Inspiratory muscles

Your Voice: An Inside View by Scott McCoy, DMA

Inspiratory muscles (primary)
We now know that the thorax must be made larger to induce inhalation. We also know that muscles are capable of only one motion: contraction. How is it, then, that something is made larger through contracting—isn’t this a paradoxical contradiction?
The most important muscle of inhalation is the diaphragm. This is the second largest muscle in the human body—in most people, only the gluteus maximus muscles are larger. Shaped like a dome or parachute with two small humps, the diaphragm bisects the body, separating the contents of the thorax from the abdomen. In this location, it serves as the floor to the thorax. Once again with the help of the pleurae, motion of the diaphragm is transferred directly to the lungs. On contraction, the diaphragm lowers and becomes somewhat flatter. It therefore increases the volume capacity of the thorax (and therefore, lungs) much like withdrawing the plunger of a syringe.

Many people have misconceptions about the location and size of the diaphragm. Since it lies deep within the abdominal/thoracic cavity and cannot be directly felt, it is often believed to be smaller in diameter and lower in placement than is correct. Often these misconceptions arise from the best intentions of voice teachers or choir directors who teach breathing by placing a hand on the tummy and telling the student to “breathe from the diaphragm.” The student naturally assumes the outward movement of the abdominal wall is the actual diaphragm. Even well educated singers often believe the diaphragm lies lower in the torso than it actually does. CD Example 8/7 presents a view of an entire torso; the diaphragm can be seen to reside at a position about one-third the distance from the clavicles to the pubis (Figure 8-5).
The diaphragm attaches in the front to the sternum, at the sides to the costal cartilages and ribs seven through twelve, and at the back to the upper lumbar vertebrae through the pillars of the diaphragm. The pillars (or crura) are two, long and thick muscle bands that run vertically from the spine to the posterior of the diaphragm. Muscle fibers in the diaphragm originate from all these attachment points and insert into the central tendon, a strong, fibrous portion of the muscle that is shaped some- what like a boomerang (Figure 8-6).

Since this muscle completely bisects the body, openings must be provided for the passage of blood and food. Fresh blood is carried to the lower body by the aorta, the largest artery in the body. The aorta passes through the diaphragm at the aortic hiatus, which is located in the center, posterior of the muscle, quite close to the spinal column. Diaphragmatic movement during respiration is minimal at this location. Oxygen-depleted blood returns from the lower body to the lungs and heart through the vena cava. This passes through the diaphragm at the foramen vena cava, which is located in the left-center portion of the central tendon (in superior aspect). The esophagus, which transports food to the stomach, passes through the diaphragm at the esophageal hiatus. It is not uncommon for people to experience problems with the esophageal hiatus. Many will develop a condition known as a hiatal hernia, in which the esophagus or upper stomach painfully bulges through the hiatus to the wrong side of the diaphragm. Severe cases of hiatal hernia can be successfully treated surgically.

While the diaphragm is one of only two unpaired muscles in the human body, it does possess some aspects of duality. For example, it is provided with two separate blood supplies. Signals from the brain are sent to it through the two phrenic nerves, which are branches of the vagus nerve (10th cranial nerve). The nerves that innervate the larynx are also branches of the vagus.

As previously stated, on contraction, the diaphragm lowers and becomes somewhat flatter. The range of motion is about one-and-a-half centimeters during quiet breathing to as much as six or seven centimeters (a little less than three inches) during deep breathing. In an average-sized adult, about 350 cubic centimeters of air will be inhaled for every centimeter the diaphragm lowers (Zemlin, 1998). Therefore, during deep breathing, diaphragmatic movement alone should draw nearly two-and-a-half liters of air into the lungs (7cm multiplied by 350cc).

Strictly speaking, the diaphragm is a voluntary muscle. It is not, however, a muscle over which most people have direct, conscious control. Singers learn to control its movement through sensations in the abdomen and ribcage. When it contracts, the diaphragm lowers into the abdomen, pressing down against its contents, which are called the abdominal viscera. Unlike the spongy, elastic lungs, the viscera are mostly incompressible—like water in a water balloon or hot water bottle. When something presses against it, it cannot be squished into a smaller space but must be displaced. The result of this displacement is the bulging abdomen (or back) often seen during inhalation in trained singers. We are not directly aware of the action or location of the diaphragm during this process; we are only aware of what it has done to something else.
Biologically, the diaphragm is exclusively a muscle of inspiration. In the vast majority of people, it is entirely passive during exhalation. Its normal action is to contract for inhalation and relax during exhalation, quickly returning to its resting position. The return to resting position occurs without force and does not significantly contribute to expiratory effort. Some singers, however, seek to maintain diaphragmatic contraction during exhalation to use as a muscular antagonist in breath support. Fluoroscopic (video x-ray) studies by Richard Miller and others have shown that while some people can indeed control their diaphragms in this manner, many cannot. Regardless, it is pedagogically incorrect (and physiologically impossible) to say that the diaphragm pushes air out of the body during singing.
The diaphragm, as important as it is, is not the only significant inspiratory muscle. It is strongly assisted by a group of muscles called the external intercostal muscles. They are called external muscles not because they are on the exterior of the thorax, but because they are the outermost layer of muscles on the inside the thorax. Each external intercostal originates from the rib above and inserts into the rib below. On contraction, the lower rib is drawn upward and outward in a swinging motion. As a result, the entire thorax is lifted and expanded somewhat in diameter.

The external intercostals run in an oblique direction down toward the midline of the body. You can approximate their direction of travel by placing your right hand on the right side of your chest with your fingers pointing toward your navel. Your fingers will now be at a diagonal angle to your body in about the same orientation as the external intercostals.

As can be seen, the external intercostals cover a substantial surface area within the thorax. Viewing the thorax in horizontal cross section with the spine in the 12:00 position (as on the face of a clock), external intercostals extend throughout the regions between 12:00 and 4:00 on the right side and 8:00 through 12:00 on the left. The external intercostals, and indeed almost all the remaining respiratory muscles, are strongly voluntary. Most people are quickly able to learn their direct control independent of the breathing process simply by deliberately expanding the circumference of the rib cage.

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