The smallest contractile unit of muscle fiber is the sarcomere. Sarcomeres are stacked throughout muscle tissue. These units are so small that some muscle cells contain thousands of sarcomeres.
Within muscles, sarcomeres are organized into structures called myofibrils. These give muscles their striped appearance. Eighty percent of a muscle cell is made up of myofibrils, which are responsible for muscle contractions.
Sarcomeres contain proteins that are able to vary their lengths, and this changes the length of the muscle. Each sarcomere has thin filaments called actin and thick filaments known as myosin. When these filaments undergo physical and chemical interactions, they shorten, which leads to muscle contraction.