Accessory muscles of respiration are those not typically involved in the restful, quiet act of breathing. They are recruited only when the body’s demand for air increases significantly. These supplementary muscles step in to help expand or contract the chest cavity beyond the capacity of the primary breathing muscles. Their activation signals an increased respiratory effort, which can be due to a voluntary need for more air or a struggle to meet the body’s oxygen requirements.
Distinguishing Primary and Accessory Muscles
Normal, quiet breathing, often referred to as eupnea, relies almost entirely on the primary muscles of respiration. The primary muscle is the diaphragm, a dome-shaped sheet that separates the chest cavity from the abdomen. When the diaphragm contracts, it moves downward, increasing the vertical volume of the thoracic cavity and drawing air into the lungs.
The external intercostal muscles, located between the ribs, also assist in quiet inspiration by pulling the ribs upward and outward. Quiet exhalation is a passive process requiring almost no muscle contraction. Air is pushed out as the diaphragm relaxes and the elastic recoil of the lung tissue returns the chest to its resting size. Accessory muscles are only called upon when this resting mechanism is insufficient, such as during exercise or respiratory distress.
Accessory Muscles for Deep Inhalation
When a person takes a deep breath or experiences difficulty inhaling, the body recruits several neck and chest muscles to lift the rib cage. The sternocleidomastoid muscles, which run from the breastbone and collarbone up to the skull, become active inspiratory accessories. Their contraction elevates the entire rib cage structure, significantly increasing the volume inside the chest.
The scalene muscles, located on the sides of the neck, are important for deep breathing. They attach to the first two ribs and, upon contraction, lift them upward. This action further expands the upper part of the thoracic cage to draw in a greater volume of air. The pectoralis minor, a smaller muscle in the upper chest, can also assist by raising the third, fourth, and fifth ribs, particularly when the shoulder blades are fixed.
Accessory Muscles for Forced Exhalation
Unlike quiet breathing, which is passive, forced exhalation is an active process that requires powerful muscle contraction to rapidly decrease lung volume. The primary accessory muscles for this action are the abdominal muscles, including the rectus abdominis, the internal and external obliques, and the transversus abdominis. These muscles wrap around the abdomen and contract forcefully to compress the abdominal contents.
This compression increases the pressure inside the abdomen, which pushes the relaxed diaphragm high up into the chest cavity. The increased pressure and upward displacement of the diaphragm rapidly and forcefully expels air from the lungs. Additionally, the internal intercostal muscles, located deep to the external intercostals, are recruited to pull the ribs downward and inward. This action actively depresses the rib cage, supplementing the work of the abdominal muscles.
Context of Accessory Muscle Use
The use of accessory muscles is a clear physical sign that the body is attempting to overcome an increased respiratory load. A person might voluntarily engage these muscles during vigorous physical activity, such as sprinting, when the demand for oxygen outstrips the capacity of the diaphragm alone. They are also used for actions requiring a forceful burst of air, like coughing, sneezing, or playing a wind instrument.
The visible use of these muscles at rest is a significant clinical indicator of labored breathing or respiratory distress. Conditions like an asthma flare-up, chronic obstructive pulmonary disease (COPD) exacerbation, or pneumonia force the body to use these supplementary muscles constantly to maintain adequate airflow. Observing the sternocleidomastoid muscles visibly bulging in the neck, or seeing the skin between the ribs or below the rib cage pull inward with each breath (known as retractions), serves as a visual cue that the body is struggling to meet its basic oxygen needs.