Answer:
A myosin head bound to actin
Explanation:
Cross bridge cycling explains the sliding filament theory according to which thin and thick filament slide against each other for muscle contraction. The two myosin heads extend form the thick filament. One myosin head has binding site for ATP and other head has binding site for actin from thin filament.
When myosin head binds to actin a cross bridge is formed. When ATP binds to the other head the bridge dissociates. Actin binding site is free and myosin tries to bind with other actin during which ATP is hydrolysed into ADP and Pi. Myosin along with the ADP and free Pi successfully binds to another actin molecule. The cross bridge formed generates a powerful force and ADP and Pi are displaced from it. The cross bridge is now free again for binding of next ATP molecule and the cycle continues.
