Final answer:
A cross-bridge in the molecular interaction of muscle contraction involves the binding of myosin heads to actin filaments, essential for muscle fibers to contract.
Step-by-step explanation:
In the context of molecular interactions known as "cross bridges" during muscle contraction, the two binding components represented by letters are myosin and actin filaments. Specifically, myosin heads, which have binding sites for ATP or its hydrolysis products (ADP and Pi), attach to actin filaments to form a cross bridge. This interaction is imperative for the muscle contraction cycle where the myosin heads pull on the actin filaments, causing them to slide and the muscle to contract.