The principal muscle of inspiration is the diaphragm, a domed sheet of muscle that separates the thoracic and abdominal cavities. The diaphragm attaches to the lower ribs, as well as to the lumbar vertebrae of the spine. When the diaphragm contracts, the dome flattens, moving downward into the abdominal cavity like a piston (think of a syringe barrel). This movement increases the volume of the thoracic cavity, creating a negative pressure that is proportional to the extent of its movement, and thus, to the force of contraction. Diaphragm contraction also induces the lower ribs to move upward and forward, which also increases thoracic volume. The ribs move outward because the central tendon of the diaphragm (at the crown of the dome) pushes down onto the liver and stomach, which act like a fulcrum. This has the effect of raising the edges of the diaphragm, which are connected to the rib margins, forcing them upward and outward. When the diaphragm moves downward into the abdominal compartment, it also raises intraabdominal pressure and assists the abdominal muscles in stabilizing the spine.
The muscles of the rib cage are known as the intercostal muscles because they are located in the space between adjacent ribs. Each space contains a layer of inspiratory and a layer of expiratory muscle fibers. The inspiratory intercostal muscles form the outer layer, and they slope downward and forward; contraction causes the ribs to move upward and outward, similar to the raising of a bucket handle. Contraction of these muscles also serves to stabilize the rib cage, making it more rigid, as well as bringing about twisting movements. The stiffening of the rib cage enables it to oppose the tendency to collapse slightly under the influence of the negative pressure generated by the movement of the diaphragm. Without this action, the rib cage would distort, and the action of the diaphragm would be less mechanically efficient, thus wasting energy. Intercostal muscle contraction also brings about stiffening of the rib cage during lifting, pushing, and pulling movements, which makes the intercostal muscles an important contributor to these movements.
Some muscles in the neck region also have an inspiratory action. The scalene and sternocleidomastoid muscles (also known as sternomastoid) are attached to the top of the sternum, upper two ribs, and clavicle at one end; at the other end, they are attached to the cervical vertebrae and mastoid process. When these muscles contract, they lift the top of the chest, but the scalene muscles are also involved in flexion of the neck.
The principal muscles of expiration are those that form the muscular corset of the abdominal wall. The most well known and visible of these (at least in male models!) is the rectus abdominis (or “six pack”); the other three muscles are less visible but arguably more functionally important to sports—the transversus abdominis and the internal and external oblique muscles. When these muscles contract, they pull the lower rib margins downward, and they compress the abdominal compartment, raising its internal pressure. The pressure increase tends to push the diaphragm upward into the thoracic cavity, inducing an increase in pressure and expiration. However, these muscles only come into play as breathing muscles during exercise or during forced breathing maneuvers; resting exhalation is a passive process brought about by the recoil of the lungs and rib cage at the end of inspiration (due to stored elastic energy).
The four abdominal muscles involved in breathing also have important functions as postural muscles, in rotating and flexing the trunk, and when coughing, speaking (or singing), and playing wind instruments. The compression and stiffening of the abdominal wall generated by contraction of the abdominal muscles also optimize the position of the diaphragm at the onset of inspiration. This also enhances spinal stability and postural control.
The rib cage also contains muscles with an expiratory action. These are the internal intercostal muscles, which slope backward; contraction causes the ribs to move downward and inward, similar to the lowering of a bucket handle. Both internal and external intercostal muscles are also involved in flexing and twisting the trunk.
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