In a fully contracted muscle, there is no ___zone and instead a thicker line called__appears.

Answer

To solve the question, we need to understand the structure of a sarcomere and the changes that occur during muscle contraction.

### Step-by-Step Solution:

1. **Understand the Sarcomere Structure**: A sarcomere is the basic unit of muscle contraction and consists of thick and thin filaments. The thick filaments are made of myosin, and the thin filaments are made of actin.

2. **Identify the Zones in a Sarcomere**: In a relaxed muscle, the sarcomere has several distinct zones:
- **I Band**: This is the region containing only thin filaments (actin).
- **A Band**: This is the region containing both thick (myosin) and thin filaments.
- **H Zone**: This is the area within the A band where there are only thick filaments (myosin) and no thin filaments (actin).
- **Z Line**: This is the boundary between adjacent sarcomeres.

3. **Understand Muscle Contraction**: During muscle contraction, the thin filaments slide over the thick filaments, which brings the Z lines closer together. This process is known as the sliding filament theory.

4. **Identify Changes During Full Contraction**: When a muscle is fully contracted:
- The I band decreases in size as the thin filaments slide over the thick filaments.
- The H zone disappears completely because the thin filaments overlap the thick filaments entirely.
- The A band remains the same length but appears thicker due to the overlapping filaments.

5. **Fill in the Blanks**: Based on the understanding of the sarcomere during contraction:
- In a fully contracted muscle, there is no **H zone** and instead a thicker line called **A band** appears.

### Final Answer:
In a fully contracted muscle, there is no **H zone** and instead a thicker line called **A band** appears.

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