What are the two factors that are necessary in cross bridge formation?

The two factors that are necessary in cross bridge formation are adequate supplies of calcium (Ca2+) ions and ATP.

Calcium ions are released from the sarcoplasmic reticulum in response to an action potential. Calcium binds to regulatory protein troponin, causing it to change shape and expose the active sites on the actin filaments, enabling myosin heads to bind to the actin filaments.  Cross-bridges cannot be formed without calcium changing the shape of troponin. 

ATP provides the energy necessary for cross-bridge cycling and muscle contraction. It first activates the myosin head so it can bind to the actin filament. Once the myosin is bound to the actin filament, ATP is hydrolyzed, releasing energy which is used to power the myosin head toward the M-line, pulling the actin filament with it and causing a muscle contraction.