The smallest functional unit of a muscle fiber responsible for contraction is the sarcomere. Sarcomeres are repeating contractile segments arranged end to end inside myofibrils.
Each sarcomere extends from one Z-line, or Z-disc, to the next. It contains thin actin filaments and thick myosin filaments. During contraction, myosin heads bind to actin and pull the thin filaments inward, causing the Z-lines to move closer together.
This is the sliding-filament mechanism. The filaments themselves do not simply shrink; instead, their overlap changes. Calcium released from the sarcoplasmic reticulum binds to troponin, shifting tropomyosin and exposing binding sites on actin. ATP then powers repeated cross-bridge cycles between myosin and actin.
The answer is sometimes confused with myofibril, myofilament, or Z-line. A myofibril is a larger rod-like structure made of many sarcomeres. Myofilaments are the protein filaments within a sarcomere, while the Z-line forms its boundary. Sarcomeres give skeletal and cardiac muscle their characteristic striated appearance; smooth muscle is not organized into repeating sarcomeres.