Which of the following occurs during muscle contraction?

Options

A. A) Reduction of ATP liberates oxygen
B. B) Z lines move closer together
C. C) Filaments of myosin slide past filaments of actin
D. D) Muscle fibres lengthen

Explanation

To answer the question "Which of the following occurs during muscle contraction?", we will analyze the options provided and determine the correct one based on our understanding of muscle physiology.

### Step-by-Step Solution:

1. **Understanding Muscle Contraction**: Muscle contraction occurs when myofibrils within muscle fibers shorten, leading to the overall shortening of the muscle. This process is primarily driven by the interaction between two types of filaments: actin (thin filaments) and myosin (thick filaments).

2. **Analyzing the Options**:
- **Option A**: "Reduction of ATP liberates oxygen" - This statement is incorrect. ATP is used for muscle contraction, but it does not liberate oxygen.
- **Option B**: "Z-lines move closer together" - This statement is correct. During muscle contraction, the Z-lines (which mark the boundaries of the sarcomere) move closer together as the actin and myosin filaments slide past each other.
- **Option C**: "Filaments of myosin slide past filaments of actin" - This statement is also correct. The sliding filament theory explains that during contraction, myosin heads attach to actin and pull, causing the filaments to slide past one another.
- **Option D**: "Muscle fibers lengthen" - This statement is incorrect. During contraction, muscle fibers shorten, not lengthen.

3. **Conclusion**: Based on the analysis, the best answer to the question is **Option B: Z-lines move closer together**. This is a direct observation of what occurs during muscle contraction.

### Final Answer:
**B: Z-lines move closer together**

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