Final answer:
During a muscle contraction, the sarcomere is the structure that shortens. The sarcomere is a component of myofibrils within the muscle fiber, which shortens as myosin pulls on actin filaments.
Step-by-step explanation:
The structure that shortens during a muscle contraction is the sarcomere. Muscle contraction is the process whereby muscle fibers become shorter, this is crucial to understand when considering the functioning of muscle fibers. Each muscle fiber contains organelles known as myofibrils, which in turn are composed of actin and myosin protein filaments.
The region between two Z lines in a myofibril is called a sarcomere, and it is this part of the muscle fiber that shortens during contraction. When a muscle contracts, myosin filaments within the sarcomere pull on the actin filaments, resulting in the sliding filament model of muscle contraction. This action brings Z lines closer together, shortening the sarcomere, but the A band length remains constant while the overlap of thick and thin filaments increases.