The muscle fatigue is due to :

Options

A. `Na^+`
B. `K^+`
C. Lactic acid
D. Citric acid

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Fatigue**: Muscle fatigue is defined as a decrease in the power production of muscles in response to contractile activity. This means that when muscles are overworked or used for an extended period, they lose their ability to generate force effectively.

2. **Analyzing the Options**:
- **Option 1: Sodium**: Sodium plays a crucial role in muscle contraction by facilitating the influx of calcium ions, which are essential for muscle contraction. Therefore, sodium is not responsible for muscle fatigue.
- **Option 2: Potassium**: Potassium helps maintain the electrochemical balance across the cell membrane. While it is important for muscle function, it does not directly cause muscle fatigue.
- **Option 3: Lactic Acid**: This is the correct answer. Muscle fatigue is primarily caused by the accumulation of lactic acid in the muscles during intense exercise. When muscles are used anaerobically (without enough oxygen), lactic acid builds up, leading to fatigue.
- **Option 4: Citric Acid**: Citric acid is involved in energy metabolism and does not contribute to muscle fatigue. It is not a byproduct of muscle activity that would lead to fatigue.

3. **Conclusion**: Based on the analysis of the options, the correct answer is that muscle fatigue is due to the accumulation of lactic acid in the muscles.

**Final Answer**: Muscle fatigue is due to the accumulation of lactic acid.

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