Which statement best describes the sliding filament theory?

• A. During contraction, actin slides past myosin, shortening the sarcomere; filaments do not shorten.
• B. During contraction, thick and thin filaments shorten; A band length shortens.
• C. During contraction, actin and myosin lengthen and sarcomere expands.
• D. During contraction, only actin shortens; myosin remains static.

Answer: (A)

At the heart of how muscles contract is the idea that actin filaments slide past myosin filaments, pulling the Z discs closer together and shortening the sarcomere. The filaments themselves don’t shorten; instead, their overlap increases as cross-bridges form and pull actin inward. Because the filaments don’t change length, the A band stays the same, while the I band and the H zone shorten as more actin overlaps with myosin. So this description—actin sliding past myosin to shorten the sarcomere without shortening the filaments themselves—best captures the sliding filament mechanism.