Which theory describes the mechanism of muscle contraction?

The sliding filament theory is the fundamental explanation for how muscle contraction occurs at the molecular level. This theory describes the interaction between thick filaments (myosin) and thin filaments (actin) within the muscle fibers. When a muscle is stimulated to contract, myosin heads attach to specific binding sites on the actin filament, forming what are known as cross-bridges.

As the myosin heads pivot, they pull the actin filaments toward the center of the sarcomere—the basic structural unit of muscle. This action shortens the muscle fibers, leading to contraction. The sliding filament theory emphasizes that the lengths of the filaments do not change; instead, they slide past one another to facilitate contraction.

While excitation-contraction coupling refers to the process by which a muscle fiber converts an electrical stimulus into a mechanical response, and the cross-bridge cycle describes the stepwise interactions of myosin heads with actin during contraction, the sliding filament theory encompasses the overall mechanism and explains how these individual processes work together to produce muscle contraction. The all-or-nothing law pertains to the firing of action potentials and is not directly related to the mechanical action of muscle contraction.