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Forced expiration is a breathing mode that can occur during exercise or activities that require active manipulation of breathing, such as singing or playing an instrument. It involves forceful exhalation, which is a sharp and sudden expulsion of air. During forced expiration, several muscles contract to assist in the process. These include the internal intercostal muscles, rectus abdominis, and abdominal muscles. The internal intercostal muscles help to pull the ribs downward and inward, reducing the size of the thoracic cavity. The rectus abdominis increases intra-abdominal pressure, pushing the diaphragm upwards, while the abdominal muscles contract and force the abdominal organs upward, aiding in pushing the diaphragm further into the thorax and expelling more air.
| Characteristics | Values |
|---|---|
| Muscles assisting in forced expiration | Internal intercostal muscles, Rectus abdominis, External oblique, Internal oblique, Transverse abdominis, Quadratus lumborum, Serratus posterior inferior, Sternocleidomastoid, Scalene, Serratus anterior |
| Function of internal intercostal muscles | Altering the anteroposterior dimension of the chest cavity |
| Function of rectus abdominis | Increase intra-abdominal pressure, pushing the diaphragm upwards |
| Function of external and internal oblique | Contract to force abdominal organs upward against the diaphragm |
| Function of scalene | Contract and lift the thoracic wall, increasing lung volume |
| Function of sternocleidomastoid | Elevate the sternum and clavicle, lifting the ribs during inhalation |
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What You'll Learn
Internal intercostal muscles
The internal intercostal muscles are a group of 11 muscles that lie between the ribs in the intercostal spaces. They are part of the muscles of the thoracic wall and are located deep to the external intercostal muscles, separated by a thin fascia. They play a crucial role in breathing and forced expiration.
During forced expiration, the internal intercostal muscles contract and work together with other muscles to facilitate a sharp and forceful exhalation. They achieve this by depressing and retracting the ribs, which compresses the thoracic cavity and expels air from the lungs. This mechanism is particularly evident during activities such as coughing or exercising, when forceful exhalation is required.
The internal intercostal muscles originate from the inferior margin of the costal cartilage and the inner lip of the costal groove. Their muscle fibres extend and insert onto the superior border of the rib below. At the posterior angle of the rib, the muscle fibres blend with the internal intercostal membrane, contributing to the structural integrity of the thoracic cage.
The internal intercostal muscles are innervated by the intercostal nerves, specifically T1-T11, which are the anterior rami of the thoracic spinal nerves. Their blood supply is provided by the anterior and posterior intercostal arteries, with venous blood drained by the corresponding intercostal veins into the superior vena cava.
The function of the internal intercostal muscles is not limited to forced expiration. They also play a role in altering the anteroposterior dimension of the chest cavity and stabilising the position of the ribs during normal respiration. Additionally, they work in conjunction with the external intercostal muscles and other accessory muscles, such as the sternocleidomastoid and scalene muscles, to facilitate a full range of respiratory movements.
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the front of the abdomen, often referred to as the "abs muscle" or "six-pack". It is separated from its fellow muscle on the opposite side by the linea alba, a connective tissue. The rectus abdominis is part of the core muscles, which include the transversus abdominis, erector spinae, and obliques. Together, these muscles act as a natural weight belt to protect the lower back from injury.
The rectus abdominis is responsible for flexing the spinal column and tensing the anterior wall of the abdomen. It also assists in compressing the contents of the abdomen and plays a role in respiration during forceful exhalation. During forced expiration, the ventral respiratory group in the brainstem activates and sends impulses to the rectus abdominis through the thoracoabdominal nerves. This results in an increase in intra-abdominal pressure, pushing the diaphragm upwards and aiding in forced exhalation.
The rectus abdominis is a paired muscle, with each muscle subdivided into smaller muscle bellies by tendinous intersections. These fibrous bands create the characteristic "six-pack" appearance in individuals with low body fat. The muscle is innervated by the thoracoabdominal nerves, which are continuations of the T7-T11 intercostal nerves. The blood supply to the rectus abdominis comes predominantly from the inferior and superior epigastric arteries, with contributions from other vessels as well.
The strength of the rectus abdominis can be assessed through exercises such as the sit-up, where the patient raises their trunk against gravity from a supine position. This muscle is susceptible to strains and tears, which can lead to abdominal pain and hematomas. Overall, the rectus abdominis plays a crucial role in core stability, respiration, and spinal flexibility.
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Sternocleidomastoid
The sternocleidomastoid muscle, often abbreviated as SCM, is a powerful neck muscle that allows you to bend, turn, and tilt your head. SCM is one of over 20 pairs of muscles acting on the neck. It is a superficially palpable muscle that serves as a primary landmark of the neck, dividing it into anterior and posterior cervical triangles. This muscle is thick and can be felt on both sides of the neck when a person moves their head to the opposite side.
The SCM originates from the manubrium of the sternum and the sternal end of each clavicle, inserting on the mastoid process of the temporal bone and the superior nuchal line of the occipital bone. This muscle is responsible for elevating the sternum and clavicle, subsequently lifting the ribs during inhalation to create space for the lungs to take in air. It also helps stabilize the neck and maintain posture even when one is not moving.
The SCM is innervated by the accessory nerve of the same side, supplying only motor fibres. The signalling process to contract or relax the SCM begins in Cranial Nerve XI, the accessory nerve, with the origin of lower motor neuron fibres in the anterior horn of the spinal cord around C1-C3. The fibres from the accessory nerve nucleus travel upward to enter the cranium via the foramen magnum, reaching both the SCM and trapezius muscles. Acetylcholine (ACH) is released, causing an increase in resting potential and initiating an action potential that travels along the muscle fibre, resulting in SCM contraction.
SCM syndrome is a condition involving neck stiffness, pain, and other symptoms such as trigger points, which may require surgery if more conservative treatments are ineffective. Treatment for SCM-related issues includes managing stress, maintaining good posture, and physical therapy, with exercises to stretch and strengthen the muscle.
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Accessory abdominal muscles
Accessory muscles are those that are used in addition to the diaphragm, which is the primary muscle for breathing. They are used when a person takes a deep breath, such as during exercise, swimming underwater, or blowing out birthday candles. They are also used in people with respiratory conditions, such as chronic obstructive pulmonary disease (COPD), pneumonia, or fluid buildup in the lungs.
The abdominal wall muscles are the main accessory muscles of expiration. When they contract, they push the diaphragm up into the chest, reducing the size of the thoracic cavity and forcing more air out of the lungs. The internal intercostal muscles are also accessory muscles of expiration. They run along the bodies and costal cartilages of the ribs between the sternum and the angle of the ribs. When they contract, they pull the ribs downwards and inward, further compressing the lungs and forcing air out.
Other accessory muscles of expiration include the external oblique, internal oblique, transverse abdominis, and quadratus lumborum muscles. These muscles work together with the abdominal wall muscles and internal intercostal muscles to assist in forced expiration, ensuring that air is effectively expelled from the lungs during exhalation.
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Diaphragm
The diaphragm is a large, dome-shaped muscle located below the lungs. It is the main muscle of respiration, responsible for the rhythmic and continual contraction and relaxation that facilitates inhalation and exhalation. During inhalation, the diaphragm contracts and flattens, enlarging the chest cavity and creating a vacuum that pulls air into the lungs. Conversely, upon exhalation, the diaphragm relaxes and returns to its dome-like shape, allowing air to be forced out of the lungs.
The diaphragm plays a crucial role in maintaining the integrity of the respiratory system. During expiration, it acts as a brake to prevent lung collapse. This function is achieved through the coupling of electrical activity in the diaphragmatic muscle cells with the mechanical force generated during expiration. The diaphragm's efficiency in this role is influenced by lung volume and pressure conditions. Specifically, at low lung volumes and pressures, the diaphragm exerts higher force and becomes more efficient in generating force.
The diaphragm's function during forced expiration is closely associated with the action of other muscles, including the internal and external intercostals. The internal intercostal muscles, consisting of 11 pairs, run along the ribs between the sternum and the angle of the ribs. They work together with the diaphragm to alter the dimensions of the chest cavity during breathing. During normal expiration, the external intercostals and the diaphragm relax, allowing air to be expelled from the lungs passively.
During forced expiration, the diaphragm contracts, and its activity is coordinated by the respiratory centre in the brain stem. Impulses from the ventral respiratory group activate the diaphragm through the thoracoabdominal nerves, leading to forced exhalation. This process increases intra-abdominal pressure, pushing the diaphragm upwards and contributing to the forceful expulsion of air.
In summary, the diaphragm is a vital muscle for respiration, and its function during forced expiration is closely integrated with the activity of other respiratory muscles. Its contraction during forced exhalation increases intra-abdominal pressure and contributes to the forceful expulsion of air, while its unique properties help prevent lung collapse.
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Frequently asked questions
The internal intercostal muscles, rectus abdominis, external oblique, internal oblique, transverse abdominis, and quadratus lumborum are some of the muscles that assist in forced expiration.
No, the external intercostal muscles also play a role in forced expiration, along with the internal intercostals.
The rectus abdominis increases intra-abdominal pressure, pushing the diaphragm upwards and aiding in forced expiration.
Yes, accessory muscles such as the obliques, scalene, and sternocleidomastoid can also assist in forced expiration by forcing abdominal organs upwards and compressing the rib cage.
Quiet expiration is a passive process that relies on the elastic recoil of the lungs and rib cage. Forced expiration, on the other hand, requires the active contraction of expiratory muscles, including the internal intercostals and abdominal muscles, to generate a forceful exhalation.
