The short answer
Breathing training mouthpieces promise a “champion’s diaphragm” and more oxygen in your muscles. Inspiratory training has real evidence behind it, but its effect is modest and mainly apparent in less-trained athletes.
Respiratory muscle training, particularly inspiratory training against resistance, can increase maximal inspiratory pressure and improve certain endurance performances. The effect is generally more apparent in less-trained people, those with weak breathing muscles or during prolonged efforts. It is an add-on, not a way to “oxygenate your mitochondria” without doing cardio.
A 2012 meta-analysis examined 46 studies in healthy people. The eight controlled trials eligible for the meta-analysis showed improved performance in certain tests, particularly constant-load efforts, time trials and intermittent tests. Standard incremental tests detected no benefit. Responses were greater in less-trained participants.
So the right question is not “how can I breathe more?” but “is respiratory fatigue limiting my effort, and does a measured training block improve a relevant outcome?”.
What breathing training actually trains
The diaphragm, intercostals and several accessory muscles drive ventilation. As intensity increases, they must generate more pressure and move more air. Their work consumes energy and can contribute to the feeling of breathlessness.
An inspiratory threshold device provides relatively stable resistance during inhalation. The principle resembles strength training: a sufficient, repeated and progressive load trains the muscles involved. The most common measure is maximal inspiratory pressure, often abbreviated as MIP or PImax.
This work does not directly change mitochondrial density in the quadriceps, stroke volume or running technique. Those adaptations still require whole-body training. To work on your cardiorespiratory ceiling, the 12-week VO2 max programme · FR remains more directly applicable.
The respiratory metaboreflex, without the hype
When breathing muscles work hard and become fatigued, their afferent signals can contribute to a sympathetic and vascular response. This phenomenon is called the respiratory metaboreflex. It does not mean the brain abruptly “cuts off” blood to the legs to save the diaphragm.
A 2023 trial in 16 young adults showed that five weeks of training at 50% of maximal inspiratory pressure increased inspiratory strength and reduced the rise in blood pressure during a loaded breathing task. The effect persisted after five weeks without training. The sample was small, and the laboratory test did not replicate an endurance competition.
The mechanism is plausible, but practical decisions should still be based on performance, breathlessness and tolerability, not a story about “blood stolen from your legs”.
The Bohr effect and false “CO2 tolerance”
Carbon dioxide and pH alter haemoglobin’s affinity for oxygen. This is a normal part of exercise physiology. It does not follow that you should hold your breath, deliberately underventilate or avoid mouth breathing to force oxygen into your muscles.
During exercise, CO2 production increases and ventilation adjusts to maintain acid–base balance. Voluntary hyperventilation at rest can cause hypocapnia, dizziness and tingling. Conversely, insufficient ventilation during intense exercise limits performance. The goal is not to accumulate CO2, but to let ventilation respond to demand.
Commercial breath-hold tests do not accurately measure “metabolic CO2 tolerance”. They also depend on lung volume, attention, anxiety and willpower. They are not necessary for planning endurance training.
Nasal breathing: possible comfort, unproven advantage
The nose warms, humidifies and filters air. Nasal breathing can be comfortable during a walk or easy exercise. It also adds resistance and limits airflow, which may force you to slow down.
The sinuses contribute nitric oxide to inhaled air, but this mechanism does not prove improved performance or muscle oxygenation. A review aimed at distinguishing science from pseudoscience in commercial breathing interventions concludes that claims about nasal breathing should be assessed cautiously.
Use your nose as a preference, not a rule. The talk test is a more practical intensity guide: at a moderate pace, you should be able to speak in sentences. An open mouth does not automatically mean you have left a particular zone.
Who is most likely to benefit
An inspiratory training block makes more sense in the following situations:
- breathlessness limits you before your legs do during prolonged exercise
- a sport with high ventilatory demands, such as swimming, rowing, cycling or running
- reliably measured low inspiratory pressure
- a supervised return after an illness that affected breathing
- a performance plateau despite an already structured aerobic training programme
In a highly trained athlete without respiratory muscle weakness, the effect may be small. If breathlessness is new, disproportionate or accompanied by wheezing, chest pain or feeling unwell, investigate the cause before buying a device.
A progressive six-week protocol
This protocol is a cautious synthesis of studied approaches. Trials vary in device, load and number of repetitions. A small trial in 20 runners competing over 800 metres used 30 inhalations twice a day, five days a week, with a progressive load of 50–80% of maximal inspiratory pressure. It found increased inspiratory strength and improved 800-metre times after four weeks, but the sample size and specific population mean this cannot be treated as a universal dose. The progression below deliberately stops at a lower load.
An adjustable threshold device is preferable to a simple tube whose resistance depends heavily on airflow.
Before you start
Read the device instructions and measure maximal inspiratory pressure if the device allows it. If that is not possible, start with a resistance that allows 30 controlled inhalations without neck tension or dizziness. Record three variables:
- resistance used
- difficulty of the final set out of 10
- breathlessness during a standardised endurance session
Weeks 1 and 2
Practise five days a week, well away from an intense session. Perform one set of 30 inhalations, then a second after one to two minutes if the first is still well tolerated. Start at around 30% of maximal inspiratory pressure when it has been measured.
Each inhalation should be firm but controlled. Exhale normally, without forcing your lungs empty. Stop if you hyperventilate or your neck muscles take over all the work.
Weeks 3 and 4
Gradually increase towards 40% of maximal inspiratory pressure, then 50% if your technique remains stable. Keep to one or two sets of 30 inhalations, five days a week.
Progress in small increments. A difficulty of 7–8 out of 10 at the end of a set is enough. The goal is not to push your breathing muscles to failure.
Weeks 5 and 6
Maintain a load of around 50% or the highest setting that allows 30 clean inhalations. Do not increase resistance and repetitions at the same time. After six weeks, repeat the measurement and the same sports performance test.
For maintenance, two to three sessions a week can be tried. Evidence on the minimum maintenance dose remains less robust than evidence on initial training blocks.
Measuring an effect that matters
An increase in inspiratory pressure shows improvement in the specific task. It is not enough to prove a sporting benefit. Also choose an outcome close to your goal:
- time in a reproducible time trial
- average power during a standardised effort
- perceived breathlessness at a fixed pace
- ability to maintain technique towards the end of an effort
Test under comparable conditions and avoid hard inspiratory training immediately beforehand. An isolated difference may come from sleep, temperature, motivation or wind. Look for a trend over at least two tests.
Breathing recovery after exercise
Deliberately slowing your breathing for a few minutes may help you feel less keyed up. It does not “crush” cortisol or guarantee faster heart rate recovery. Walk slowly, then let your exhalation become a little longer if that remains comfortable.
A fixed pattern of four seconds inhaling and eight seconds exhaling can create air hunger in some people. Instead, start by inhaling normally and exhaling for one to two seconds longer. If you feel dizzy, return to spontaneous breathing.
Safety and contraindications
Do not use a shared device without appropriate cleaning. Do not practise hyperventilation followed by breath-holding in water, in the shower, while driving or while standing unsupported.
Seek medical advice or advice from a respiratory physiotherapist before loaded training if you have lung disease, poorly controlled asthma, uncontrolled hypertension, heart disease, a history of pneumothorax, recent surgery, a high-risk pregnancy or unexplained symptoms.
Stop immediately if you experience chest pain, feel unwell, have blurred vision, a sudden headache, unusual breathlessness or a marked drop in oxygen saturation in someone being monitored. The device does not treat anaemia, asthma, infection or heart disease.
The Verdict
Inspiratory training is one of the few “breathing tools” with a clear mechanism, a measurable load and human performance trials. Its effect is neither universal nor spectacular. It mainly improves respiratory muscle strength and may carry over to certain endurance efforts.
Try a six-week block, keep your aerobic training and measure a sporting outcome. Drop the promises of mandatory nasal breathing, therapeutic breath-holding and oxygen “unlocked” by CO2. If your sports performance test does not improve despite better inspiratory strength, your diaphragm probably was not your main bottleneck.
