The muscular system is composed of 639 named muscles and six trillion muscle fibers (cells). At a width thinner than a human hair, each fiber can support up to 1000 times its own weight.
Each long, microscopic muscle fiber contains protein filaments and is encased in a layer of fascia (connective tissue) called the endomysium. Each muscle fiber is grouped with adjacent muscle fibers to form bundles called fascicles, which are wrapped in fascia. These bundles are bound to other fascicles to form the muscle belly, which is also enveloped by fascia. These layers of fascia merge at either end of the muscle to form a tendon, which attaches the muscle to a bone.