Skeletal muscle is also known as striated muscle because of its striped appearance under the microscope and as voluntary muscle because it can be controlled at will. These muscles are attached to, and move, the bones, and are arranged in antagonistic pairs that enable movement in opposite directions. For example, the biceps flexes the lower arm while the triceps extends it.

A muscle is like a set of Russian nesting dolls. Each level of structure has a smaller element within it. A muscle such as the biceps is composed of many long fibers running in parallel. Each muscle fiber contains many myofibrils, and each myofibril contains many copies of two types of myofilaments, actin, or thin filaments; and myosin, or thick filaments. These, in turn are composed of aggregates of actin and myosin molecules, respectively.

The actin and myosin filaments are arranged in repeating units called...