External intercostalis: Elevates ribs during inspiration; depresses ribs during expiration.
The external intercostalis is a muscle located between the ribs and plays a crucial role in the mechanics of breathing. During inspiration, the external intercostalis contracts to elevate the ribs, expanding the thoracic cavity and allowing air to flow into the lungs. This action is essential for inhalation. Conversely, during expiration, the muscle relaxes, allowing the ribs to descend, which contributes to the compression of the thoracic cavity and facilitates the expulsion of air from the lungs.
The primary function of the external intercostalis is to assist in the respiratory process by dynamically adjusting the space within the thoracic cavity during breathing cycles. While the muscle is not directly involved in abdominal compression, waist flexion, or arm movements, its respiratory role is fundamental to the efficient functioning of the respiratory system.