Which of the following muscles contracts during normal expiration ?

Options

A. Internal intercostal muscles
B. Diaphragm
C. Abdominal muscles
D. None of these

Answer

To solve the question "Which of the following muscles contracts during normal expiration?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Normal Expiration**:
- Normal expiration is the process of releasing carbon dioxide from the lungs to the outside environment. It occurs passively under normal conditions.

2. **Muscle Activity During Expiration**:
- During normal expiration, the primary muscles involved are the internal intercostal muscles and the abdominal muscles. However, it is important to note that during normal expiration, the diaphragm relaxes.

3. **Role of Internal Intercostal Muscles**:
- The internal intercostal muscles assist in forced expiration by contracting to pull the ribs downwards and inwards, but they do not play a significant role in normal expiration, which is primarily a passive process.

4. **Role of Abdominal Muscles**:
- The abdominal muscles can contract during forced expiration, but they are not actively contracting during normal expiration.

5. **Conclusion**:
- Since the diaphragm relaxes and the internal intercostal muscles and abdominal muscles do not contract during normal expiration, the correct answer to the question is that none of the listed muscles contract during normal expiration.

### Final Answer:
- The correct answer is **None of these** (Option 4).