What is the sliding filament model of contraction?

The sliding filament model states that contraction occurs when thin actin filaments slide past thick myosin filaments toward the center of each sarcomere, increasing their overlap. Neither filament type changes length. Myosin heads bind to actin to form cross bridges, which attach and detach like ratchets to generate tension and pull the thin filaments inward. As sarcomeres shorten, I bands shorten, H zones disappear, Z discs move closer together, and A bands remain the same length.

In a relaxed muscle fiber, thin and thick filaments overlap only at the ends of the A band. When the muscle fiber is stimulated, myosin heads latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins. These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the center of the sarcomere. Because this occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens. As the thin filaments slide centrally, the Z discs to which they are attached are pulled toward the M line. As a result, the I bands shorten, the distance between successive Z discs shortens, the H zones disappear, and the contiguous A bands move closer together while their length stays unchanged. Contraction ends when the cross bridges become inactive, tension declines, and the muscle fiber relaxes.