What are the repeating segments found along myofibrils called?

The correct choice is "sarcomeres" because these are the fundamental repeating units found within myofibrils, which are the contracting elements of muscle cells. Sarcomeres are composed of overlapping thick and thin filaments, primarily made up of the proteins myosin and actin, respectively. This structure is essential for muscle contraction, as the actin and myosin filaments slide past each other during the contraction process, enabling the muscle to shorten and generate force.

Each sarcomere is bounded by Z discs, which serve as anchors for the thin filaments, and the arrangement of these sarcomeres along the length of a myofibril leads to the characteristic striated appearance of skeletal muscle. Understanding sarcomeres is crucial for grasping the mechanics of muscle function and the overall physiology of muscular contraction.

In contrast, myocytes refer to muscle cells in general, fascicles denote bundles of muscle fibers, and muscle fibers typically refer to the individual muscle cells themselves. Each of these terms describes a different aspect of muscle structure but does not specifically denote the repeating contractile units that are essential to muscle functioning.