The source of energy for muscle contraction is ATP (Adenine Triphosphate). An enzyme called myosin ATPase present on the head of myosin molecule breaks down into ADP and inorganic phosphate in the presence of magnesium and calcium ions and releases energy in the head of the myosin.

i.e., ATP -> ADP + Pi + Energy

This energy from the ATP causes the myosin to form a cross-bridge by binding to the actin filament. The movement along the cross-bridge causes the sliding of thin and thick myofilaments which initiates muscle contraction.