Which statement is correct for muscle contraction?

Options

A. Length of H-zone decreases
B. Length of A-band decreases
C. Length of two Z-line increases
D. Length of I-band remains constant

Answer

To determine which statement is correct regarding muscle contraction, let's analyze the options provided:

1. **Length of H zone decreases**:
- During muscle contraction, the H zone, which is the area in the A band where there is no overlap of thick and thin filaments, does indeed decrease as the filaments slide past each other.

2. **Length of A band decreases**:
- The A band, which contains the thick filaments (myosin), remains the same length during contraction. Therefore, this statement is incorrect.

3. **Length of two Z lines increases**:
- The Z lines mark the boundaries of the sarcomere. During contraction, the Z lines move closer together, which means the distance between them decreases, making this statement incorrect.

4. **Length of I band remains constant**:
- The I band, which is the region of thin filaments (actin) that does not overlap with thick filaments, decreases in length during contraction. Thus, this statement is also incorrect.

After analyzing all the options, the only correct statement is:

- **Length of H zone decreases**.

### Step-by-Step Solution:
1. **Identify the components of a sarcomere**: Understand the structure of a sarcomere, which includes the A band, I band, H zone, and Z lines.
2. **Understand muscle contraction**: During contraction, the sliding filament theory explains how thick and thin filaments slide past each other.
3. **Analyze each statement**:
- A band: Remains constant.
- H zone: Decreases in length.
- I band: Decreases in length.
- Z lines: Move closer together, so their distance decreases.
4. **Determine the correct statement**: The only correct statement is that the length of the H zone decreases during muscle contraction.