The Lungs You Never Trained. Breathing Muscles Fatigue Like Any Other Muscle, And They Decide The Last Two Minutes.
Every athlete has a point in a hard effort where the legs go. A good share of the time the legs are not the first thing that failed. The diaphragm is skeletal muscle with motor units, a fatigue curve and a reflex that pulls blood away from the limbs when it tires, and almost nobody programs for it. This is the respiratory side of conditioning, including the part of the aisle that sells you nothing.
Opening. The last two minutes, every time
Marc is thirty one and competes. He knows his numbers better than most people know their own address. Threshold pace, heart rate at threshold, the split he can hold for twenty minutes and the one he cannot.
There is a pattern in his hard efforts that none of those numbers explain. Somewhere around the eight minute mark of anything genuinely difficult, his breathing changes character. It goes shallow and fast and stays there. Within ninety seconds his legs arrive at a wall that feels nothing like the wall he gets in a heavy squat. Not local burning. A kind of general withdrawal, as though the limbs had been quietly taken offline.
He has been told this is cardio. He has been told it is mental. He has spent two years adding intervals to address something he never located.
Here is the part nobody mentioned. The muscles that move air into his chest are skeletal muscles. They have motor units. They fatigue. And when they fatigue hard enough they set off a reflex that reduces the blood supply to his legs, which is why the legs feel switched off rather than worn out.
Section one. The breathing apparatus is a muscle group
Separate two things that get collapsed into one word. The lungs are an organ, essentially a very large gas exchange surface, and they are not trainable in any structural sense. The breathing muscles are a muscle group, and they are entirely trainable.
The diaphragm is the main one. It is a dome of skeletal muscle sitting under the lungs, and when it contracts it flattens downward, enlarging the chest cavity and drawing air in. Inspiration is active work. At rest, expiration is mostly passive recoil, which is why breathing out costs you almost nothing on the couch and costs you a great deal at the end of a four hundred metre effort, where the abdominals and internal intercostals are driving air out on purpose.
Around it sit the external intercostals between the ribs and a set of accessory muscles including the scalenes and sternocleidomastoid in the neck, which is why someone who has been breathing hard for an hour often finishes with sore neck muscles and no idea why.
The diaphragm is unusually fatigue resistant. It has a high density of capillaries and a high share of fatigue resistant fibres, because it is built for a job that never stops. Fatigue resistant is not fatigue proof, and the difference between those two phrases is this entire article.
Section two. The evidence that they genuinely fatigue
This was an open question for a long time, because the respiratory system was assumed not to limit exercise in healthy people at sea level.
The group led by Jerome Dempsey at the University of Wisconsin settled a good part of it. Work published in the Journal of Physiology in 1993 by Johnson, Babcock, Suman and Dempsey used phrenic nerve stimulation to measure diaphragm force directly, taking a baseline and repeating it immediately following sustained heavy cycling. The force producing capacity had dropped. The diaphragm had fatigued, in healthy trained people, from exercise alone.
Two other findings matter to a competitor. At high work rates the act of breathing itself consumes a meaningful share of your total oxygen uptake, which is energy not available to the legs. And the breathing muscles reach their fatigue threshold at sustained intensities above roughly what most people call threshold, which is to say in exactly the efforts that decide races.
That share varies with intensity, with the individual and with the method of measurement, and the published range is wide. Regard it as significant and unquantified rather than as a number.
Section three. The reflex that explains the legs
Inside a working muscle there are small sensory nerve fibres, grouped as type three and type four afferents, that respond to the local chemical environment produced by hard work. The breathing muscles have them too. When they accumulate enough metabolites those afferents fire, and the reflex response is an increase in sympathetic outflow that narrows the blood vessels supplying the limbs.
The name for this is the respiratory muscle metaboreflex, described in detail by Dempsey, by Markus Amann and by Lee Romer among others. The logic is not hard to see. Ventilation is non negotiable, so if the breathing muscles are in trouble, blood is diverted toward them and away from everything else.
From inside the effort this does not feel like a breathing problem. It feels like a leg problem. The legs lose blood flow at the exact moment they most need it, and the fatigue arrives faster than the work rate should have produced.
It connects to the delivery network we wrote about in The Vascular Floor. Local blood flow is the currency of endurance, and there is a muscle group with the authority to tax it.
Section four. Three things the market has wrong
The lungs are the limiter. For healthy people at sea level, generally not. Lung volume is largely set by your frame and training does not expand it. The honest version is that the muscles moving the lungs can limit you, which is a more useful claim because it points at something you can work on.
Breath work is the answer. Slow breathing practice has real uses for settling the nervous system, and we wrote about that in our piece on stillness. The evidence that breathing exercises performed at rest improve hard performance is thin. Loading a muscle is what trains a muscle, and sitting quietly does not load the diaphragm.
Restriction masks simulate altitude. They do not. Altitude is a lower partial pressure of oxygen, and a mask over your face does not change the composition of the air. What it does is make breathing harder, which is a crude form of respiratory loading. Research by Porcari and colleagues published in the Journal of Sports Science and Medicine in 2016 put the elevation style training mask through a six week study and found the training itself produced the gains, with no advantage attributable to the mask. Purpose built devices do the same job with a load you can actually set.
There is a fourth, quieter error. The diaphragm has a second job. It is a central part of the bracing system, the subject of our earlier piece on intra abdominal pressure. On a heavy set it is holding your trunk stiff and trying to ventilate you at once, and those demands compete. That is why the fifth rep of a heavy squat produces a sensation of airlessness that has nothing to do with fitness.
Section five. What actually trains them, and the honest ceiling
Two routes, and the first one is free.
Sustained hard conditioning trains the breathing muscles because it loads them. Rowing, hard cycling, sled work, loaded carries and high repetition squatting all drive ventilation up and hold it there. If your conditioning is short intervals with full recovery, the breathing muscles are never loaded long enough to adapt.
The second route is specific inspiratory muscle training with a pressure threshold device, which you inhale against. The protocol that appears most often in the literature, developed in large part through the work of Alison McConnell, is thirty breaths twice daily against a load of roughly half your maximal inspiratory pressure, for four to six weeks, after which two or three sessions a week holds the adaptation.
Now the honest part. A systematic review with meta analyses by HajGhanbari and colleagues, published in the Journal of Strength and Conditioning Research in 2013, found that respiratory muscle training reliably improved respiratory muscle strength, with smaller and more variable effects on performance itself. The signal is strongest in endurance events with a heavy ventilatory demand and in people whose respiratory muscles were relatively weak to begin with. It is weakest in short efforts and in strength sports, and the studies are mostly small.
So this is a marginal gain with a clear mechanism rather than a transformation. It belongs in the same drawer as the last two percent of anything.
One separate line of research is worth naming and leaving alone. Daniel Craighead and colleagues at the University of Colorado Boulder have published on high resistance inspiratory training and cardiovascular measures in clinical and ageing populations. That is a conversation for a doctor, not a recommendation from a retailer.
Section six. A defensible protocol for the competitor
First, work out whether this is your limiter. Three honest signs. Breathing goes shallow and fast early in a hard effort and never recovers its depth. You cannot speak a full sentence two minutes from the end of an effort you can otherwise complete. And the limb fatigue arrives alongside air hunger rather than local burning.
If the answer is no, spend your attention elsewhere. There is no point loading a system that is not the constraint.
If the answer is yes, run six weeks. A pressure threshold inspiratory device, thirty breaths twice a day at a load you can just complete with good depth, progressed when thirty breaths stop being difficult. Then hold it at three sessions a week.
Build the free side at the same time. Two sessions a week of sustained work at an intensity where breathing is uncomfortable but holdable, twenty to thirty minutes, which is the middle layer conditioning we covered in The Anaerobic Threshold.
Fix the pattern under load. Depth before rate. A shallow fast pattern moves air in and out of the airways without reaching much of the gas exchange surface. Lengthen the exhale on the recovery portion of an interval. Nasal breathing at easy intensity is fine, and there is no reason to defend it once the demand rises.
Train it in position. If you compete seated, load it seated.
On inputs, nothing in a tub trains a muscle. Adequate carbohydrate, electrolytes when the work is long or warm, creatine monohydrate to underwrite the training, and sleep so the adaptation consolidates. No supplement acts on the diaphragm, and anyone implying otherwise has invented a category.
And a boundary. Breathlessness that is new, disproportionate or frightening, or any history of asthma or another respiratory condition, belongs with a doctor before it belongs with a device.
Section seven. The mastermind frame
There is a habit in serious athletes of working hardest on the system they can already feel.
The legs announce themselves. The lungs do not, or rather they announce themselves in a language we translate as weakness, as cardio, as a character problem. So the athlete adds intervals to a system that was never the constraint and gets a little better, slowly, for a lot of work.
The useful skill is not more effort. It is locating the actual limiter and loading that. Sometimes it is the delivery network. Sometimes it is the fuel system. Sometimes it is a dome of muscle under your ribs that has worked every minute of your life without a single session written for it.
Find the constraint. Load the constraint. Everything else is enthusiasm, and the body does not reward enthusiasm. It rewards the work aimed at the right place.
Frequently asked questions
Is an inspiratory training device worth buying?
It is worth considering if breathing is genuinely your limiter in long hard efforts, and it is close to pointless if it is not. The devices reliably make the breathing muscles stronger. Whether that shows up in your result depends on your sport, your event length and how weak those muscles were to start with. Expect a small gain, honestly described.
Will this help my squat or my bench?
Not meaningfully. Strength work is short, and the limiting factors are elsewhere. The one indirect benefit is that a stronger diaphragm supports the bracing system on heavy sets, which is a secondary effect rather than a reason to buy anything.
Does a restriction mask do the same job?
Partially, badly, and at a higher price. It loads breathing in an unmeasured way, it does not simulate altitude because it cannot change the oxygen content of air, and the six week study by Porcari and colleagues found the training rather than the mask produced the gains. A purpose built pressure threshold device gives you a load you can actually set.
How long until I notice anything?
Respiratory muscle strength measures tend to shift inside three to four weeks of daily loading. Whether you feel it in an effort is a different question and may take the full six weeks, and some people never feel it because breathing was not their constraint.
I get a stitch in hard efforts. Is that the diaphragm?
Possibly, and the mechanism is still debated. The leading explanations involve irritation of the membranes lining the abdominal cavity rather than diaphragm fatigue. What helps is eating further from the session, sensible fluid volume, and breathing deeper rather than faster. Persistent or severe pain is a doctor conversation.
Written for the Supplement Superstore Vault. We sell the supplements that support the work. We do not sell the work. Information here is educational and is not medical advice. Speak with a qualified health professional before changing any protocol, especially during pregnancy, breastfeeding, competitive training or while managing any clinical condition.
