Final answer:
During muscle action, the cross-bridges attach to the actin filaments, forming a strong connection between the thick and thin filaments.
Step-by-step explanation:
During muscle action, the cross-bridges attach to the actin filaments. Cross-bridge formation occurs when the myosin head attaches to the actin, forming a strong connection between the thick and thin filaments. This attachment allows the myosin heads to interact with the actin and move the filaments, resulting in muscle contraction.