During contraction, thin filaments slide past thick filaments causing actin and myosin to overlap more; cross bridges form and break several times, ratcheting thin filaments toward the center of the sarcomere. Which model describes this process?

• A. Sliding filament model of contraction
• B. Cross-bridge cycling model
• C. Z-line shortening model
• D. Titin-spring model

Answer: (A)

The sliding filament model describes muscle shortening by actin filaments sliding past myosin filaments as cross-bridges repeatedly form and break. In contraction, myosin heads attach to actin, pull (power stroke), detach when a new ATP binds, and reattach further along. This repeated cycling ratchets the thin filaments toward the center of the sarcomere, increasing overlap and shortening the muscle fiber. The description you gave—filaments sliding, multiple cross-bridge events, and ratcheting inward—fits this model perfectly because it links the microscopic cross-bridge cycling to the macroscopic shortening of the sarcomere. The other ideas are less accurate: focusing on Z-line shortening would misstate how contraction occurs, and the titin-spring concept emphasizes passive elasticity rather than active shortening.