The trunk decides what breathing costs you

Two athletes ride at threshold for the same hour at the same power. One finishes ready to ride another hour. The other is destroyed.

They have the same fitness numbers on paper. The same FTP. The same VO₂max. The difference between them does not live in their lungs or their hearts. It lives in their trunks.

This is the variable that almost no training program talks about.

The body has to solve two problems at once under load. It has to produce force. It has to move air. If the trunk becomes a single rigid block trying to do both jobs through bracing alone, it will manage for a while, but every breath costs more than it should. Heart rate runs hot for the wattage. Respiratory rate climbs higher than it needs to. The cardiovascular system is asked to compensate for a breathing system that has lost its mechanical efficiency.

For the past two weeks I have written about what blocks breath under load. Mouth breathing has four hidden costs that accumulate silently. Three structures — spine, ribs, and belly — decide how much room the diaphragm has to work. If you read those articles, you have the components. What you do not yet have is the pattern that holds them together, and the reason that pattern decides whether your body spends more air than it has to.

How the trunk actually works during breathing

The trunk is not one block. It is a pressure system with five players that have to coordinate: the diaphragm, the ribcage, the spine, the abdominal wall, and the pelvic floor.

When the diaphragm descends, intra-abdominal pressure rises. The abdominal wall has to yield to receive that pressure. The ribs have to widen sideways to give the diaphragm room. The pelvic floor has to participate in the pressure system rather than fight it. The spine has to allow a soft wave of movement that distributes pressure rather than collecting it.

This alternation is the real mechanism. Pressure rises in one chamber, falls in another, then reverses. The trunk is not stable in the static sense. It is stable in the dynamic sense, the way a wave is stable. The diaphragm gets its mechanical leverage not from one fixed position, but from a pressure system that can adjust around it.

This is what coaches and physios mean when they talk about dynamic stability. Not rigidity. Not looseness. A trunk that holds its position while the parts inside it continue to move.

In walking and running, this alternating pressure becomes visible from the outside as a spiral. The pelvis rotates one direction, the ribcage rotates the other, and the breath rides this counter-rotation. The spiral is not the cause of efficient breathing. It is what efficient breathing looks like when the internal pressure system is working.

On the bike, where the pelvis is partly fixed by the saddle and the feet are fixed by the pedals, the same internal pressure system has to do its job inside a much smaller window of external movement. The principle stays the same. Its expression changes.

Why most athletes have lost it

The athletes I work with did not lose their trunk alternation by accident. They lost it through the same training that built everything else they value.

Strength training that emphasized bracing. Core stability work that taught the abdominals to grip and hold rather than yield and shape-change. Sustained training positions — the saddle, the aero bars, the racing tuck — that locked the pelvis and ribcage into one block for hours at a time. Years of cultural conditioning to hold the stomach in and pull the chest up.

The body learned to fuse the upper and lower halves of the trunk into a single stable unit.

For force production in static positions, this fusion works. For sprinting out of the saddle, pulling under threshold, pressing or lifting, a braced trunk transfers power efficiently. The training was not wrong. It built half of what the body needed. The other half — the ability to alternate, to let pressure shift across the trunk in a wave — got buried under decades of stability work that nobody was paying attention to.

Here is the part that makes this hard to undo. The body does not just learn the bracing pattern muscularly. It builds the bracing into its connective tissue. Fascia — the continuous network that wraps every muscle and links the pelvis to the ribcage as one integrated system — adapts to whatever pattern you repeat. Years of bracing teach the fascial network to hold the brace as its default shape.

The pattern stops being something you do and becomes something you are.

This is why most athletes who try to "relax their core" cannot. The instruction reaches the muscle, but the muscle lives inside a connective-tissue and nervous-system environment that has been trained for years to protect, hold, and limit movement. Tone can shift quickly. Tissue and the nervous system's default coordination take longer.

This is also why breathing technique alone hits a ceiling. Box breathing, nasal training, CO₂ work — they all operate inside whatever room your trunk currently has. They can improve control. They cannot change the room.

How to recognize it in your sport

You have already felt this difference, whether you named it or not.

In walking, watch how your step length and your breath length connect. When the hips can rotate freely and the shoulders swing softly against them, the breath drops deep and the stride lengthens. When the trunk locks, the stride shortens, the breath shortens, and the whole gait gets tighter. The body is telling you exactly where the alternation is broken. Most athletes never listen because they think breathing is a lung question and walking is a leg question. They are the same question.

In running, the cleanest stride is an X across the body. Right knee comes up, left arm drives back, the trunk rotates between them. Lose that rotation and the stride collapses into vertical bounce. You feel it as heavy legs. The diagnostic is breathing. If you cannot keep your breath nasal through an easy run, the trunk alternation is not running cleanly.

On the bike, the picture is different. The pelvis is partly anchored to the saddle, the feet are fixed to the pedals, and the hands create a third stabilizing contact through the bars. The trunk does not need, and cannot produce, the visible rotation of walking or running. What it needs is something subtler — dynamic stability that still allows the ribs, the diaphragm, the abdominal wall, and the spine to make small, rhythmical adjustments without collapsing into a rigid brace.

The best cyclists are not loose in the trunk. They are not rigid either. They hold a quiet platform that delivers power into the pedals while leaving the ribcage enough room to breathe. When that balance fails, the rider can still hold the watts, but the markers of rising cost appear: the shoulders creep up, the chest lifts vertically instead of widening, the belly hardens, the breath climbs into the upper chest, and the cardiovascular system starts drifting upward for the same wattage.

Research on cyclists supports exactly this picture.

What happens to the trunk when fatigue accumulates is just as revealing.

This is why two athletes at the same FTP can have completely different sustainable hours at threshold. One holds dynamic stability with respiratory freedom. The other braces, and pays for every breath.

What changes when the trunk reorganizes

After three months of somatic work focused on restoring spinal mobility, ribcage expansion, abdominal yield, and the alternating coordination across the trunk, I repeated the same cycle ergometer protocol at the same wattage. Not more fitness training. Structural reorganization of the trunk that delivers fitness.

At 360 watts — 6.2 watts per kilogram — the same load on both tests, the numbers were these.

These are not fitness gains. The variable I trained was trunk-breath coordination, and the data are consistent with a system that has stopped paying for inefficient breathing. The fitness was already there. The body just stopped spending more air than it needed to produce the same power. Breathing was nasal for the entire test.

The intervention that gets here is not what most athletes look for at this point.

The mistake is to add more exercises. Stretching the thoracic spine. Mobility drills for the ribs. Breath holds. CO₂ tolerance work. All of these can help. None of them touches the actual problem, which is that your trunk has been trained to brace and the tissue environment around it has learned to hold that brace as its default shape.

The intervention is to retrain the alternation itself. To let the pelvis and ribcage participate in the breath again. To let the abdominal wall yield instead of grip. To let the spine wave instead of stay fused. Tissue needs repeated input before it lets go of an old pattern. This is why one session does not change the body. Months do.

The cheaper the breath, the longer the legs last

Once you retrain the alternation, breathing under load stops being a battle and starts being what it was always designed to be.

The trunk decides what breathing costs you. The cheaper the breath, the longer the legs last.

JB Method · Reliable Under Pressure