It is the functional unit of a muscle fiber.

The answer to this question is a bit more complex, because there is not one single name for the three types of muscle fibers in the human body. It is important thus to remember that the human system has three types: skeletal muscle, or voluntary muscle, smooth muscle, and cardiac muscle fibers.

In the case of skeletal muscle, the functional unit of myofibrils, the ones responsible for movement, is called a sarcomere, and they are striated. These sarcomeres combine in units of myofibrils that when stimulated by motor neurons, produce movement.

The second case is smooth muscle. In this case, we have single-unit cells, elongated, to form the linings of organs such as the bladder, and their movement is not voluntary. This system's unit, then, is called a myocytes.

The third type of muscle, which is cardiac muscle, is the one responsible for the movements of organs such as the heart, and their unit is better known as cardiomyocyte.