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The trunk, or torso, is the central part of the body to which the head and limbs are attached. Trunk muscles are divided into two large groups, depending on their location: the muscles of the thoracic cage and the muscles of the abdominal wall. The anterior trunk muscles, which are associated with the front of the body, include chest and abdominal muscles. Chest muscles function in respiration, while abdominal muscles are responsible for torso movement and the maintenance of balance and posture. The intercostal muscles, which are located in the spaces between the ribs, play a key role in respiration. The diaphragm, a dome-shaped muscle that separates the thorax from the abdomen, is also crucial for respiration. The erector spinae group of muscles, located on each side of the vertebral column, is responsible for extending the vertebral column to maintain an erect posture.
| Characteristics | Values |
|---|---|
| Definition | Muscles of the thorax, abdomen, and back |
| Location | The trunk or torso (central part of the body) |
| Function | Protection of thoracic and abdominopelvic viscera, breathing, movement, defecation, and micturition |
| Groups | Muscles of the thoracic cage, muscles of the abdominal wall, anterior trunk muscles, and posterior trunk muscles |
| Examples of Thoracic Cage Muscles | Pectoralis major, pectoralis minor, serratus anterior, subclavius, intercostal (external, internal, and innermost), subcostal, and transversus thoracis muscles, including the diaphragm |
| Examples of Abdominal Wall Muscles | Obliques, rectus abdominis ("six-pack abs"), linea alba |
| Examples of Anterior Trunk Muscles | Chest and abdominal muscles |
| Examples of Posterior Trunk Muscles | Latissimus dorsi, superficial posterior muscles, intermediate muscles, intrinsic back muscles |
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What You'll Learn
Intercostal muscles
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They are divided into three groups, from superficial to deep: external, internal, and innermost intercostals. All three groups of muscles support the rib cage and are accessory respiratory muscles that participate in the process of forced breathing.
The external intercostal muscles are the most superficial intercostal muscles. They originate from the inferior aspect of the rib above and insert onto the superior aspect of the rib below in an inferomedial direction. During inspiration, the external intercostals contract and raise the lateral part of the ribs, increasing the transverse diameter of the thorax. They are responsible for the elevation of the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity.
The internal intercostal muscles form the middle layer of the intercostal musculature. They originate from the costal groove of one rib, course inferolaterally, and insert to the superior border of the immediate rib below. The internal intercostals depress the rib cage during forced expiration, reducing the size of the thorax and lungs. They are responsible for the depression of the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity.
The innermost intercostal muscles are the deepest intercostal muscles. They originate from the medial aspect of the costal groove of the rib above and insert onto the internal aspect of the rib below. The innermost intercostal muscles also originate from the costal groove of the ribs and insert onto the superior border of the rib below. They are deep layers of the internal intercostal muscles, separated from them by a neurovascular bundle.
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Thoracic cage muscles
The thoracic cage, or thorax, is the central part of the body to which the head and limbs are attached. The thoracic wall forms its anterior, posterior, and lateral boundaries. The thoracic cage houses all the vital organs of the human body, except for the brain. The muscles of the thoracic cage support the thoracic cage and are involved in movements of the upper limbs and respiration.
The thoracic wall is made up of five muscles: the external intercostal, internal intercostal, innermost intercostal, subcostalis, and transversus thoracis. There are eleven pairs of intercostal muscles in each of the intercostal spaces, arranged from superficial to deep. The external intercostal muscles are the most superficial layer. They originate from the inferior aspect of the rib above and insert onto the superior aspect of the rib below in an inferomedial direction. During inspiration, the external intercostals contract and raise the lateral part of the ribs, increasing the transverse diameter of the thorax. The internal intercostal muscles extend in an inferoposterior direction from the costal groove of the ribs to the superior border of the rib below. The innermost intercostals also originate from the costal groove of the ribs and insert onto the superior border of the rib below.
The subcostal muscles are strips of muscle located on the internal surface of the lower ribs, sharing a plane with the innermost intercostals. They extend from the internal surface of the lower ribs to the internal surface of a rib located 2-3 levels below. They support the intercostal spaces and thoracic cage and depress the ribs during forced expiration. The transversus thoracis muscle is a thin, radiating muscle located on the internal aspect of the anterior thoracic wall. It originates from the inferoposterior surface of the sternum and xiphoid process.
In addition to the intercostal muscles, there are other muscles that do not form the thoracic wall but attach to it. These are termed accessory muscles of respiration and include the pectoralis major, pectoralis minor, serratus anterior, and scalene muscles. The diaphragm is another muscle in the thorax that serves as the main muscle of inspiration. It forms a partition between the thorax and the abdomen and makes up the floor of the thorax, separating the contents of the chest from those of the abdomen.
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Abdominal wall muscles
The trunk (torso) is the central part of the body to which the head and limbs are attached. The trunk houses all the vital organs of the human body, except the brain. The abdominal wall, formed of skin, fascia, and muscle, encases the abdominal cavity and viscera. The abdominal wall muscles can be divided into two sections: the anterolateral abdominal wall and the posterior abdominal wall.
The anterolateral abdominal wall consists of four main layers: skin, superficial fascia, muscles and associated fascia, and parietal peritoneum. The muscles of the anterolateral abdominal wall can be divided into two main groups: flat muscles and vertical muscles. Flat muscles include external oblique, internal oblique, and transversus abdominis. The external oblique is the largest and most superficial flat muscle in the abdominal wall. The internal oblique lies deep to the external oblique and is smaller and thinner in structure. The transversus abdominis is the deepest of the flat muscles, with transversely running fibres. Deep to this muscle is a well-formed layer of fascia, known as the transversalis fascia. There are two vertical muscles located in the midline of the anterolateral abdominal wall – the rectus abdominis and pyramidalis. The rectus abdominis is a long, paired muscle, found either side of the midline in the abdominal wall.
The posterior abdominal wall includes the psoas major and quadratus lumborum bilaterally. The abdominal muscles contribute to movements of the trunk, including flexion, extension, lateral flexion, and rotation. The abdominal muscles may also act as accessory muscles of respiration by depressing the ribs to cause active expiration.
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Back muscles
The back muscles are skeletal muscles that are part of the musculoskeletal system. They are the main structural support for the trunk or torso, helping to maintain posture and enabling movement. These muscles start just under the skull, covering the shoulders and running down the spine to the lower back, just above the hips. They are woven layers of groups that work together to reposition the torso and allow for movements like bending over, twisting, turning the head and extending the back.
The intrinsic back muscles can be further subdivided into their own superficial, intermediate, and deep layers. These muscles collectively work to facilitate movements of the vertebral column and control posture. The deep muscles of the back are well-developed, extending from the sacrum to the base of the skull. They are associated with movements of the head and neck and are located on the posterolateral aspect of the neck, covering the deeper neck muscles.
The erector spinae group of muscles, located on each side of the vertebral column, is a large muscle mass that extends from the sacrum to the skull. These muscles are primarily responsible for extending the vertebral column to maintain an erect posture. The intercostal muscles, located in the spaces between the ribs, contract during forced expiration, and elevate the ribs during the inspiration phase of breathing.
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Anterior trunk muscles
The trunk, or torso, is the central part of the body to which the head and limbs are attached. Trunk muscles are divided into two large groups, depending on their location: the muscles of the thoracic cage and the muscles of the abdominal wall. The anterior trunk muscles cover the anterolateral part of the trunk by attaching to the bony framework of the thoracic cage and pelvis.
The muscles of the thoracic cage include the pectoralis major, pectoralis minor, serratus anterior, subclavius, intercostal (external, internal and innermost), subcostal and transversus thoracis muscles, including the diaphragm. The pectoralis major is a large, fan-shaped, superficial muscle located on the anterior thoracic wall. It forms the bulk of the chest area and can be easily seen in some people, such as weightlifters. The muscle has three points of origin: the clavicular head, which originates from the anterior surface of the clavicle; the sternocostal head, which originates from the anterior surface of the sternum and costal cartilages of ribs 1-6; and the abdominal (rectus) head, which originates from the anterior layer of the rectus sheath.
The intercostal muscles consist of three layers of muscles, from superficial to deep: external, internal, and innermost intercostals. The external intercostals are the most superficial of the intercostal muscles and are continuous with the external oblique muscle of the abdomen. They elevate the ribs during forced inspiration, expanding the thorax and lungs. The internal intercostals lie below the external intercostals and are continuous with the internal oblique muscle of the abdomen. They can elevate or depress the ribs. The innermost intercostals are the deepest intercostal muscles and are similar in structure to the internal intercostals. They originate from the costal groove of the ribs and insert onto the superior border of the rib below.
The transversus thoracis is a thin, radiating muscle located on the internal aspect of the anterior thoracic wall. It originates from the inferoposterior surface of the sternum and xiphoid process and inserts on the internal surface of the 2nd to 6th costal cartilages. It is a weak depressor of the rib cage, assisting expiration.
There are five muscles that form the abdominal part of the anterior trunk: rectus abdominis, pyramidalis, external abdominal oblique, internal abdominal oblique, and transversus abdominis. The rectus abdominis are the muscles often referred to as the "six-pack abs" and are involved in numerous aspects of trunk stabilization and bending. The obliques are abdominal muscles that assist during bending and twisting of the torso.
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Frequently asked questions
Truncal muscles are the muscles of the torso or trunk, which is the central part of the body to which the head and limbs are attached. They are divided into two groups based on their location: the anterior trunk muscles and the muscles of the thoracic cage.
The anterior trunk muscles are associated with the front of the body, including the chest and abdominal muscles. The pectoralis major is a fan-shaped muscle covering the chest, while the pectoralis minor lies beneath it. The intercostal muscles are another group of muscles that lie between the ribs and form the chest wall.
The truncal muscles have several important functions. They protect the thoracic and abdominopelvic viscera and assist in essential body activities such as breathing, movement, defecation, and micturition. The chest muscles function in respiration, while the abdominal muscles help with torso movement and maintaining balance and posture.
