During skeletal muscle contraction, actin and myosin filaments unite and slide toward each other, causing shortening.

• A. True
• B. False
• C. Sometimes true
• D. Not related

Answer: (A)

This describes the sliding filament mechanism: during skeletal muscle contraction, cross-bridges form between myosin heads and actin filaments and these bridges pull the thin filaments inward, shortening the sarcomere. Calcium released inside the muscle fiber exposes binding sites on actin, allowing myosin to attach. The myosin heads pivot in a power stroke, pulling actin toward the center and causing the filaments to slide past each other. This repeated cycle, fueled by ATP, shortens the sarcomere even though the thick and thin filaments don’t fuse together permanently. The net effect is contraction, so the statement is true.