Muscle cells—whether the skeletal muscles in the arms or legs, the smooth muscles that line the digestive tract and other organs, or the cardiac muscle cells in the heart—work by contracting. Skeletal muscle cells are comprised of thousands of contracting units known as sarcomeres. The proteins actin (thin filament) and myosin (thick filament) are the major components of the sarcomere. These units perform work by moving structures closer together through space; in the skeletal muscles, they pull parts of the body through space relative to each other (for example, when you walk or swing your arms).

To visualize how a sarcomere works, try the following: Interlace the fingers of your two hands with the palms facing toward you (represents actin, myosin); push the fingers together so that the overall length from one thumb to the other is decreased (sarcomere length decreases). And from there, realize that any object attached to either thumb would be pulled through space as the fingers move together (sliding filament theory).