Final answer:
Calcium facilitates the binding of myosin heads to actin during muscle contraction by causing conformational changes in troponin and moving tropomyosin away from the myosin binding sites on actin.
Step-by-step explanation:
The role of calcium in the cross-bridge is to facilitate the binding of myosin heads to actin during muscle contraction. The release of calcium ions causes conformational changes in troponin, which allows tropomyosin to move away from the myosin binding sites on actin. This exposes the actin-binding sites, allowing the myosin heads to bind and form cross-bridges with actin.