Dominic Stone
The process of breathing out, also known as exhalation or expiration, is usually passive when a person is at rest, with little to no muscle contraction involved. However, during vigorous exercise, several muscles become important in aiding exhalation. The diaphragm is the primary muscle responsible for breathing, and during exhalation, it relaxes and returns to its dome shape, causing air to be expelled from the lungs. The abdominal muscles are also crucial during active exhalation, contracting to raise abdominal pressure and push the diaphragm against the lungs, forcing air out. These include the rectus abdominis, transverse abdominis, external oblique, and internal oblique muscles. Additionally, the intercostal muscles, which consist of external, internal, and innermost intercostals, assist in manipulating the width of the rib cage and play a role in both inhalation and exhalation.
| Characteristics | Values |
|---|---|
| Muscles supporting exhalation | Abdominal muscles (rectus abdominis, external oblique, internal oblique, and transversus abdominis), diaphragm, intercostal muscles |
| Function | Contraction of abdominal muscles raises abdominal pressure, pushing a relaxed diaphragm against the lungs and causing air to be exhaled |
| Use | During quiet breathing, exhalation is passive and requires no muscle contraction. During vigorous exercise, abdominal muscles are used for active exhalation |
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What You'll Learn
The diaphragm
During exhalation, the diaphragm works in conjunction with the abdominal muscles, which are the most important muscles involved in this process. The abdominal muscles contract, raising abdominal pressure, and push the diaphragm against the lungs, facilitating the expulsion of air.
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Intercostal muscles
The external intercostal muscles are the outermost layer of intercostal muscles. They originate on ribs 1–11 and insert on ribs 2–12. These muscles elevate the ribs and bend them more open, expanding the transverse dimensions of the thoracic cavity. The external intercostals facilitate forced inspiration. They receive their blood supply from the anterior and posterior intercostal arteries.
The internal intercostal muscles are the intermediate layer of intercostal muscles. They originate from the costal groove of one rib and insert into the superior border of the immediate rib below. These muscles depress the ribs and bend them inward, decreasing the transverse dimensions of the thoracic cavity. The internal intercostals aid in forced expiration. They are innervated by the intercostal nerves, just like the external intercostals.
The innermost intercostal muscles are the deepest layer of intercostal muscles. They originate from the costal groove of one rib, posteriorly to the origin of the internal intercostals, and insert into the superior border of the immediate rib below. The innermost intercostals assist the internal and external intercostals in their function. They follow the same vascularization and innervation pattern as the internal intercostals.
During vigorous exercise, the abdominal muscles are the most important muscles involved in exhalation. They contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be exhaled. However, the intercostal muscles, particularly the internal and innermost layers, also play a role in forced expiration by aiding in the mechanical aspect of breathing and helping to collapse the lungs.
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Abdominal muscles
Exhalation, or expiration, is usually a passive process when a person is at rest. During vigorous exercise, however, a number of muscles participate in exhalation, with the abdominal muscles being the most important.
The abdominal muscles contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be pushed out. The muscles used in breathing can contract only if the nerves connecting them to the brain are intact.
The abdominal muscles that support exhalation are the rectus abdominis, external oblique, internal oblique, and transversus abdominis. The rectus abdominis pulls the ribs down during active expiration. Its point of origin is the pubic symphysis and pubic crest, and it attaches to the xiphoid process and the 5th to 7th costal cartilages. This pair of muscles is separated by the linea alba.
The expiratory activity of the rectus abdominis and external oblique muscles increases with exercise intensity. During cycling, rectus abdominis activity was observed in six out of seven subjects, while external oblique activity was observed in four of the seven subjects. During constant work rate exercise, EMG activities increased to 40-50% and 5-10% of peak activity in rectus and external oblique muscles, respectively.
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Thoracic muscles
The thoracic cage and its walls form a protective bony framework for the respiratory system, allowing ventilation to take place. While the thoracic cage provides a resistant yet flexible structure, it is the action of the thoracic muscles that makes breathing possible.
The diaphragm is a crucial muscle that sits beneath the thoracic cage, separating the thoracic and abdominal cavities. During inhalation, the diaphragm contracts and moves downwards, increasing the vertical diameter of the thoracic cavity and expanding the lungs. During exhalation, the diaphragm relaxes, and the volume of the thoracic cavity decreases, forcing air out of the lungs.
The intercostal muscles are another important group of thoracic muscles that assist in breathing. These muscles are located in the intercostal spaces between the ribs. There are three types of intercostal muscles: external, internal, and innermost intercostals. The external intercostals are the most superficial layer, while the internal and innermost intercostals lie deeper. The external and internal intercostals work together to alter the anteroposterior diameter of the chest cavity, thereby assisting in ventilation. During inhalation, the external and internal intercostals contract and pull upward, elevating the rib cage and expanding the lungs. During exhalation, these muscles relax and depress the rib cage, decreasing the volume of the thoracic cavity and aiding in air expulsion.
In addition to the diaphragm and intercostal muscles, other muscles that contribute to forceful exhalation include the abdominal muscles, subcostals, and intercostalis intimi. The abdominal muscles, specifically the rectus abdominis, pull the ribs down during active exhalation. During vigorous exercise, the abdominal muscles contract, raising abdominal pressure and pushing against the diaphragm, which aids in forcing air out of the lungs.
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Accessory expiratory muscles
The accessory expiratory muscles are those that support exhalation, or breathing out. During normal breathing, exhalation is a passive process that relies on the elastic recoil of the lungs. However, during vigorous exercise or in cases of reduced lung elasticity, active exhalation is achieved through the contraction of accessory expiratory muscles.
The abdominal muscles are the most important accessory expiratory muscles. They contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be exhaled. The abdominal muscles that support exhalation include the rectus abdominis, external oblique, internal oblique, and transversus abdominis.
In addition to the abdominal muscles, the lowest fibres of iliocostalis and longissimus in the thoracolumbar region also support exhalation. Other accessory expiratory muscles include the serratus posterior inferior and quadratus lumborum.
The use of accessory muscles during breathing is often a sign of respiratory distress. Patients with chronic obstructive lung disease, respiratory muscle fatigue, or severe obstructive disease often exhibit accessory muscle use. For example, in patients with chronic obstructive lung disease, the clavicle lifts during inspiration, indicating the use of accessory muscles.
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Frequently asked questions
The abdominal muscles are the most important muscles for exhalation. These include the rectus abdominis, transverse abdominis, external oblique muscle, and internal oblique muscle.
During quiet breathing, exhalation is a passive process, and the abdominal muscles are usually not involved. However, during vigorous exercise, these muscles contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be exhaled.
The diaphragm, along with the intercostal muscles, are also involved in exhalation. The diaphragm relaxes during exhalation, allowing the elastic recoil of the lungs and chest wall to return to their resting volume and forcing air out of the lungs.
