What is considered the contractive part of a muscle?

The belly of a muscle is considered the contractive part because it is the thick, central portion that contains the muscle fibers responsible for contraction. This expansion and contraction of the belly enable movement and force generation when the muscle is activated. The muscle belly is where the majority of the muscle's action occurs; as it contracts, it shortens and pulls on the tendons, which then exert force on the bones to create movement.

In contrast, the origin and tendon serve different functions. The origin is the fixed attachment point of the muscle, typically at the beginning of the muscle, while the tendon connects the muscle to the bone and transmits the force generated by the muscle. Inertia is not a part of the muscle structure but rather a science term that refers to the tendency of an object to resist changes to its state of motion. Understanding the distinction between these elements clarifies why the belly is recognized as the primary component for the muscle's contractive abilities.