you strong on the hill. Clench your middle for an hour and you lose the breath you were trying to protect.

You don't need a separate core workout. Yoga, kettlebells, running, riding and swimming build the barrel, as long as you use it. You can run, swing and swim for years without ever bracing, so the work is to switch the barrel on in every session. These are the places to practise it, from the easiest to the hardest.

Yoga is where you find it. The pace is slow and nothing heavy is in your hands. Stand on one leg, breathe low and wide, and hold the pressure while you stay still.

Kettlebells are where you push through it. In a swing or a press, a barrel that holds sends the force straight into the bell. One that leaks shows up at once, as an arching back and a lift that feels weak.

Running and riding ask you to hold a tall, relaxed posture for an hour or more. Every hill tests it, and on a bike the test is not letting your middle sag over the bars.

Swimming is the hardest. The water sets the rhythm of your breath, your body rotates on every stroke, and your arms and legs move on their own. With the barrel on, your hips stay up and your body stays long. With it off, your hips drop and every stroke fights your own drag. Hold the barrel here and you can hold it anywhere.

Breathe low and wide, into your belly, sides and back. Before you lift, swing or push off the wall, brace the barrel. On long efforts, stay relaxed and tall and keep the breath low.

Train the barrel and you train everything at once. Every step, every lift, every stroke goes through your middle, and when your middle holds, all of it gets stronger.

• Hodges and Gandevia 2000. Journal of Applied Physiology 89:967-976. Changes in intra-abdominal pressure during postural and respiratory activation of the human diaphragm.
• Hodges, Eriksson, Shirley and Gandevia 2005. Journal of Biomechanics 38:1873-1880. Intra-abdominal pressure increases stiffness of the lumbar spine.
• Hodges and Richardson 1998. Journal of Spinal Disorders 11:46-56. Delayed postural contraction of transversus abdominis in low back pain associated with movement of the lower limb.