Which muscles relax during normal expiration ?
A. Diaphragm
B. External intercostal muscles
C. Internal intercostal Muscles
D. Abdominal muscles

Options

A. A and B
B. C and D
C. A and C
D. No muscles contract during expiration

Answer

To determine which muscles relax during normal expiration, let's analyze the process of expiration step by step.

### Step-by-Step Solution:

1. **Understanding Expiration**: Normal expiration is primarily a passive process that occurs when the diaphragm and intercostal muscles relax. This leads to a decrease in the volume of the thoracic cavity.

2. **Diaphragm**: The diaphragm is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. During expiration, the diaphragm relaxes and moves upward, reducing the volume of the thoracic cavity.

3. **External Intercostal Muscles**: These muscles are located between the ribs and assist in inhalation by raising the rib cage. During expiration, these muscles also relax, allowing the rib cage to return to its resting position.

4. **Internal Intercostal Muscles**: These muscles are primarily involved in forced expiration. They contract during forced expiration to pull the ribs down and inward. However, they do not play a significant role in normal expiration, where they are not actively contracting.

5. **Abdominal Muscles**: These muscles can assist in forced expiration by pushing the diaphragm up, but during normal expiration, they do not necessarily contract.

6. **Conclusion**: During normal expiration, the diaphragm and external intercostal muscles relax. Therefore, the correct answer to the question is:
- A. Diaphragm
- B. External intercostal muscles

### Final Answer:
The muscles that relax during normal expiration are:
- A. Diaphragm
- B. External intercostal muscles