Actin and myosin filaments are essential contractile elements found in muscle cells. They are essential because they conduct muscle contraction. A molecule of actin is made up of small microfilaments, which give them a very thin appearance. Myosin is made up of long polypeptide chains that join together to form a thick filament; therefore, actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments.

All muscle cells, regardless of type, contain both actin and myosin filaments. Muscle contraction is not possible without the presence of both contractile elements. Organization of these molecules can vary, as smooth muscle does not contain striations, but the molecules are still responsible for contractile actions. Troponin, calcium, and tropomyosin are all required to initiate the contact between myosin and actin. Calcium binds to troponin, which subsequently removes tropomyosin from actin (thin filaments). None of these interact with myosin, the thick filaments.

When a muscle is shortened, the force decreases as the filaments slide past one another and lose room to form new cross-bridges. When a muscle is lengthened, there is less filament overlap which leads to fewer cross-bridges. Stretching a muscle before contracting it will not affect the force produced, nor will shortening a muscle before lengthening it. Titin is a protein responsible for some of the elastic properties of muscle, but is not involved in force production.

At rest, the muscle has the potential to form the maximum number of cross-bridges, resulting in the maximum amount of force production. For further review, go over the length-tension curve for a muscle fiber.

A sarcomere is one contractile unit of a muscle fiber, and contains two half-filaments of actin and a full filament of myosin. The ends of the sarcomere are the Z-discs and the center is the center is the M-line (the middle of the myosin filament). The H-band lies between the two half-actin filaments where there is only myosin; however, it does not correspond to the full myosin filament. The I band corresponds to the region where only actin is present and the A-band correspond to the full length of the myosin filament.

The A band is the only element that remains constant during a muscle contraction. It represents the segment of the sarcomere that contains the length of the thick filament. The H zone refers to the part of the sarcomere where there are only thick filaments, and no superimposing thin filaments. Conversely, the I band refers to the area where there are only thin filaments and no superimposing thick filaments. As filaments overlap, both the H zone and I band will shorten. The N line does not exist in musculoskeletal physiology.

A sarcomere is the functional unit of the skeletal or cardiac muscle cell, and is made of interlocking myofibrils. A sarcomere is the smallest unit in the muscle cell to contract and relax.

Note that smooth muscle cells still contract using actin and myosin filaments, but do not organize these filaments into sarcomeres as skeletal and cardiac muscle do. This is why smooth muscle is not striated.