Skeletal muscle comprises the largest single organ of the body. It is highly compartmentalized, and we often think of each compartment as a separate entity (such as the biceps muscle).
Each of these individual muscles is composed of single cells or fibers embedded in a matrix of collagen. At either end of the muscle belly, this matrix becomes the tendon that connects the muscle to bone.
Muscle cells contain most of the structures common to all cells. Each cell is enclosed by a cell membrane or plasmalemma; they contain mitochondria for the oxidative metabolism of nutrients; and all the machinery necessary for protein synthesis. Skeletal muscle fibers are multinucleated and can be as much as two centimeters long.
The principal force generating components are actin and myosin molecules. These myofilaments are arranged in interdigitating matrices capable of sliding across each other. To produce force, crossbridges from the myosin filaments associate with the actin filament, then rotate slightly to pull the filaments across each other (much like the oars of a rowboat pull across the water).
Muscle fibers, though, are just the building blocks for whole muscles. The precise way in which fibers are arranged into muscle is referred to as architecture.