ABSTRACT
The diaphragm and accessory (external and internal intercostals, abdominal, scalenes, and sternomastoid) muscles of respiration are embryologically, morphologically, and functionally skeletal muscles. These respiratory muscles are the only skeletal muscles that contract with a regular rhythm throughout the life span. During walking or running, skeletal muscles of the arms and legs contract rhythmically as the muscles of respiration do during breathing, but when the walking or running ceases, breathing continues. In this way, continuous rhythmic activity of the respiratory muscles is analogous to the activity of the heart.
