Final answer:
The external oblique muscle is associated with forced expiration, aiding in forcibly expelling air from the lungs during activities that require active breathing effort.
Step-by-step explanation:
The external oblique is a muscle involved in forced expiration. It is the most superficial of the three flat skeletal muscles located in the antero-lateral wall of the abdomen, positioned such that they extend inferiorly and medially, mirroring the direction of sliding your four fingers into pants pockets. This muscle plays a pivotal role in forced breathing, particularly during activities such as exercising or singing, which require the active manipulation of breathing.
During forced expiration, the external oblique contracts along with other accessory muscles, forcing abdominal organs upward against the diaphragm. This action pushes the diaphragm further into the thorax, thereby expelling more air out of the lungs. Unlike during normal breathing, which involves primarily the diaphragm and the external intercostal muscles, forced breathing engages additional muscles to manipulate the volume and pressure within the thoracic cavity to a greater degree.