The muscles you never think about – until they stop working

Athletes can benefit from respiratory muscle training, but can it help unhealthy people, too?

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We breathe in, we breathe out, every minute of every day, and for most of us breathing is not given much consideration. That’s because breathing is under autonomic nerve control. The signals are sent from the brain automatically. The brain determines when we need to breathe based on the signals it receives from our organs and nerves. Although we can breathe in and out on command – for example, during pulmonary function testing or when completing breathing exercises – respiratory muscle function is primarily an automatic task.

What are the respiratory muscles?

We have two types of respiratory muscles, inspiratory and expiratory, to accomplish this task. The inspiratory muscles contract to draw air into the lungs. The most important muscle of inspiration is the diaphragm; however, the external intercostals assist with normal quiet breathing. Contraction of the diaphragm increases the space in the thoracic cavity and the lungs fill with air from the external environment. Accessory muscles of inspiration – sternocleidmastoids, scalenes, serratus, pectoralis – contribute less during normal breathing periods and more during active breathing periods, e.g., during exercise and forced breathing maneuvers. Expiration is a passive process because the lungs naturally want to recoil inward and collapse. During expiration, the lungs deflate without much effort from our muscles. However, the expiratory muscles – internal intercostals, rectus abdominis, external and internal obliques, transversus abdominis – can contract to force air out of the lungs during active breathing periods.

Respiratory muscles for inspiration and expiration. Note: serratus and pectoralis muscles are not shown here. (Source: Alison McConnell, <em>Respiratory Muscle Training; Theory and Practice</em>, Elsevier, Oxford, 2013)

Breathing is a necessary function of life, and most of the time it is effortless. However, if you have lung disease or if you exercise regularly, you may think about breathing more often. That’s because the respiratory muscles (RMs) can fatigue just like any other skeletal muscle. And if the muscles are overloaded or weak because of disease, then breathing may be the only thing you can think about. In the presence of a medical condition that impacts the lungs, such as chronic obstructive pulmonary disease (COPD) and high thoracic/cervical spinal cord injury, our breathing is altered because of functional and structural changes to the respiratory muscles. When demand increases, the RMs must respond. The initial response is an increase in contraction of the diaphragm and recruitment of accessory muscles. However, if the muscles are weak or functionally overloaded the demand may not be met.

What happens when muscles fatigue?

Fatigue of the diaphragm causes a decrease in muscle contraction and alternating movements of the abdomen and rib cage, known as paradoxical breathing. The typical breathing pattern of a person experiencing RM fatigue is rapid and shallow. This type of breathing pattern is not sustainable for long periods of time. Inspiratory tidal volumes are unable delivery fresh oxygen to the alveoli and remove enough carbon dioxide waste. As the RMs contract more frequently, they require more oxygen and produce more carbon dioxide. The oxygen delivered to the RM gets robbed from other skeletal muscles. So, those muscles begin to experience fatigue.

How can we exercise the lungs?

Tiara Flores, a respiratory care graduate student at Texas State University, performs respiratory muscle training with the PowerLung Sport model. (Photo by Christopher Russian)

Respiratory muscle training (RMT) is a way to improve strength or endurance of the inspiratory or expiratory muscles. Strength training is accomplished by breathing against a resistance. Two common examples of resistance strength training include breathing through a small opening (flow resistance) and breathing around a spring-loaded valve (pressure resistance). In both examples, the resistance load can be adjusted to change the workout, e.g., by using a progressively smaller hole (increased flow resistance) or a tightly compressed spring (increased pressure resistance).

Endurance training involves breathing at an above normal respiratory rate and tidal volume for a prolonged period of time, usually 30 minutes. This type of RM training is also known as voluntary isocapnic hyperpnea (VIH). The VIH method requires sophisticated equipment and oversight; therefore, resistance training is used most often when attempting RM training.

Focusing specifically on strength training, there are devices available to perform inspiratory muscle training (IMT) only, expiratory muscle training (EMT) only, and both inspiratory and expiratory muscle training (also known as concurrent respiratory muscle training). Inspiratory muscle training has received more attention in the medical literature compared to expiratory or concurrent muscle training.

Here are some of the more common devices:

Importance and application of respiratory muscle training

There is plenty of literature to support the use of RM training. An Internet search will return a large number of articles, and YouTube can provide videos of device use. Dr. Alison McConnell, an international expert on RMT, has written extensively on the benefits of RMT for both healthy and non-healthy individuals. In her book Respiratory Muscle Training: Theory and Practice (Elsevier, 2013), she discusses the structural and functional RM changes that occur when using a RM training device, such as increases in diaphragm thickness, changes in muscle fiber type (greater fatigue resistance), and improvements in strength. The benefit extends to athletes in most sports and multiple disease diagnoses.

Focusing specifically on COPD, there is a typical pattern that exists. The lungs are overinflated, the diaphragm tends to be flat at rest and the respiratory muscles are functionally and structurally weak. These changes contribute to complaints of dyspnea – difficulty breathing or breathlessness – during exercise. Dyspnea associated with physical activity can be quantified using the modified Borg Scale. Avoidance of physical activity because of dyspnea can lead to additional skeletal muscle weakness. Respiratory muscle weakness and fatigue are contributing factors to the breathlessness sensation. A stronger muscle can contract more forcefully and potentially resist fatigue.

A meta-analysis in Respiratory Medicine conducted by Geddes et al in 2008 reported significant improvements in inspiratory muscle strength, inspiratory muscle endurance, exercise capacity, dyspnea and quality of life in COPD patients following IMT. Complete support that RMT can offer additional benefit over pulmonary rehabilitation (PR) alone is lacking. A 2013 official statement by the American Thoracic Society/European Respiratory Society on adjuncts to pulmonary rehabilitation questions the benefits of having inspiratory muscle training (IMT) as an adjunct to a whole-body exercise program; although, the authors recognize the benefits of IMT in general. Additional scientific research is needed to determine the exact role RMT should play for COPD patients enrolled in a PR program.

It is possible that the lack of support is related to study design and training protocol versus questionable benefits of RMT. Given the overall support for IMT in the Geddes et al meta-analysis and by the American Thoracic Society/European Respiratory Society, it seems that future research should focus on two fundamental questions:

  • When should RMT be started?
  • What is the best training protocol to use?

Related article

This article is freely available in Elsevier’s open archive:

E. Lynne Geddes et al: “Inspiratory muscle training in adults with chronic obstructive pulmonary disease: An update of a systematic review,” Respiratory Medicine (2008)

We can test the strength of the respiratory muscles using a basic pressure manometer or sophisticated equipment. From this test we can determine if the person has RM weakness using predicted equations. But should RMT begin when the person’s RM strength is at 40 percent of predicted, or 60 percent or 75 percent? Obviously, weaker muscles result in a greater benefit from RM training. Therefore, we can expect to see bigger gains in some individuals. But should we expect the same amount of improvement in someone who is at 40 percent of predicted and someone who is at 75 percent? Also, what training intensity should be used? In general, training intensity should be no lower than 30 percent of the maximum strength of the muscles. But what is best? Should we start at 60 percent of the maximum strength, or 70 percent of the maximum strength?

COPD complicates things even further because of the GOLD stage classification system. Each GOLD stage indicates a more advanced disease process. Theoretically, the respiratory muscles are negatively impacted to a greater degree when comparing GOLD stage 1 to GOLD stage 4. So should our training regimen change based on GOLD stage 1 versus 2, 3 or 4? As always, additional research is needed. And that new research will surely generate additional questions on the best way to train the respiratory muscles. That’s the wonderful thing about research; there is always something new to read thus something new to learn.

What is a respiratory therapist, and how can I be one?

A respiratory therapist provides specialized care for patients with cardiopulmonary disorders. RTs are involved in the care of babies that are born prematurely and have underdeveloped lungs, individuals with asthma or cystic fibrosis, individuals with a spinal cord injury or neuromuscular weakness, older patients with COPD, and anyone that is experiencing difficulty breathing. RTs manage the mechanical ventilators that provide the breath of life for anyone who cannot breathe on their own. RTs give medications by nebulizer and provide airway clearance therapies to make breathing easier. RTs are specialists in conducting breathing tests, called pulmonary function tests, to aid the physician in diagnosing pulmonary complications. Ultimately, RTs are involved in the diagnosis, treatment and rehabilitation of patients with difficulty breathing.

In the United States, respiratory therapists are the primary healthcare provider tasked with testing and training the respiratory muscles. Assessing the strength of the respiratory muscles and providing the training is an expected part of your day, whether it is implemented on a patient that is newly extubated from the mechanical ventilator, a patient with a spinal cord injury, or a COPD patient recovering from a pulmonary exacerbation.

Respiratory therapists must be familiar with the equipment used to complete respiratory muscle testing and training. In some hospitals a simple pressure gauge will be used to assess respiratory muscle strength. In other locations more sophisticated digital equipment is used to generate respiratory muscle strength assessments. Also, the devices that are available to strengthen the respiratory muscles should be reviewed prior to implementing a training program. Available devices can provide pressure resistance or flow resistance. An invaluable resource is the American Thoracic Society/European Respiratory Society 2002 joint statement on Respiratory Muscle Testing. This document provides a complete review of respiratory muscle testing and should be available to all therapists and students in training. 

To become a respiratory therapist you must graduate from an accredited respiratory care program that offers at least an associate’s degree. Upon graduation, you will be eligible to sit for board exams offered by the National Board for Respiratory Care (NBRC). Upon passing NBRC board exams to become a certified respiratory therapist (CRT) or registered respiratory therapist (RRT), you are eligible to complete the requirements for state licensure and to enter practice as a respiratory therapist. Most respiratory therapists are employed in acute care hospitals around the United States and Canada. Respiratory therapists can be found in other countries (Philippines, United Arab Emirates, Saudi Arabia); however, most countries consider respiratory therapy a form of specialty training for physicians, nurses and physiotherapists. No matter the location, there will always be a need for high quality respiratory therapy to help patients breathe easier.

Every respiratory therapy education program will offer several different curricular experiences on respiratory muscle testing and training. As a student, you attend lectures and get hands-on practice during didactic and laboratory course work. You learn how to use the devices that are available, the proper way to test the strength of the respiratory muscles, and when and how to initiate training.

As a second phase of instruction, you provide testing and training on actual patients in the acute care and rehabilitation hospital clinical rotations. Respiratory muscle testing and training can occur in the pulmonary function laboratory or at the patient’s bedside.

As a final level of assessment, national board exams for respiratory therapists include content related to respiratory muscle testing and training. You must be familiar with normal and abnormal respiratory muscle strength numbers. Respiratory muscle strength will vary based on the patient’s age and gender.


  • The Committee on Accreditation for Respiratory Care (CoARC) contains information on all accredited respiratory care programs.

Elsevier Connect Contributor

Christopher J. Russian, PhD, RRT-NPS, RPSGT, RST Dr. Christopher J. Russian is an Associate Professor and Director of Clinical Education in the Department of Respiratory Care at Texas State University, which offers a BS degree in Respiratory Care and a graduate certificate program in Polysomnography.

He completed a bachelor’s degree in Kinesiology from Sam Houston State University in 1995 and another bachelor’s degree in Respiratory Care in 1998. He started teaching as a clinical instructor for the Texas State Respiratory Care program and realized teaching was his calling. He completed a master’s degree in Education in 2002 and his PhD in Adult, Professional and Community Education in 2014.

As a therapist he earned the Registered Respiratory Therapist (RRT) and Neonatal Pediatric Specialty (NPS) credentials from the National Board for Respiratory Care. He also has experience in polysomnography, earning his Registered Polysomnography Technologist (RPSGT) and Registered Sleep Technologist (RST) credentials. He has a variety of research interests but has a passion for respiratory muscle training and testing related to spinal cord injury, COPD and sleep.

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