Maximus Gage
The torso, or trunk, is the central region of the body that connects the upper and lower extremities. It contains over 30 pairs of skeletal muscles, which support the spine and trunk, assist with breathing, movement, and defecation, and protect the vital organs. The abdominal muscles, which are part of the torso, include the external and internal obliques, transversus abdominis, rectus abdominis, and pyramidalis. These muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. The back muscles also play a crucial role in torso compression by supporting the torso, enabling movement, and aiding in breathing.
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What You'll Learn
External oblique muscles
The external abdominal oblique muscle is the largest and outermost of the three flat abdominal muscles of the lateral anterior abdomen. It is a pair of muscles, one on each side of the rectus abdominis, and is broad, thin, and irregularly quadrilateral. The external oblique is situated on the lateral and anterior parts of the abdomen, extending from the lower half of the ribs around and down to the pelvis. The muscle arises from eight fleshy digitations, each from the external surfaces and inferior borders of the fifth to twelfth ribs (lower eight ribs). These digitations are arranged in an oblique line which runs inferiorly and anteriorly, with the upper digitations being attached close to the cartilages of the corresponding ribs, the lowest to the apex of the cartilage of the last rib, and the intermediate ones attached at some distance from their cartilages.
The external abdominal oblique is innervated by the anterior rami of the thoracic spinal nerves T7-T12. The intercostal nerves T7- T11 supply the superior-most part of the muscle, and the subcostal nerve (T12) innervates the lower part. The muscle receives sensory innervation from the lumbar plexus, via the iliohypogastric nerve (L1). The upper two-thirds of the muscle receive blood supply from the branches of the lower posterior intercostal and subcostal arteries, while the deep circumflex iliac artery supplies the inferior third of the muscle.
The external oblique functions to pull the chest downwards and compress the abdominal cavity, increasing intra-abdominal pressure. It also performs ipsilateral (same-side) side-bending and contralateral (opposite side) rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa. When acting unilaterally and in synergy with the contralateral internal abdominal oblique, it rotates the trunk to the opposite side. Working together with the ipsilateral abdominal and back muscles, it contributes to the lateral flexion of the trunk on the same side.
The abdominal muscles, including the external obliques, work together to produce movements of the spine as well as to compress the abdominal viscera. They help hold internal organs in place and protect them. They also help with movement between the ribcage and pelvis, allowing the trunk to be in constant motion and supporting the spine and trunk during walking, sitting down, standing up, or twisting from side to side.
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Internal oblique muscles
The internal oblique muscles are thin, flat muscles located on the lateral side of the abdomen. They are one of three layers of muscles that make up the abdominal wall, alongside the external oblique and transversus abdominis muscles. The internal oblique muscles are much thinner and smaller than the external oblique muscles, which sit below them.
The internal oblique muscles are important for several functions of the body. Firstly, they are accessory muscles of respiration, acting as antagonists to the diaphragm. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs and allowing them to fill with air. In contrast, when the internal oblique muscles contract, they compress the organs of the abdomen, pushing them upwards into the diaphragm and reducing the volume of the lungs, resulting in exhalation.
Secondly, the contraction of the internal oblique muscles causes ipsilateral rotation and side-bending. They work together with the external oblique muscles to achieve torsional movements of the trunk. For example, the right internal oblique and left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip.
The internal oblique muscles have multiple sites of origin, including the inguinal ligament, the anterior iliac crest, the iliopectineal arch, and the posterior end of the iliac crest. They insert onto the costal cartilages of the lower ribs, the abdominal aponeurosis, and the linea alba, a fibrous band of connective tissue.
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Transversus abdominis
The transversus abdominis is a broad paired muscular sheet found on the lateral sides of the abdominal wall. It is the deepest layer of the lateral abdominal wall, lying beneath the internal and external abdominal oblique muscles. The transversus abdominis is one of five main abdominal muscles, which help to hold internal organs in place and support the body during movement.
The transversus abdominis has several origin points: the lateral one-third of the superior surface of the inguinal ligament and the associated iliac fascia; the anterior two-thirds of the inner lip of the iliac crest; the thoracolumbar fascia between the iliac crest and the 12th rib; and the internal aspects of the lower six ribs and their costal cartilages. From these origin points, the transversus abdominis fibres course horizontally over the lateral abdominal wall towards the midline, perpendicular to the linea alba.
The transversus abdominis serves to compress and retain the contents of the abdomen, providing thoracic and pelvic stability. It also assists in exhalation, contracting to force air out of the thorax. This muscle is vital for back and core health, helping to pull in what would otherwise be a protruding abdomen, earning it the nickname the "corset muscle".
Weakness in the transversus abdominis can lead to abdominal pain and an inability to hold in the stomach, particularly after a large meal or when gassy. Low back fatigue after prolonged standing or walking can also be a sign of a weak transversus abdominis. The drawing-in technique is an effective way to activate and strengthen this muscle.
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the whole length of the front of the abdomen. It is separated from its fellow on the opposite side by the linea alba, a connective tissue. The rectus abdominis is commonly referred to as the ""abs muscle"" or the "six-pack". It is visible beneath the skin in people with low body fat.
The rectus abdominis is part of the core muscles, which include the transversus abdominis, erector spinae, and obliques. These muscles act like a natural weight belt to protect the lower back from injury. The rectus abdominis specifically assists in compressing the contents of the abdomen. It also flexes the spinal column and tenses the anterior wall of the abdomen.
The muscle is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. These nerves pierce the anterior layer of the rectus sheath. The blood supply to the rectus abdominis comes predominantly from the inferior and superior epigastric arteries, with contributions from other vessels.
The rectus abdominis is paired and runs vertically on either side of the linea alba. It is attached by two tendons: the larger one connects to the pubic crest, from the pubic tubercle to the pectineal line, and the smaller, medial tendon attaches to the pubic symphysis. The fibres of the rectus abdominis extend vertically superiorly and insert into the xiphoid process of the sternum and the costal cartilages of the 5th to 7th ribs.
In terms of function, the rectus abdominis is an important postural muscle. It flexes the lumbar spine, as seen in a crunch. It also assists with breathing and plays a role in respiration during forceful exhalation, such as after exercise or in certain conditions.
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Back muscles
The back muscles are essential for maintaining posture, facilitating movement, and supporting the spine. They are organised into three main groups: superficial, intermediate, and deep muscles, each serving distinct functions. The superficial muscles, located closest to the skin surface, play a crucial role in movements such as lifting, bending, and rotating. They run from the vertebral column to the rib cage and assist in elevating and depressing the ribs. These muscles also aid in respiration.
The intermediate muscle groups, also known as extrinsic muscles, are considered immigrant muscles as they represent muscles of the upper limb that migrated to the back during fetal development. They are separated from the deep muscles by the thoracolumbar fascia.
The deep muscles of the back are well-developed and extend from the sacrum to the base of the skull. They are associated with the movements of the vertebral column and control posture. These muscles are covered by deep fascia, which is crucial for their organisation. The transversospinales, a deep group of back muscles, lie deep to the Erector Spinae and assist in bending the back when contracted bilaterally. When unilateral contraction occurs, they facilitate lateral bending and rotation.
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Frequently asked questions
The abdominal muscles, back muscles, and obliques are the primary muscles that enable torso compression. The abdominal muscles support the trunk, hold organs in place, and facilitate movement. The back muscles aid in respiration, posture, and repositioning the torso. The obliques, both internal and external, enable the torso to twist and turn.
The internal and external obliques are a pair of muscles that flank the rectus abdominis, with the internal obliques sitting on top of the external obliques. The internal obliques are much thinner and smaller than the external obliques.
The rectus abdominis is a muscle that acts as a flexor of the trunk and an accessory muscle during expiration. It is also responsible for compressing the abdominal contents to protect them.
To strengthen the torso compression muscles, you can perform exercises that target the abdominal muscles, such as plank, crunches, leg raises, and oblique twists. For the back muscles, exercises like the bird dog, supermans, and deadlifts can be beneficial. Additionally, core-strengthening exercises like Pilates and yoga can also enhance torso compression.
