According to the sliding filament model of muscle contraction, during muscle contraction, a myosin head attaches to a binding site on the actin filament, forming a cross-bridge
This binding causes the head to bend, pulling on the actin filament, and moving it toward the center of the sarcomere
The head then releases, and attaches to the next binding site on the actin, pulling this site toward the center
As this occurs again and again, the filaments increase their overlap, and the sarcomere shortens from both ends
When many sarcomeres shorten at the same time, the muscle fiber shortens
Energy from the conversion of ATP to ADP is provided to the cross-bridges by the enzyme ATPase; ATP breakdown causes the heads to return to the “cocked” position, ready to bind to another actin binding site