During fatigue

Options

A. muscle fails to relax
B. muscle fails to be stimulated
C. blood circulation in muscles stops
D. motor nerve does not respond to external stimulus

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Fatigue**: Muscle fatigue refers to the condition where muscles become tired and are unable to perform effectively. This can occur after prolonged physical activity.

2. **Energy Source for Muscles**: Muscles primarily derive energy from glycogen, which is stored in muscle cells. During activity, glycogen is broken down into glucose.

3. **Glycolysis Process**: The glucose undergoes glycolysis, a metabolic pathway that converts glucose into pyruvic acid, producing energy in the form of ATP.

4. **Oxygen Deficiency**: During intense or prolonged exercise, the demand for oxygen may exceed the supply, leading to a state of oxygen deficiency in the muscles.

5. **Anaerobic Fermentation**: In the absence of sufficient oxygen, pyruvic acid is converted into lactic acid through a process called anaerobic fermentation. This process allows for continued ATP production, albeit less efficiently.

6. **Lactic Acid Accumulation**: The accumulation of lactic acid in the muscle cells is a significant factor contributing to muscle fatigue. High levels of lactic acid can lead to discomfort and pain in the muscles.

7. **Nerve and Muscle Interaction**: Muscle fatigue is also characterized by a reduction in the force of contraction after prolonged stimulation. This occurs at the neuromuscular junction, where the nerve communicates with the muscle.

8. **Evaluating Options**:
- Option 1: "Muscles fail to relax" - This is incorrect because muscles can relax, but their ability to contract effectively is diminished.
- Option 2: "Muscles fail to be stimulated" - This is the correct answer as the presence of lactic acid reduces the responsiveness of the muscle to stimulation.
- Option 3: "Blood circulation in the muscle stops" - This is incorrect; blood circulation actually increases during exercise.
- Option 4: "Motor nerve does not respond to external stimulus" - This is also incorrect; the nerve can respond, but the muscle may not contract effectively.

9. **Conclusion**: The correct answer is that during fatigue, "muscles fail to be stimulated" due to the accumulation of lactic acid and reduced responsiveness.

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