According to sliding filament theory of muscle contraction, the filament that move to shorten a muscle are

Options

A. Myosin
B. Actin
C. Collagen
D. Creatine phosphate

Answer

**Step-by-Step Solution:**

1. **Understanding the Sliding Filament Theory**: The sliding filament theory explains how muscles contract. It states that during contraction, the thick filaments (myosin) slide past the thin filaments (actin) without changing their lengths.

2. **Identifying the Filaments**: In muscle fibers, there are two main types of filaments involved in contraction:
- Thick filaments: Myosin
- Thin filaments: Actin

3. **Mechanism of Contraction**: When a muscle contracts, the sarcomere (the basic unit of a muscle) shortens. However, the lengths of the myosin and actin filaments do not change. Instead, the myosin filaments pull the actin filaments closer together.

4. **Movement of Filaments**: During contraction, it is the actin filaments that move. The myosin filaments remain stationary and act as the pulling force.

5. **Conclusion**: Therefore, according to the sliding filament theory of muscle contraction, the filament that moves to shorten a muscle is the actin filament.

**Final Answer**: The filament that moves to shorten a muscle according to the sliding filament theory is **actin**.

---