Exercise Intolerance: Causes, Symptoms and Respiratory Muscle Training

Exercise intolerance is more common than many people realise. When a young, otherwise healthy adult feels disproportionately breathless and cannot handle the normal demands of physical exercise, fitness is rarely the whole story. How they breathe is often part of the picture, alongside the heart, lungs, muscles and nervous system.

This article covers the causes and symptoms of exercise intolerance, why breathlessness during exercise has more than one driver, and how breathing exercises, respiratory muscle training and the Oxygen Advantage® method can help improve breathing control and reduce breathlessness.

Exercise causes pain and soreness. As we push ourselves to improve, we can feel exhausted after working out. While it often hurts, exercise is incredibly beneficial to our health. For some people, however, high levels of pain and exhaustion after exercise may be the very thing stopping them from doing anything at all.

The joints and muscles are stretched and moved during exercise. Various types of exercise can stress the muscles and joints, leaving us sore and tired. This is a natural and necessary part of training. These levels of pain and stress can increase as we age or face health challenges.

When breathing feels out of proportion to the effort, too hard, too fast or too uncomfortable for the work you are doing, it can hold you back. This can be one sign of exercise intolerance.

What Is Exercise Intolerance?

Exercise intolerance is a reduced ability to perform physical exercise that should be possible for a person's age, sex, size and muscle mass.

It describes what someone experiences rather than a diagnosis in itself. If you notice a sudden or unexplained drop in your ability to exercise, see your doctor first to rule out an underlying medical cause.

Possible Causes of Exercise Intolerance

Exercise intolerance rarely has a single cause. Several factors can contribute, often at the same time. Here are some of the most common:

Heart disease

Appropriately prescribed exercise is good for people with coronary issues. It can improve cholesterol levels and circulation. But our ability to exercise is powered by the heart, which needs its own supply of blood and oxygen.

Cardiovascular disease can limit how effectively blood reaches the working muscles, reducing a person's capacity for physical exercise. Anyone with heart disease should follow their doctor's guidance on exercise.

Lung issues

The lungs play an important role in exercise. Oxygen moves from the lungs into the blood, which travels to the heart and is pumped through the rest of the body. If gas exchange from the lungs to the blood is impaired, a person may experience strong breathlessness during exercise and a fall in blood oxygen saturation.

Breathing pattern and breathing control

Breathing itself can become a limiting factor, even when the heart and lungs are healthy. Fast, upper-chest or mouth breathing at low workloads, poor coordination of the diaphragm, and a strong reaction to the feeling of breathlessness can all make exercise feel harder than it should.

Fitness and breathing function are not the same thing. In a study of 1,933 competitive athletes in Japan, around 91% were classed as having a dysfunctional, upper-chest dominant breathing pattern on screening (Shimozawa et al., 2023).

Muscle tears

The muscles bear the stress of exercise. If muscle fibres are torn, physical exercise becomes painful and difficult. Muscles work as part of a coordinated system, so an injury in one area can affect movement as a whole.

Bruises, wounds or injuries

Cuts, bruises and other injuries can also limit exercise. Normal exercise-related discomfort is challenging enough, and adding pain from an injury can make activity feel intolerable.

Other medical conditions, illness and recovery from infection can also reduce exercise capacity. This is another reason a medical check comes first.

Why Breathlessness During Exercise Has More Than One Cause

Breathlessness is often treated as one feeling with one cause. In reality, it is several sensations, produced by several interacting systems.

Researchers describe at least three distinct sensations (Parshall et al., 2012; Banzett et al., 2021):

• Air hunger: the feeling "I need more air." It tends to arise when the brain's drive to breathe is high relative to the breathing actually achieved.
• Work or effort: the feeling "breathing is hard work," linked to how hard the breathing muscles are working.
• Chest tightness: the feeling that the chest is constricted, often reported with airway narrowing such as asthma.

During hard exercise, air hunger and breathing effort can rise together.

What drives breathing during exercise is also more complex than a single chemical trigger. Breathing rises almost immediately when exercise begins, before arterial CO₂ needs to rise. Signals from the brain's motor areas (central command), feedback from the working muscles and lungs, metabolic CO₂ production, and chemical feedback from CO₂, oxygen and pH all contribute (Welch and Mitchell, 2024; Dempsey and McCrimmon, 2026). Exactly how breathing is matched so closely to the body's needs during exercise is still not fully understood.

Patrick McKeown teaches six drivers of breathlessness during exercise:

1. Chemical sensitivity (how strongly you respond to rising CO₂)
2. Breathing pattern (how fast, how much, and which muscles you use)
3. Acid-base balance
4. Oxygen delivery to the muscles
5. Respiratory muscle fatigue
6. Psychological perception of breathlessness

Breathing training and physical training influence these drivers in different ways, and people differ in which ones matter most. That is why reducing exercise intolerance to one mechanism, such as CO₂ sensitivity, oversimplifies it.

Perception matters too. Respiratory sensations are processed by the brain, and attention, expectation and emotion can amplify or reduce how threatening breathlessness feels (Banzett et al., 2021). That does not make the sensation imaginary. It means perception is part of respiratory physiology. As Patrick puts it, "If your breath says, this is hard, your brain is going to listen."

Symptoms of Exercise Intolerance

Exercise intolerance symptoms can present themselves in a number of ways. Each symptom alone does not necessarily mean you have the condition, but a combination of symptoms is something to investigate further.

If you notice a number of the following symptoms while exercising, it is worth looking into further.

1. Unusual and severe fatigue

It is normal to get tired after exercise. But severe fatigue after working out is a possible symptom of exercise intolerance. Compare how tired you used to get after exercise with how exhausted you get now. If you feel far more fatigued than usual after a workout of the same duration, it is worth discussing with your doctor.

2. Uncontrolled breathing

Breathing that feels out of proportion to the effort is one of the most common signs. You might find yourself gasping, switching to mouth breathing at low intensities, or struggling to settle your breathing afterwards.

Several factors can contribute: your breathing pattern, how strongly your body responds to rising CO₂ and other chemical signals, the strength and endurance of your breathing muscles, and how your brain interprets the sensation of breathlessness.

Harder and faster breathing also increases the work of the breathing muscles. During heavy, sustained exercise, tiring breathing muscles can trigger a reflex that reduces blood flow to the working limbs, adding to leg fatigue and the sense of effort (Romer and Polkey, 2008).

Fitness does not necessarily change chemical sensitivity. One study found that endurance athletes did not differ from sedentary people in their sensitivity to CO₂, but appeared to use additional strategies during exercise to reduce its influence on their breathing and perceptions (Harrison et al., 2022).

Your comfortable breath hold time, measured by the BOLT score, can give you useful feedback. BOLT measures how quickly you feel the first clear urge to breathe during a comfortable breath hold after a normal exhalation. The score is influenced by several factors, including chemical sensitivity, starting blood gases, breathing pattern, respiratory sensation, lung volume and psychological state.

A shorter BOLT can be associated with dysfunctional breathing, but it works best as part of a broader assessment that includes symptoms and breathing pattern (Kiesel et al., 2017). It does not isolate CO₂ sensitivity, and it is not a diagnostic test. It is also not a validated predictor of athletic performance: a 2024 study using the Oxygen Advantage BOLT protocol in 49 highly trained speed skaters found no relationship between BOLT and VO₂max or anaerobic test performance (Kowalski et al., 2024).

Oxygen Advantage® exercises train breathing regulation across several dimensions: breathing pattern and mechanics, how you respond to air hunger, and respiratory muscle function. With consistent practice, many people find their breathing feels calmer and more controlled during exercise, with less breathlessness at the same workload.

3. Pain in the chest region

If you notice unusual chest pain while you exercise, it may be a sign of exercise intolerance. This pain can also spread to the neck, arms and joints. With any chest pain, the first thing to do is get the all-clear from your doctor. Chest pain during exercise that is severe, or comes with dizziness or faintness, needs urgent medical attention.

Once medical causes have been ruled out, chest discomfort can sometimes be associated with breathing pattern disorders, possibly due to overuse of the upper-chest breathing muscles.

4. Prolonged muscular cramps

Muscle cramps are common during exercise and can happen for many reasons. They usually last a few minutes. Cramps that are prolonged or keep returning can make exercise very difficult and are worth discussing with a doctor.

5. Unusual sweating

It is normal to sweat during exercise. But with exercise intolerance, just a little exercise can leave you soaked in sweat. Heavy sweating on its own is not specific to exercise intolerance, but alongside other symptoms it is worth noting.

How to Manage and Improve Exercise Intolerance

Getting the right assessment early leads to better management. Once medical causes have been ruled out or are being treated, the following approaches can help:

Practise slow breathing exercises

Nose and slow breathing trains control of your breathing rate and pattern. At rest, light, quiet breathing helps reduce unnecessary ventilation, and slow breathing influences heart rhythm and the nervous system. At low to moderate exercise intensities, nasal breathing can reduce breathing frequency and total ventilation while the body still meets the demands of the work (Dallam et al., 2018; Mapelli et al., 2025).

Slowing your breathing rate is not the same as breathing less air. If each breath gets bigger to compensate, total ventilation can go up. Slow isn't the goal. Light is.

Functional breathing is breathing that is appropriate to the situation: quiet and economical at rest, and large enough to meet metabolic demand during exercise. Practising it during training can improve breathing efficiency and control, and many people find their breathing settles more quickly after effort.

Reduce exercise intensity and allow nose breathing to set the pace

For recreational exercisers, breathing through the nose during low to moderate intensity training is a simple way to regulate effort. Allow the nose to determine the intensity of your session. One study found that restricting athletes to nasal-only breathing during self-selected low intensity training did not change how they distributed their training intensity (Rappelt et al., 2023).

If the need for air is too strong and you need to switch to mouth breathing, slow down the intensity of movement. As you continue exercising with your mouth closed over the following weeks, many people find the air hunger becomes easier to manage and breathing feels calmer.

The mechanism is still being researched. It may involve changes in breathing pattern and mechanics, greater familiarity with air hunger, less anxiety about breathlessness, and possibly changes in chemical sensitivity. Research on athletes suggests that how the brain interprets and responds to breathing sensations plays an important role (Harrison et al., 2022).

Nasal breathing also has limits. In one study, exclusive nasal breathing during a maximal exercise test reduced peak ventilation, peak oxygen uptake and peak workload compared with normal breathing, because the nose could not move enough air at maximal effort (Mapelli et al., 2025). Use nasal breathing as a training constraint at suitable intensities, not a rule for every effort.

Ease mental stress

Mental and emotional state affects how exercise feels. Stress and anxiety can speed up breathing, shift it into the upper chest and amplify how threatening breathlessness feels, so you may tire more easily.

The mind and body work together. A part of the nervous system monitors your breathing and asks, am I safe? Fast, hard, upper-chest breathing signals threat. Use functional breathing before and after your exercise to settle your breathing and nervous system.

Respiratory Muscle Training and Diaphragm Fatigue

Have you ever tailed off at the end of a race because you could not breathe? Then you already understand why strong breathing muscles matter for exercise performance, particularly in endurance events.

Respiratory muscle training aims to improve the strength and endurance of the breathing muscles. It can help athletes who want to reduce breathing effort and people whose breathing muscles tire easily. Here is how it works and how you can integrate it into your training.

What Is Respiratory Muscle Training?

Respiratory muscle training involves loading the breathing muscles so they adapt, much like strength training for other muscles. The aim is stronger, more fatigue-resistant breathing muscles and less breathing effort during exercise. Research on loading the inspiratory muscles, including as part of a warm-up, suggests it can improve respiratory muscle function and, in some studies, exercise performance (Cirino et al., 2023).

What Is the Main Respiratory Muscle?

The diaphragm is the main breathing muscle. It is a dome-shaped sheet of muscle that sits beneath the lungs, attached to the lower ribs. During inhalation, it contracts and flattens, drawing air into the lungs. During exhalation, it relaxes and returns to its dome shape. Efficient, healthy breathing at rest is mainly diaphragmatic.

A healthy diaphragm also contributes to core stability. As it descends during inhalation, pressure increases in the abdomen. This intra-abdominal pressure helps stabilise the spine and pelvis, supporting functional movement.

The diaphragm is a skeletal muscle. Like other skeletal muscles, it can be trained, and it can also fatigue.

The Role of the Diaphragm During Exercise

During exercise, breathing volume rises steeply, and the diaphragm and other respiratory muscles do much more work to move air in and out of the lungs. This brings in oxygen for the blood to carry to the working muscles, and removes the extra carbon dioxide the body produces.

What Are the Symptoms of a Weak Diaphragm?

When you exercise, breathing volume increases and the breathing muscles have to move a lot more air. During heavy, sustained exercise, they can fatigue. Fatiguing respiratory muscles can trigger a reflex, called the respiratory muscle metaboreflex, that narrows blood vessels in the working limbs. This reduces blood flow to the legs, adds to leg fatigue and increases the sense of effort (Romer and Polkey, 2008).

Poor respiratory muscle function and dysfunctional breathing mechanics have been associated with symptoms including:

• Breathlessness on exertion
• Diaphragm muscle pain that interferes with breathing
• Lower back pain and injury
• Poor balance
• Poor core muscle strength
• Pelvic floor problems
• Reduced blood flow to the legs during heavy exercise

These associations do not mean breathing is the only cause, and they are worth assessing with a qualified professional.

How Do I Make My Breathing Stronger?

Strengthening your breathing starts with paying attention to it. As with any muscle, the breathing muscles adapt when they are given an appropriate load. Here are three ways to train them:

1. Nasal breathing as respiratory muscle training: In its simplest form, respiratory muscle training involves adding a load to breathing. You can begin by breathing only through your nose. The nose adds natural resistance to airflow and encourages the diaphragm to engage.
2. Oxygen Advantage® breath hold exercises: The Oxygen Advantage method is a programme of functional breathing exercises. Breath holds after exhalation during movement create a temporary rise in CO₂, a fall in blood oxygen and a stronger air hunger, providing controlled exposure to respiratory discomfort without any equipment.
3. Oxygen Advantage® SportsMask: The SportsMask adds resistance to breathing, increasing both the mechanical and chemical load: the breathing muscles work harder, and the reduced airflow creates air hunger. By adjusting the valve on the mask, airflow can be restricted during rest or physical exercise, adding an extra load to the breathing muscles to help strengthen them. The user breathes in and out through the nose, encouraging better recruitment of the diaphragm.

How Do Breathing Exercises Work?

The exercises involve holding your breath after a passive exhalation, often while walking or jogging, until you feel a moderate to strong air hunger. Carbon dioxide accumulates in the blood while the muscles continue to extract oxygen, producing temporary hypoxia and hypercapnia (lower blood oxygen and higher CO₂).

This gives you repeated, controlled exposure to strong respiratory sensations. Over time, it may change how you perceive and respond to air hunger. It is less clear whether it also reduces chemical sensitivity to CO₂, so it is more accurate to say breath holds train your response to breathlessness.

As a breath hold continues, the drive to breathe rises and the breathing muscles may begin to contract involuntarily. Together with nasal breathing and resistance breathing, this adds load to the respiratory muscles. One study in elite swimmers, described below, found that breath-hold training increased respiratory muscle strength.

What to Expect from Respiratory Muscle Training

Results vary between individuals, but benefits reported with respiratory muscle and breathing training include:

• Less perceived breathing effort
• Reduced or delayed respiratory muscle fatigue
• A healthier, more controlled breathing pattern
• Less breathlessness at a given intensity
• Better tolerance of hard efforts in some athletes
• Lower perceived effort during exercise

Inspiratory muscle training may be particularly useful in endurance sports such as running or cycling, where breathing effort builds over long efforts. Less breathlessness can make it easier to sustain your pace, although performance depends on many other factors too.

Note: Breath hold and air hunger exercises are powerful and not suitable for everyone. If you have heart disease, high or low blood pressure, a respiratory condition, long COVID, chronic fatigue syndrome, dysautonomia, anxiety or panic disorder, are pregnant, or have any other medical condition, consult a medical doctor before practising strong breath holds. Never practise breath holds in or under water. If possible, learn the exercises with a certified Oxygen Advantage® instructor.

What Is Diaphragm Fatigue?

Diaphragm fatigue is a temporary loss of the diaphragm's ability to generate force after heavy or prolonged work. In healthy people, it tends to occur during heavy, sustained exercise rather than at easy workloads (Romer and Polkey, 2008). Along with fatigue of other respiratory muscles, it is one factor that can limit exercise tolerance.

How Does Diaphragm Fatigue Affect Exercise Tolerance?

When the diaphragm fatigues, it cannot contract as forcefully, so other breathing muscles have to work harder and breathing feels more effortful. Respiratory muscle fatigue can also reduce blood flow to the limbs through the metaboreflex described above, contributing to leg fatigue and breathlessness.

What Causes Diaphragm Fatigue?

The main factors are:

1. High respiratory muscle work during heavy or prolonged exercise, when breathing volume is very high.
2. Competition for blood flow between the breathing muscles and the working limbs during intense exercise (Romer and Polkey, 2008).
3. Inefficient breathing mechanics, such as upper-chest breathing, which may increase the work of breathing.

Diaphragm weakness can also have medical causes, including:

• Damage to the diaphragm due to injury or surgery
• Nerve damage or dysfunction affecting the diaphragm
• Respiratory infections

If you feel unusual fatigue or shortness of breath during physical exercise, stop and rest until symptoms settle.

How Can Diaphragm Fatigue Be Treated?

Exercise-related diaphragm fatigue is temporary and usually recovers with rest. Easing off the activity that caused it is the first step. Over time, respiratory muscle training and good breathing mechanics can help the breathing muscles cope better with hard efforts.

Persistent breathlessness or diaphragm weakness that does not resolve with rest is different. It needs medical assessment, as it can have other causes that require specific treatment.

How Can Diaphragm Fatigue Be Prevented?

• Build training intensity gradually so your breathing muscles, like the rest of your body, have time to adapt.
• Practise breathing with good diaphragm recruitment and lower-rib movement to improve coordination of the respiratory muscles. Good posture supports this.
• Include respiratory muscle training, such as nasal breathing at suitable intensities or resistance breathing.
• Stay well hydrated before, during and after training as part of good general practice.

The diaphragm plays a critical role in respiration, and when it fatigues, performance can suffer. Training the breathing muscles through nasal breathing and breath hold training is one practical, long-term way to support them.

Improving Respiratory Muscle Strength: The Science

Respiratory muscles such as the diaphragm are susceptible to fatigue and weakness, like any other skeletal muscle. When they fatigue, they cannot contract as forcefully, and breathing becomes harder work.

Respiratory system limitations can affect exercise performance in highly trained individuals (Boutellier et al. 1992), especially at high intensities, where the increased work of breathing can compromise exercise performance (Harms et al. 2000).

Respiratory muscle fatigue not only decreases the ability to ventilate adequately (Lomax and McConnell 2003), but also increases limb muscle sympathetic nerve activity (St Croix et al. 2000), which has been shown to reduce blood flow to the peripheral muscles (Sheel et al. 2001). The shortness of breath experienced may limit an individual's ability to continue to exercise at the required intensity (Altose et al. 1985). These findings are summarised by Wells et al. (2005).

Adding a load to breathing during exercise, for example with nasal breathing or a resistance device, gives the respiratory muscles extra work, which can help strengthen them.

Study: Breath-Hold Training and Respiratory Muscle Strength in Elite Swimmers

A study examined the effects of an 8-week hypercapnic-hypoxic (breath holding) training programme on respiratory muscle strength and 100-metre front crawl swimming performance. It involved 26 elite male Croatian swimmers (experimental group n=12, control group n=14).

Both groups followed the same swimming training programme. The experimental group also completed breath-hold training while running on a treadmill, three times a week.

Compared with the control group, the swimmers in the breath-hold group showed significantly greater improvements in inspiratory and expiratory muscle strength. They also reduced their breathing frequency during a 100 m front crawl swim and improved their 100 m time by 3.6%, compared with 1.1% in the control group.

The researchers concluded that the breath-hold training significantly increased respiratory muscle strength. They suggested this may reflect a thicker diaphragm and other adaptations, but these explanations were not directly tested. This was a small study in one group of elite athletes, so larger studies are needed.

Source: Dajana KARAULA, Jan HOMOLAK, Goran LEKO. Effects of hypercapnic-hypoxic training on respiratory muscle strength and front crawl stroke performance among elite swimmers. Turkish Journal of Sport and Exercise. 2016; 18(1): 17-24.

Strengthen Your Exercise Tolerance with Oxygen Advantage®

Exercise intolerance usually involves several systems at once. The Oxygen Advantage® method works on the breathing side of the picture across three dimensions: biomechanical (how the diaphragm, ribs and airway move), biochemical (matching breathing to what the body needs) and psychophysiological (how you perceive and respond to breathlessness).

Nasal breathing, paced breathing and breath hold exercises each provide a different training stimulus. Together, they aim to improve breathing control, strengthen the breathing muscles and reduce breathlessness during exercise.

Start by measuring your BOLT score. It gives you a simple baseline for how quickly you feel the urge to breathe at rest, and a way to track your own progress over time. Use it as feedback alongside how your breathing feels during exercise, not as a diagnosis.

If you are interested in trying the OA method for yourself, why not try our online breathing course, become a certified breathwork instructor, or find an Oxygen Advantage instructor near you.

FAQs

Q: What causes exercise intolerance?
A: Exercise intolerance can have several causes, including heart and lung conditions, injury, illness and dysfunctional breathing patterns. Breathlessness itself comes from several interacting factors, such as breathing pattern, chemical signals, respiratory muscle fatigue and how the brain perceives the sensation. If your exercise capacity drops without a clear reason, a medical check is the first step.

Q: Can breathing exercises help with exercise intolerance?
A: Breathing exercises can help when breathing pattern, breathing control or respiratory muscle function is part of the problem. With consistent practice, many people report calmer breathing and less breathlessness at the same workload. Breathing training complements, rather than replaces, medical care for underlying conditions.

Q: Does a low BOLT score mean I have exercise intolerance?
A: No. BOLT measures how quickly you feel the first clear urge to breathe during a comfortable breath hold, and it is influenced by several factors. A 2024 study found no relationship between BOLT and exercise performance in highly trained athletes. It is useful feedback on your breathing, not a diagnostic test.

Q: Should I breathe through my nose during exercise?
A: Nasal breathing works well at low to moderate intensities, where it can reduce breathing rate and ventilation and help you regulate effort. At maximal intensity it can limit how much air you move, so mouth breathing during very hard efforts is normal. Use nasal breathing as a training tool at suitable intensities.

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