Abdominal muscles are the most important muscles of forced expiration. During the contraction of these muscles, the pressure of the intra-abdominal region is raised, and the diaphragm is pushed upward. During coughing, vomiting, and defecation, these muscles also contract forcefully. By pulling the ribs downward and inward, the internal intercostal muscles assist active expiration. This decreases thoracic volume.