<P> A skeletal muscle refers to multiple bundles of cells called muscle fibers (fascicles). The fibres and muscles are surrounded by connective tissue layers called fasciae . Muscle fibres, or muscle cells, are formed from the fusion of developmental myoblasts in a process known as myogenesis . Muscle fibres are cylindrical, and have more than one nucleus . It has multiple mitochondria to meet energy needs . </P> <P> Muscle fibers are in turn composed of myofibrils . The myofibrils are composed of actin and myosin filaments, repeated in units called sarcomeres, which are the basic functional units of the muscle fiber . The sarcomere is responsible for the striated appearance of skeletal muscle, and forms the basic machinery necessary for muscle contraction . </P> <P> Connective tissue is present in all muscles as fascia . Enclosing each muscle is a layer of connective tissue known as the epimysium; enclosing each fascicle is a layer called the perimysium, and enclosing each muscle fiber is a layer of connective tissue called the endomysium . </P> <P> Muscle fibres are the individual contractile units within muscle . A single muscle such as the biceps brachii contains many muscle fibres . </P>

How many skeletal muscle fibers do we normally use at any one time