Your lungs are specialized organs designed for efficient absorption of oxygen and removal of carbon dioxide from your body. By constantly filtering blood through these tissues, the pulmonary system creates an interface between the circulation and the atmosphere. As carbon dioxide levels increase during strenuous exercise, additional muscles of respiration are called to action in order to avoid large changes in blood-gas concentration.

Your tissues are constantly metabolizing nutrients and producing carbon dioxide as a byproduct. Even as you sit reading this article, all the cells of your body are producing a baseline level of carbon dioxide; the amount produced in this state determines your normal respiratory rate. The diaphragm, the primary muscle of breathing, is the only one required for this level of respiration.

Running perpendicular to your chest ribs, the intercostal muscles serve to stiffen the chest wall during normal breathing so the lungs can effectively exchange air. However, during exercise these muscles take on a different role. By facilitating rib cage expansion, the intercostal muscles contract during strenuous exercise, allowing the lungs to accommodate more air with each breath. These adaptations in muscle function increase the efficiency of gas exchange and help take some of the work of breathing off the diaphragm.

Several muscles located in the neck may also contribute to the work of breathing during exercise. The sternocleidomastoid muscle, which runs from the back base of your skull down to your chest, may be called to action during periods of strenuous exercise. This muscle acts to pull your entire rib cage upward, facilitating lung expansion and gas exchange. Another muscle group in the neck, known as the scalene muscles, also elevates the rib cage; this action complements the sternocleidomastoid muscle in promoting lung expansion.

Strenuous breathing is a normal response during exercise and is important for maintaining normal blood gas levels. If you find yourself breathing heavily at rest, however, this may indicate an underlying medical condition that should be evaluated by a physician, according to MedlinePlus. Also known as hyperventilation, rapid or deep breathing at rest may be caused by anxiety, panic, or other medical conditions such as heart or lung disease.