The muscle fatigue occurs due to the accumulation of:

Options

A. lactic acid
B. glycogen
C. creatine phosphate
D. carbon dioxide

Explanation

**Step-by-Step Solution:**

1. **Understanding Muscle Fatigue**: Muscle fatigue refers to the decline in the ability of a muscle to generate force. It can occur during prolonged exercise or exertion.

2. **Identifying Conditions Leading to Fatigue**: Muscle fatigue is often associated with anaerobic conditions, which occur when there is insufficient oxygen available for aerobic respiration.

3. **Role of Lactic Acid**: Under anaerobic conditions, muscles switch to anaerobic respiration, which leads to the production of lactic acid as a byproduct.

4. **Accumulation of Lactic Acid**: As exercise continues and oxygen levels remain low, lactic acid accumulates in the muscle tissues. This accumulation can lead to an increase in acidity in the muscle cells.

5. **Impact of Acidity on Muscle Function**: The increased acidity (lower pH) in the muscle cells can inhibit enzyme activity, which is crucial for muscle contraction. When enzymes are not functioning optimally, muscle contractions become less effective, leading to fatigue.

6. **Conclusion**: Therefore, the primary cause of muscle fatigue during anaerobic conditions is the accumulation of lactic acid.

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

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