Final answer:
When a muscle contracts, the attachment to the less moveable bone is called the insertion.
Step-by-step explanation:
When a muscle contracts, the bone that is attached to the LESS moveable end of the muscle is called the muscle's insertion. The insertion is the part of the muscle that attaches to the bone being pulled.
For example, during forearm flexion, the muscle called the brachialis contracts to bend the elbow. The brachialis attaches to the ulna bone, which is the less moveable bone in the forearm, making it the insertion of the muscle.
So, in summary, the attachment site for the less moveable bone when a muscle contracts is called the insertion.