Accessory Muscles of Breathing Illustration

What are the Accessory Muscles of Breathing? (2024)

by | Updated: Jan 15, 2024

The accessory muscles of breathing are a crucial but often overlooked component of the respiratory system.

While the diaphragm and intercostal muscles are the primary drivers of respiration, the accessory muscles, including the sternocleidomastoid, scalene muscles, and various abdominal muscles, play a pivotal role when breathing demands increase.

Whether it’s during physical exertion or respiratory distress due to medical conditions like asthma or COPD, these muscles assist in expanding and contracting the thoracic cavity to facilitate adequate gas exchange.

Understanding the function and importance of these accessory muscles is key for clinicians, patients, and anyone interested in the complexities of human physiology.

What are the Accessory Muscles of Breathing?

The accessory muscles of breathing are muscles that are not primarily responsible for respiration but can assist in the act of breathing when there is increased demand, such as during exercise, or when the primary muscles of respiration are not sufficient to achieve adequate gas exchange, as might occur during disease states like chronic obstructive pulmonary disease (COPD) or asthma.

The primary muscles of breathing are the diaphragm and the intercostal muscles. When breathing becomes difficult or laborious, the accessory muscles are recruited to assist in the respiratory effort.

Here are some of the accessory muscles of breathing:

Inspiratory Accessory Muscles

  • Sternocleidomastoid: Helps lift the sternum and increases the anteroposterior diameter of the chest, thereby allowing for greater expansion of the lungs.
  • Scalene Muscles (Anterior, Middle, and Posterior): These muscles help to lift the first two ribs and also contribute to the anteroposterior expansion of the chest.

Expiratory Accessory Muscles

  • Abdominal Muscles (Rectus Abdominis, Obliques): These muscles contract to push the diaphragm upwards, which increases intra-abdominal pressure and helps to expel air from the lungs.
  • Internal Intercostal Muscles: Unlike the external intercostals, which are more active during inspiration, the internal intercostals assist in forced expiration by depressing the ribs and reducing the dimensions of the thoracic cavity.

Note: The accessory muscles are generally not used much during quiet, relaxed breathing but are increasingly recruited as the demand for oxygen increases or when there is a compromise in lung function. Medical conditions that necessitate the use of accessory muscles for breathing generally warrant prompt medical evaluation and intervention.

What are the Primary Muscles for Breathing?

The primary muscles involved in breathing are the diaphragm and the intercostal muscles.

These muscles are crucial for the fundamental act of breathing and are responsible for the majority of air movement in and out of the lungs during quiet, relaxed breathing. 

Diaphragm

The diaphragm is a dome-shaped muscle that separates the thoracic cavity, which houses the lungs and heart, from the abdominal cavity.

During inspiration (inhaling), the diaphragm contracts and flattens, enlarging the volume of the thoracic cavity. This decreases the pressure inside the cavity relative to the outside atmosphere, allowing air to flow into the lungs.

During expiration (exhaling), the diaphragm relaxes and returns to its original dome shape, reducing the volume of the thoracic cavity and pushing air out.

Diaphragm Muscle Labeled Vector Illustration

Intercostal Muscles

The intercostal muscles are located between the ribs and are divided into external and internal layers.

  • External Intercostal Muscles: These muscles are primarily responsible for inspiration. When they contract, they elevate the rib cage, increasing the volume of the thoracic cavity and allowing air to be drawn into the lungs.
  • Internal Intercostal Muscles: These muscles are more active during forced expiration. Their contraction depresses the rib cage, reducing the volume of the thoracic cavity and aiding in the expulsion of air from the lungs.

These primary muscles of respiration are critical for normal breathing and are automatically controlled by the respiratory centers in the brainstem, which adjust the rate and depth of breathing based on factors such as blood CO2 levels, pH, and oxygen needs.

Intercostal Muscles Labeled Vector Illustration

Inspiration vs. Expiration

Inspiration and expiration are the two fundamental processes that constitute the act of breathing or respiration.

Each phase involves a distinct set of physiological actions aimed at either drawing air into the lungs or expelling it out.

Inspiration vs. Expiration vector illustration

Inspiration

  • Muscles Involved: The primary muscles involved in inspiration are the diaphragm and the external intercostal muscles. Accessory muscles like the sternocleidomastoid and scalene muscles may be recruited during strenuous activity or respiratory distress.
  • Mechanical Process: During inspiration, the diaphragm contracts and moves downward while the external intercostal muscles lift the rib cage upward and outward. This increases the volume of the thoracic cavity.
  • Pressure Changes: The increase in thoracic volume leads to a decrease in intrathoracic pressure relative to atmospheric pressure.
  • Air Flow: The pressure gradient between the atmosphere and the thoracic cavity prompts air to flow into the lungs.
  • Gas Exchange: The incoming air replenishes oxygen levels in the alveoli, where gas exchange with the bloodstream occurs.
  • Energy Requirement: Inspiration is generally an active process that requires energy.

Expiration

  • Muscles Involved: During quiet breathing, expiration is often a passive process and may not require muscular effort. However, during forced expiration, internal intercostal muscles and abdominal muscles can be used.
  • Mechanical Process: In passive expiration, the diaphragm and external intercostal muscles relax, reducing the volume of the thoracic cavity. In forced expiration, the abdominal and internal intercostal muscles contract to further reduce thoracic volume.
  • Pressure Changes: The decrease in thoracic volume increases intrathoracic pressure relative to the external atmosphere.
  • Air Flow: Air flows out of the lungs due to the pressure gradient.
  • Gas Exchange: Expired air carries waste products like carbon dioxide out of the body.
  • Energy Requirement: Passive expiration generally does not require energy, while forced expiration does.

Summary: Inspiration involves the active contraction of muscles to draw air into the lungs, whereas expiration can be either a passive or an active process aimed at expelling air out of the lungs. Both are vital for gas exchange, the ultimate purpose of which is to supply the body with oxygen and remove waste products like carbon dioxide.

Causes of Accessory Muscle Use for Breathing

The use of accessory muscles for breathing is generally a sign that the respiratory system is under stress or that normal breathing mechanics are compromised.

Here are some common causes that may necessitate the use of accessory muscles:

Respiratory Diseases

  • Asthma: During an asthma attack, the airways become narrowed, making it difficult to breathe and often requiring the use of accessory muscles.
  • Chronic Obstructive Pulmonary Disease (COPD): This condition, which includes chronic bronchitis and emphysema, impairs the lungs’ ability to exchange gases, often requiring additional muscular effort for breathing.
  • Pneumonia: Inflammation and fluid in the lungs can make breathing laborious, leading to accessory muscle use.
  • Pulmonary Edema: Fluid accumulation in the lungs can increase the work of breathing, requiring accessory muscles.

Acute Medical Conditions

  • Acute Respiratory Distress Syndrome (ARDS) This is a severe form of lung injury that requires significant effort to maintain adequate oxygen levels.
  • Pleural Effusion: Accumulation of fluid in the pleural space can impair lung expansion, making breathing difficult.
  • Pneumothorax: A collapsed lung can severely limit respiratory function, necessitating the use of accessory muscles.

Physical Exertion and Environmental Factors

  • Exercise: Intense or prolonged physical activity increases the body’s demand for oxygen, which can lead to the recruitment of accessory muscles.
  • High Altitude: The reduced oxygen availability at high altitudes may make breathing more difficult, necessitating the use of accessory muscles.

Neuromuscular Disorders

  • Muscular Dystrophy: Weakness in the diaphragm or other primary respiratory muscles can make it difficult to breathe, leading to reliance on accessory muscles.
  • Amyotrophic Lateral Sclerosis (ALS): Progressive muscular weakness may impair the normal mechanics of breathing, requiring the use of accessory muscles.

Other Factors

  • Anxiety and Panic Attacks: The perception of difficulty in breathing may lead to increased respiratory effort, including the use of accessory muscles.
  • Obesity: Excess weight, particularly in the abdominal region, can increase the work of breathing, possibly leading to accessory muscle use.

Remember: The use of accessory muscles for breathing is often a clinical indicator that immediate medical evaluation and intervention may be necessary, as it signals an increased work of breathing that could compromise oxygen and carbon dioxide exchange.

FAQs About the Accessory Muscles of Breathing

Is it Normal to Use Accessory Muscles When Breathing?

No, it is not generally considered normal to use accessory muscles for breathing during quiet, relaxed respiration.

The use of accessory muscles typically indicates increased work of breathing and may be a sign of respiratory distress or disease.

However, during intense physical exertion or specific situations like high altitude, temporary use of accessory muscles can be considered normal.

Where are the Accessory Muscles of Breathing?

The accessory muscles of breathing are located in various regions of the upper body. The sternocleidomastoid is located in the neck and helps lift the sternum.

The scalene muscles are also found in the neck and assist in lifting the first two ribs.

For forced expiration, the abdominal muscles (rectus abdominis, obliques) are located in the abdominal region and help push the diaphragm upwards to expel air.

If the Accessory Muscles of Breathing Are Active, an Individual Must Be Experiencing What?

If the accessory muscles of breathing are active, an individual is likely experiencing increased work of breathing. This could be due to a variety of reasons ranging from respiratory diseases like asthma or COPD to acute conditions like pneumonia or ARDS.

It could also be seen in situations of intense physical exertion or high emotional stress. The activation of accessory muscles is generally a sign that immediate medical evaluation may be necessary.

What Is the Process of Passive Expiration?

Passive expiration occurs during relaxed, quiet breathing and does not generally require muscular effort.

In this process, the diaphragm and external intercostal muscles simply relax, allowing the elastic recoil of the lungs and thoracic cavity to decrease the volume, thereby increasing the pressure inside the thoracic cavity.

This pressure gradient between the inside and outside atmosphere facilitates the outward flow of air, expelling it from the lungs.

Final Thoughts

The accessory muscles of breathing serve as vital support mechanisms for the primary respiratory muscles.

Their role becomes especially pronounced during periods of increased oxygen demand or compromised lung function, often providing the much-needed boost to achieve effective respiration.

Whether aiding in forced inspiration or facilitating more efficient expiration, these muscles are integral to the robustness and adaptability of our respiratory system.

Their importance extends beyond academic interest, offering practical insights for respiratory therapy and the management of pulmonary diseases.

Therefore, a comprehensive understanding of these accessory muscles is not only scientifically enriching but also clinically imperative.

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.

References

  • Sieck GC, Ferreira LF, Reid MB, Mantilla CB. Mechanical properties of respiratory muscles. Compr Physiol. 2013 Oct
  • Poole DC, Sexton WL, Farkas GA, Powers SK, Reid MB. Diaphragm structure and function in health and disease. Med Sci Sports Exerc. 1997 Jun
  • Tang A, Bordoni B. Anatomy, Thorax, Muscles. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.

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