Which muscle contracts during normal expiration?

Options

A. Diaphragm
B. EICM
C. IICM
D. None of these

Answer

**Step-by-Step Solution:**

1. **Understanding Normal Expiration**: Normal expiration is the process of breathing out, where air is expelled from the lungs. This occurs when the pressure inside the lungs becomes greater than the pressure outside.

2. **Role of Lung Volume**: For air to be expelled, the volume of the lung cavity must decrease. This decrease in volume leads to an increase in pressure inside the lungs, facilitating the movement of air outwards.

3. **Muscle Involvement**: During normal expiration, certain muscles are involved in the contraction process that helps reduce the lung volume.

4. **Identifying the Muscles**: The primary muscles that contract during normal expiration are the internal intercostal muscles. These muscles are located between the ribs and assist in pulling the rib cage downwards and inwards, thereby decreasing the volume of the thoracic cavity.

5. **Distinguishing Other Muscles**:
- The diaphragm primarily contracts during inspiration, especially during forced breathing, to increase lung volume.
- The external intercostal muscles also assist during inspiration by expanding the rib cage and increasing lung volume.

6. **Conclusion**: Therefore, the correct answer to the question "Which muscle contracts during normal expiration?" is the internal intercostal muscles.

**Final Answer**: Internal intercostal muscles.

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