 Tropomodulin, a small, 40-kilodalton actin-binding protein, is attached to the free portion of the thin filament. This actin-capping protein maintains and regulates the length of the sarcomeric actin filament. Variations in thin filament length (such as those in type I and type IIb muscle fibers) affect the length–tension relationship during muscle contraction and therefore influence the physiologic properties of the muscle.
 Desmin, a type of 53-kilodalton intermediate filament, forms a lattice that surrounds the sarcomere at the level of the Z lines, attaching them to one another and to the plasma membrane, thus forming stabilizing cross-links between neighboring myofibrils.
 Myomesin, a 185-kilodalton myosin-binding protein, holds thick filaments in register at the M line.
 C protein, one of possibly several myosin-binding proteins (140 to 150 kilodaltons), serves the same function as myomesin and forms several distinct transverse stripes on either side of the M line.
 Dystrophin, a large 427-kilodalton protein, is thought to link laminin, which resides in the external lamina of the muscle cell, to actin filaments. Absence of this protein is associated with progressive muscular weakness, a genetic condition called Duchenne’s muscular dystrophy. Dystrophin is encoded on the X chromosome, which  explains why only boys suffer from Duchenne’s muscular dystrophy. Recently, characterization of the dystrophin gene and its product has been clinically  important (Folder 11.2).
When a muscle contracts, each sarcomere shortens and becomes thicker, but the myofilaments remain the same length.
During contraction the sarcomere and I band shorten, whereas the A band remains the same length. To maintain the myofilaments at a constant length, the shortening of the sarcomere must be caused by an increase in the overlap of the thick and thin filaments. This overlap can readily be seen by comparing electron micrographs of resting and contracted muscle. The H band narrows, and the thin filaments penetrate the H band during contraction. These observations indicate that the thin filaments slide past the thick filaments during contraction.
The Contraction Cycle
Shortening of a muscle involves rapid contraction cycles that move the thin filaments along the thick filament. Each contraction cycle consists of five stages: attachment, release, bending, force generation, and reattachment.
Attachment is the initial stage of the contraction cycle; the myosin head is tightly bound to the actin molecule of the thin filament.
At the beginning of the contraction cycle, the myosin head is tightly bound to the actin molecule of the thin filament, and