Final answer:
The movement of a single Myosin II head on the actin filament is called the power stroke in muscle contraction. It occurs when the actin binding sites are uncovered, forming a cross-bridge between the myosin head and actin molecules.
Step-by-step explanation:
The movement of a single Myosin II head on the actin filament is called the power stroke in muscle contraction. When the actin binding sites are uncovered, a cross-bridge is formed between the myosin head and actin molecules.
The myosin head then releases phosphate (P₁) which allows it to use stored energy to change conformation and move towards the M line, pulling the actin along with it. This movement, known as the power stroke, results in the shortening of the sarcomere and muscle contraction.