If you read last week's article, you now know that mouth breathing is not a habit. It is a structural compensation with four measurable costs running in the background of every training session, every race, and every night of sleep.
The logical next question is: why can't I just switch to nasal breathing?
Most athletes who try already know the answer. It works at low intensity. It works on a recovery ride. But the moment effort climbs, the nose feels insufficient. The body opens the mouth because it has to.
Fig. 01 — Nose is not the limit. The ceiling is.
The standard explanation is that the nose simply cannot move enough air at high intensity. This is partially true, but it misses the actual limiting factor. The nose is not the bottleneck. The diaphragm is.
Underneath the nasal-vs-mouth debate lives one muscle that decides everything: the diaphragm. It is the primary muscle of respiration - responsible for roughly 70–80% of every breath at rest. Every other muscle that helps you breathe - neck, upper chest, intercostals - is either an assistant or a compensator. When the diaphragm cannot move, the assistants take over, and the cost shows up exactly as the four costs from last week's article.
One engine, three conditions
Three structures. One engine. The diaphragm is the muscle that actually moves air. Spine, ribcage, and belly are the mechanical conditions that determine how much leverage that engine has. When any one of them is restricted, the volume per breath drops, the frequency rises to compensate, and the system switches to mouth breathing not as a choice but as the only available option.
The diagram below is the article's keystone. Lock any structure and watch the chamber close around the diaphragm - the engine loses its room to work.
Volume / breath
high
Frequency
low
Default route
NASAL
Fig. 02 — One chamber, three locks. The keystone diagram.
The diaphragm is the engine. Spine, ribs, and belly are the room it has to work in. Lock the room - choke the engine.
The spine has to move
The spine is not a rigid pole. During breathing, it participates in a subtle wave of flexion, extension, and rotation that helps drive air through the trunk. Watch a sleeping infant breathe and you will see this clearly. The entire torso moves as a coordinated wave, not just the chest or the belly in isolation.
When the spine loses this mobility, breathing becomes mechanical. The wave disappears. Air moves vertically, in and out of the upper chest, instead of distributing through the full volume of the trunk.
Only the upper chest moves. The thorax has lost its participation.
Fig. 03 — Watch the wave disappear, and return.
Endurance athletes lose spinal mobility through predictable mechanisms. Sustained training posture - on a bike, in a boat, or over a desk - locks the thoracic spine into a fixed curve. Chronic stress reinforces the pattern by activating what Thomas Hanna described as the red light reflex, a protective contraction of the front body that compresses the chest and pulls the head forward. The green light reflex, an overactivation of the posterior chain, creates the opposite problem: an extended, rigid back that cannot flex or rotate freely.
Both patterns eliminate the spinal wave that breath depends on.
Research confirms the connection. A study on 90 adults with spinal deformity found that flexibility of the thoracic curve was an independent predictor of pulmonary function, with greater spinal mobility correlating directly with higher forced vital capacity and expiratory volume[1].
The ribcage has to expand
The ribcage is a dynamic structure designed to widen laterally on the inhale and narrow on the exhale. This lateral expansion is what creates the negative pressure that pulls air into the lungs. Without it, the body compensates by lifting the chest vertically, using the neck and shoulder muscles to create space that the ribs should be providing.
Fig. 04 — The container that should widen, doesn't.
Most athletes carry a chronically narrowed ribcage without knowing it. The causes are layered. Cultural conditioning teaches us to hold the stomach in and puff the chest out, which locks the lower ribs in a compressed position. Emotional history adds another layer. The body stores unprocessed fear, sadness, or anger as muscular contraction, and the ribcage is one of the primary sites where this holding accumulates. Over time, what began as a temporary protective response becomes permanent structural armor.
A randomized controlled trial demonstrated this relationship directly. Young women with thoracic hyperkyphosis who completed a program that reduced their kyphosis angle showed statistically significant improvements in both forced vital capacity and forced expiratory volume[2].
When the ribcage opened, the lungs worked better. The fitness did not change. The container did.
The belly has to yield
On the diaphragm itself
The diaphragm is a thin, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. It is the only muscle whose contraction directly produces inhalation. When it descends, the lungs fill. When it cannot descend, no other muscle can replace its work - only compensate around it.
It is also a postural muscle and, as somatic work consistently shows, a primary site of emotional regulation. Restoring its movement restores not only breath, but balance, presence, and the capacity to feel.
When the diaphragm contracts, it descends toward the pelvis, creating negative pressure in the thoracic cavity that draws air into the lungs. But it can only descend if there is somewhere to go. The belly has to yield, to soften and expand outward, to make room for that descent.
In most high-performing athletes, the belly does not yield. It is held. Chronically. The engine has nowhere to go.
Diaphragm travel
~70 mm
Zone of apposition
~8.0 cm
schematic — illustrative ranges
Fig. 05 — Brace the belly, lose the breath.
Core stability training, which dominates modern athletic preparation, teaches the abdominals to brace. This bracing is useful for generating force and protecting the spine under load. But it comes with a respiratory cost that is almost never discussed. A chronically braced abdominal wall compresses the diaphragm from below. The zone of apposition - the area where the diaphragm contacts the inner surface of the lower ribcage and generates its mechanical force - shrinks. The diaphragm loses leverage. Volume per breath drops.
A narrative review documented this problem explicitly. Most core stability programs for rehabilitation focus heavily on the transversus abdominis and multifidus while giving minimal attention to the diaphragm. The authors proposed that any core stabilization program should begin with restoration of normal breathing patterns before progressing to stabilization work[3].
Impaired diaphragm function - measured by decreased thickness and restricted movement - is strongly associated with balance disorders. The diaphragm is not just a breathing muscle. It is a postural muscle. When it is restricted, both functions degrade simultaneously.[4]
Kocjan et al., PLoS One, 2018The diaphragm does the work - the three decide whether it can
The diaphragm does the work. Spine, ribs, and belly decide whether it can. When all three are free, the diaphragm descends fully, the lower third of the lungs - where most of the blood and alveoli live - actually fills, and nasal breathing becomes structurally possible. When any one is locked, the diaphragm loses leverage and the assistant muscles - neck, jaw, upper chest - take over.
When any one of these structures is locked, the diaphragm compensates. When two are locked, the compensation becomes significant. When all three are restricted - the common state in endurance athletes who train in sustained postures, brace their core habitually, and carry chronic stress in their trunk - the diaphragm has almost no room to function.
Functional breath capacity
100%
Three free structures. The chamber breathes.
Fig. 06 — Restriction does not add. It compounds.
Restriction does not add. It compounds. Frequency rises. Volume drops. The mouth opens.
Why technique alone hits a ceiling
This is why breathing technique alone cannot solve the problem. You can practice box breathing, Buteyko exercises, breath holds, and CO₂ tolerance training. These approaches have value. But they all operate within the structural room that currently exists.
Fig. 07 — Practice fills the room you have.
If that room is limited by a locked spine, a compressed ribcage, and a braced belly, the technique hits a ceiling that no amount of practice can break through.
The intervention is not respiratory
The intervention that changes this is not respiratory. It is structural. It is the process of restoring mobility to the spine, expansion to the ribcage, and softness to the belly so that the diaphragm can do what it was built to do.
Restoring the diaphragm is not a breathing exercise. It is a structural project: free the spine, expand the ribs, soften the belly - and the primary muscle of respiration finally has the room to do its job.
When those three conditions exist, nasal breathing at higher intensities becomes structurally possible. Not as a technique you execute consciously. As the natural output of a system that finally has the room to breathe.
The pattern that connects all three
Spine, ribs, belly are necessary conditions. But there is one movement pattern that integrates them into a single coordinated breath - the spiral rotation between pelvis and ribcage. It is the deepest differentiator between an athlete who breathes efficiently and one who compensates.
Fig. 08 — Next week: the spiral that connects all three.
Next week
The spiral rotation between pelvis and ribcage that connects all three structures into one coordinated breathing movement - and why this single pattern is the deepest differentiator in how an athlete moves under pressure.
References
- [1] Ohashi M. et al. - Thoracic spinal flexibility as an independent predictor of pulmonary function in adults with spinal deformity. Journal of Orthopaedic Science, 2019.
- [2] Taslimipour S. et al. - Effect of corrective exercise on kyphosis angle, FVC and FEV in young women with hyperkyphosis. Journal of Sport Rehabilitation, 2020.
- [3] Sannasi R. et al. - The role of diaphragm in core stability programs: a narrative review. Journal of Bodywork and Movement Therapies, 2023.
- [4] Kocjan J. et al. - Impaired diaphragm function and balance disorders: a study on 142 participants. PLoS One, 2018.
JB Method - Reliable Under Pressure