The largest unit of a skeletal muscle is known as the muscle belly. The muscle belly is composed of many units called fascicles which comprise of bundles of muscle fibers (also known as muscle cells). Muscle fibers are filled with myofibrils - filaments containing contractile proteins. These contractile proteins are arranged in repeating units known as sarcomeres, which give skeletal and cardiac muscle tissues their striated appearance.

The cross-section of muscle consists of many tiny red tubes, called myofibrils, parallelly aligned inside a bigger red tube, called muscle fiber, that has blue spheres adhered to its outer surface. Those muscle fiber tubes are also aligned inside a bigger bundle called muscle fascicle, or cell bundle, with similar blue spheres on its outer surface. Those cell bundles are, again, aligned together into the final muscle. Inside the muscle, in between those bundles of cells, arteries, veins, and nerves are placed along. The close view of the myofibril represents a cylindrical structure that consists of alternately arranged Z line and M line. Two Z lines and one M line in the middle create a sarcomere - a basic contractile unit of muscle fiber.

Figure 1: The architecture of a skeletal muscle.

The architecture of the muscle influences skeletal muscle activity. Skeletal muscle tissue contracts rapidly and with great force, but it also tires very easily and must rest after short periods of activity. Cardiac muscle tissue, on the other hand, contracts constantly without our conscious influence.