Final answer:
The more movable bone's attachment site when a muscle contracts is the muscle's insertion.
Step-by-step explanation:
The attachment site for the more movable bone when a muscle contracts is known as the muscle's insertion. When a muscle contracts, it generally moves one bone towards another, and this movable bone is where the insertion is attached. The other end of the muscle is attached to a stable bone, referred to as the origin. During a movement like forearm flexion, which is bending the elbow, muscles such as the brachioradialis and brachialis work together to pull on the forearm bones. These muscles have their origins and insertions on specific bones that allow the motion of flexing the elbow to occur.