Sliding filament theory is a model used to explain the mechanism by which muscles contract. The contraction of skeletal muscle, which is what makes movement possible, occurs in three ways. Concentric muscle contraction involves the shortening of muscle fibers, as in the lifting phase of a bicep curl, while eccentric muscle contraction is made possible by the lengthening of muscle fibers, as in the lowering phase of a bicep curl. Isometric contraction is another possibility, during which the muscle does not change in length while sustaining a contraction, as in stopping the weight midway through a bicep curl and holding the elbow at 90 degrees. Sliding filament theory describes the process that makes these changes in muscle length, and therefore muscle contraction, possible.
Two kinds of proteins found in muscle cells, actin and myosin, work together to produce these contractions, as they are arranged in filaments that slide past each other, giving sliding filament theory its name. Within each muscle cell, actin protein chains form passive thin filaments that work in conjunction with thick filaments of myosin, a motor or movement protein that produces the force of muscle contraction. To do this, the myosin filaments slide back and forth along the actin filaments within a unit inside the muscle cell called the sarcomere. Each muscle cell can contain hundreds of thousands of sarcomeres, a band-like structure that expands and contracts as a unit as the actin and myosin filaments slide past each other. It is the bands of sarcomeres that give muscles their striated appearance.
Under sliding filament theory, myosin filaments are alternated with actin filaments in horizontal lines, much like the red and white stripes on the American flag. The myosin proteins slide along the actin, releasing calcium ions that allow the head of each myosin protein to bind to a site on the actin filament. Once the myosin binds to the actin along these sites, much like a crew of rowers in a scull pulling their oars simultaneously, the myosin pulls the two filaments past each other, resulting in an overall shortening of the sarcomere. This collective shortening is made possible by the hydrolysis of adenosine triphosphate (ATP), the body's main energy source for many cellular functions, and results in the contraction of the muscle cell.
Once the actin and myosin filaments bind and the “stroke” occurs, pulling the actin filaments toward the center of the sarcomere, the myosin heads detach from actin, and the ATP is recharged in these filaments again, causing the next stroke of the filaments. If no muscle contraction is needed and the muscle is at rest, a protein called tropomyosin wraps itself around the actin filaments, blocking the binding sites and thereby preventing the myosin from binding to the actin so that no muscle contraction may occur. Sliding filament theory also explains how cytokinesis, or cell division, takes place, with the sliding filament mechanism causing one cell to pinch off into two during mitosis.