The first source of energy that is used to reconstitute the ATP in a contracting skeletal muscle is

Options

A. Glycogen
B. Phosphocreatine
C. Lactic acid
D. Acetyl CoA

Answer

To answer the question, "The first source of energy that is used to reconstitute the ATP in a contracting skeletal muscle is," we need to understand the energy systems utilized by muscles during contraction.

### Step-by-Step Solution:

1. **Understanding Muscle Contraction**: When skeletal muscles contract, they require energy in the form of ATP (adenosine triphosphate). ATP is the immediate energy source for muscle contractions.

2. **Energy Sources for ATP Reconstitution**: The body has several systems to regenerate ATP:
- **Phosphagen System**: This system uses stored ATP and creatine phosphate (CP) for quick energy.
- **Anaerobic Glycolysis**: This system breaks down glucose without oxygen to produce ATP.
- **Aerobic Respiration**: This system uses oxygen to produce ATP from carbohydrates, fats, and proteins.

3. **Identifying the First Source**: The first source of energy that is used to reconstitute ATP during the initial stages of muscle contraction is the phosphagen system, specifically the breakdown of creatine phosphate. This process occurs rapidly and provides immediate energy for muscle contractions.

4. **Conclusion**: Therefore, the first source of energy used to reconstitute ATP in a contracting skeletal muscle is **creatine phosphate**.

### Final Answer:
The first source of energy that is used to reconstitute the ATP in a contracting skeletal muscle is **creatine phosphate**.