What is the contractile unit of skeletal muscle?

• A. Myofibril
• B. Actin
• C. Sarcomere
• D. Myosin

Answer: (C)

The contractile unit of skeletal muscle is the sarcomere. It is the repeating segment between two Z-discs inside a myofibril, and it is the part that shortens during contraction. Within a sarcomere, the thick myosin filaments and thin actin filaments interact; when calcium enables cross-bridge cycling, the myosin heads pull on actin, sliding the filaments past one another. This sliding causes the Z-discs to come closer together, shortening the sarcomere and producing a contraction. The overall shortening of many sarcomeres along a muscle fiber generates the full muscle contraction. The other options are components or larger structures: myofibrils are bundles of many sarcomeres, while actin and myosin are the filamentous proteins themselves, not the contractile unit.