Accessory muscles in the neck
and chest are the last group of inspiratory
muscles, used only for
deep and heavy breathing, such as
during intense exercise or respiratory
failure. During expiration, the diaphragm
relaxes, decreasing thoracic
cavity size and causing thelungs to deflate. With normal
breathing, expiration is purely passive.
But with exercise or forced
expiration, expiratory muscles (including
the abdominal wall and internal
intercostal muscles) become
active. These important muscles are
necessary for coughing.