Dominic Stone
Accessory muscles are additional muscles that the body activates to aid breathing. They are used when extra power is needed, such as during exercise, or in cases of respiratory distress. Typically, the diaphragm is the only muscle used during normal breathing, with expiration being a passive process that relies on the elastic recoil of the lungs. However, accessory muscles can be activated during inspiration or expiration in cases of respiratory conditions, such as COPD, or in situations where a person needs to take a deliberately deep breath, such as when swimming underwater.
| Characteristics | Values |
|---|---|
| When are accessory muscles used? | When the body needs to breathe more deeply or forcefully, such as during exercise, or when there is a compromise in lung function. |
| Why are accessory muscles used? | To increase the size of the thoracic cavity, allowing for greater expansion of the lungs and optimising lung function. |
| What are accessory muscles? | Muscles that help in expanding the thoracic cavity are called inspiratory muscles because they help in inhalation. |
| Are there different types of accessory muscles? | Yes, accessory muscles include the sternocleidomastoid, scalene, intercostal, and abdominal muscles. |
| What do these accessory muscles do? | The sternocleidomastoid lifts the sternum, facilitating an increased expansion of the chest cavity. The scalene muscles assist in elevating the ribs, further contributing to chest expansion. The abdominal muscles contract to push the diaphragm upwards, helping to expel air from the lungs. |
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What You'll Learn
Accessory muscles are used during exercise
Accessory muscles are not used during typical breathing. They are called into action when the diaphragm and outer intercostal muscles are unable to move enough air in and out of the lungs. The diaphragm and intercostal muscles are the primary muscles involved in breathing and are responsible for the majority of air movement in and out of the lungs during quiet, relaxed breathing.
During exercise, the body's demand for oxygen increases. This increased metabolic need during exercise can lead to the recruitment of accessory muscles. Accessory muscles of inspiration lift up the breastbone, upper ribs, and collarbones, causing the upper part of the chest to rise, making the lungs bigger, and allowing more air to enter.
The accessory muscles of inspiration include the scalene, sternocleidomastoid, trapezius, and pectoralis major muscles. The sternocleidomastoid, for example, helps lift the sternum and increases the anteroposterior diameter of the chest, allowing for greater expansion of the lungs. The scalene muscles help to lift the first two ribs and also contribute to the anteroposterior expansion of the chest.
Accessory muscles of expiration are also used in disorders preventing normal movement of air out of the lungs. For example, people with severe chronic obstructive pulmonary disease (COPD) often use these muscles as air that cannot be breathed out becomes trapped in the lungs. The main accessory muscles of expiration are the abdominal wall muscles.
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They are also used in respiratory distress
Accessory muscle use is often a sign of respiratory distress. Typically, the diaphragm is the only muscle used in normal breathing, and accessory muscles are not active during regular breathing in healthy people. However, accessory muscles are used during respiratory distress, which is characterised by laboured breathing or respiratory distress or dyspnea at rest.
Accessory muscle breathing means using muscles other than those typically used for breathing to take in and expel air. This can be due to deliberate breathing practices, strenuous exercise, or a health condition. In people with certain medical conditions, such as chronic obstructive lung disease or respiratory muscle fatigue, accessory muscles are used to help with breathing. This is because these conditions make it more difficult to breathe, and the body automatically activates the accessory muscles to compensate. For example, in patients with obstructed airways, the abdominal muscles help to expel air across the blockage.
Accessory muscles that are used during respiratory distress include the sternocleidomastoid, scalene, and subclavius muscles. The sternocleidomastoid and scalene muscles are involved in inspiration, while the abdominal oblique muscles are primarily involved in expiration. The subclavius muscle, located between the collarbone and the top rib, stabilises the rib cage.
The use of accessory muscles in respiratory distress can be assessed using scales such as the 7-item Respiratory Distress Observation Scale (RODS). This scale includes parameters such as fear, grunting, respiratory rate, heart rate, nasal flaring, accessory muscle use, and restlessness. It helps predict when a nurse would rate respiratory distress as moderate or severe.
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Accessory muscles can be used to compensate for respiratory conditions
Accessory muscles are recruited during physical exercise due to increased metabolic need and during dysfunction in the respiratory system. They are also used to compensate for respiratory conditions.
The primary muscles involved in the process of breathing are the diaphragm and the intercostal muscles. When breathing becomes difficult, accessory muscles are employed to assist in the respiratory effort. These include the sternocleidomastoid, scalene muscles, and various abdominal muscles. Accessory muscles help to expand and contract the thoracic cavity, facilitating adequate gas exchange. They can also be used to forcefully expel air, such as when blowing out birthday cake candles.
In the case of respiratory conditions, accessory muscles may be activated to compensate for the increased work of breathing. For example, during an asthma attack, the airways become narrowed, making it difficult to breathe, and requiring the use of accessory muscles. Similarly, in Chronic Obstructive Pulmonary Disease (COPD), the lungs' ability to exchange gases is impaired, necessitating additional muscular effort. Other conditions that may require the use of accessory muscles include pneumonia, pulmonary edema, and Acute Respiratory Distress Syndrome (ARDS).
Accessory muscles can also be used in situations of high emotional stress or intense physical exertion. For instance, during swimming underwater, a person may engage their accessory muscles to take a deliberate deep breath. In patients with obstructed airways, the abdominal muscles, a type of accessory muscle, help to expel air across the obstruction.
Additionally, different life stages can influence accessory muscle breathing. Young children with underdeveloped muscles may need to rely on accessory muscles to breathe, as their diaphragm is not yet strong enough. Similarly, in end-of-life care, doctors often observe the activation of accessory muscles in patients experiencing shortness of breath.
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They are used during forceful exhalation
Accessory muscles are used during forceful exhalation, such as when blowing out birthday candles or during exercise. These muscles are not typically used during regular breathing, but they may be called upon when taking a deliberately deep breath, such as when swimming underwater.
The accessory muscles used during forceful exhalation include the internal intercostals, intercostalis intimi, subcostals, and the abdominal muscles. The abdominal muscles involved are the rectus abdominis, transverse abdominis, external oblique muscle, and internal oblique muscle. These muscles press the abdominal organs upward into the diaphragm, reducing the volume of the thoracic cavity and forcing air out of the lungs.
The involvement of accessory muscles in breathing may also depend on the degree of respiratory effort required. For example, during quiet breathing, the scalenes are consistently physically active, while the sternocleidomastoids are quiet. However, with an increase in respiratory volume, the sternocleidomastoids also become active, and both muscles are simultaneously activated during maximal inhalation.
Accessory muscles may also be used during typical breathing in individuals with respiratory conditions, such as COPD, that make breathing more difficult. In such cases, the lungs over-inflate and air becomes trapped, rendering the diaphragm and intercostal muscles unable to move enough air in and out of the lungs, thus requiring the use of accessory muscles.
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Accessory muscles are used in infants
Accessory muscles are used in breathing when there is difficulty in taking in enough air using only the primary breathing muscle, the diaphragm. This can be due to certain medical conditions or underdeveloped muscles in young children and infants.
Infants' diaphragms are prone to dysfunction due to lower muscle mass, a flattened shape, and a decreased content of fatigue-resistant muscle fibres. The diaphragm is also more likely to fatigue in newborns, as it is not as strong as in older children or adults. When this fatigue sets in, the newborn's body recruits accessory muscles to help them keep breathing.
The infantile diaphragm and thorax are anatomically disadvantaged to handle an increased Work of Breathing (WoB). The diaphragm's flattened shape and larger angle of insertion into the chest wall result in a smaller zone of apposition and a decreased range of displacement. This means that the diaphragm acts more like a bellow, and its movement can distort the lower rib cage.
Additionally, the rib cage in newborns is already elevated and horizontal, so its elevation during inspiration has little effect on the intra-thoracic volume. The neonatal chest wall is also highly compliant and prone to distortion, especially during REM sleep when intercostal muscle activity is inhibited.
Therefore, accessory muscles are used in infants to compensate for the disadvantages of their diaphragm and thorax, ensuring they can breathe effectively.
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Frequently asked questions
Accessory muscles are additional muscles that the body activates to help inhale and exhale air into the lungs. They assist, but do not play a primary role in breathing.
Accessory muscles are used when additional power is needed, for example, during exercise or in people with airway pathologies such as COPD. They are also used when taking a deliberately deep breath, such as when swimming underwater.
There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included. Other accessory muscles include the serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, and quadratus lumborum.
Inspiratory muscles help in inhalation by expanding the thoracic cavity. Expiratory muscles induce exhalation by compressing the thoracic cavity.
The diaphragm is the major muscle responsible for breathing and is the only muscle used during normal breathing. During quiet breathing, the intercostal muscles also contribute to a lesser extent.
