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Why do muscles that produce large forces have motor units with many muscle fibers?
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muscle
The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.
Muscles that generate larger forces utilize motor units containing many muscle fibers to increase the overall strength of a contraction. A motor unit consists of a single motor neuron and the specific muscle fibers it controls.
By activating a motor unit that includes a large number of fibers, the nervous system can recruit a greater mass of contractile tissue simultaneously, thereby producing more significant force. In contrast, muscles requiring precise, delicate movements—such as those controlling the eyes or fingers—typically have motor units with relatively few fibers to allow for finer motor control. The nervous system further modulates the strength of a muscle contraction by activating additional motor units as needed.1