State the basic principle of the sliding filament theory.

• A. Actin and myosin filaments slide past each other during contraction, shortening the sarcomere
• B. Filaments shorten during contraction without sliding
• C. Z-lines move apart during contraction
• D. Myosin length increases during contraction

Answer: (A)

Contraction is driven by the sliding filament mechanism: the actin (thin) and myosin (thick) filaments slide past each other, causing the sarcomere to shorten. Myosin heads form cross-bridges with actin and pull, using energy from ATP, which pulls the filaments inward without changing their lengths. As this happens, the Z-lines move closer together and the I-band and H-zone shrink, while the A-band stays the same length because it reflects the length of the myosin filaments. This sliding action shortens the sarcomere and generates force.

The other ideas don’t fit the observed mechanism. Filaments do not shorten themselves during contraction, so shortening of filaments wouldn’t produce the same sarcomere shortening. Z-lines moving apart would lengthen the sarcomere, not shorten it. Myosin filaments do not lengthen during contraction, so that option doesn’t describe the process.