Question

which statement best describes the step in muscle contraction when the sarcomere is the shortest?

• the sarcomere is contracted, and the actin and myosin filaments are partially overlapped.
• the sarcomere is relaxed, and the actin and myosin filaments are partially overlapped.
• the sarcomere is contracted, and the actin and myosin filaments are completely overlapped.
• the sarcomere is relaxed, and the actin and myosin filaments are completely overlapped.

Explanation:

To determine the correct statement, we analyze muscle contraction and sarcomere structure:

1. Sarcomere State: A sarcomere is shortest during contraction (not relaxation). Relaxed sarcomeres are longer as actin - myosin overlap is less. So options with "relaxed" (second and fourth) are eliminated.
2. Actin - Myosin Overlap: In a fully contracted sarcomere (shortest length), the actin and myosin filaments are completely overlapped. Partial overlap occurs in less - contracted or relaxed states. So the first option (partial overlap in contraction) is incorrect, and the third option (contracted with complete overlap) is correct.

Answer:

The sarcomere is contracted, and the actin and myosin filaments are completely overlapped.