Which muscle is primarily responsible for lowering the ribs during expiration?

The internal intercostals play a crucial role in the mechanics of breathing, particularly during the process of expiration. During expiration, the internal intercostals contract to pull the ribs downward and inward, decreasing the volume of the thoracic cavity. This action helps to force air out of the lungs, allowing for effective removal of carbon dioxide and other gases.

In contrast, the diaphragm primarily aids in inhalation by flattening and increasing the volume of the thoracic cavity, while the external intercostals assist with inhalation by elevating the ribs. The rectus abdominis, although it contributes to forced expiration and plays a part in compressing the abdominal contents, does not directly pull the ribs down. Therefore, the internal intercostals are the key muscles specifically responsible for lowering the ribs during expiration.