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 (a thin filament) and myosin (a thick filament) are the major components of the sarcomere. These units perform work by moving structures closer together through space. Sarcomeres in the skeletal muscles pull parts of the body through space relative to each other (e.g., walking or swinging the arms).

To visualize how a sarcomere works:

1. Interlace the fingers of your two hands with the palms facing toward you (represents actin, myosin); fingertips touching

2. Push the fingers together so that the overall length from one thumb to the other is decreased (sarcomere length decreases); allow fingers to slide past each other without bending

3. Any object attached to either thumb would be pulled through space as the fingers move together (sliding filament theory).