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The primary muscles responsible for forced expiration are the internal intercostals and the abdominal muscles. During this process, the internal intercostals help to lower the rib cage, expelling air from the lungs more forcefully than during normal expiration. They pull the ribs closer together, which decreases the volume of the thoracic cavity and increases the pressure within the thorax, pushing air out of the lungs.

Simultaneously, the abdominal muscles play a significant role in forced expiration. They contract, pushing the diaphragm upward and compressing the abdominal contents, further assisting in the expulsion of air from the lungs. This coordinated effort is particularly evident during vigorous activities such as exercise, coughing, or heavy lifting.

While other muscle groups mentioned may contribute to respiration in various contexts, they do not primarily facilitate forced expiration to the extent that the internal intercostals and abdominal muscles do. For instance, the diaphragm is crucial for inspiration rather than expiration, and the external intercostals mainly assist with inhalation.