Answer
The myosin head must have an ATP bound to it to initiate contraction. When this happens an enzyme in the myosin head -myosin ATPase--hydrolyses ATP to get ADP and Pi molecules. The energy released by this process activates-- or cocks-- the myosin head into a high energy position ; in this position the myosin head is ready to bind an exposed active site on an actin filament. The myosin binds the actin filament, thus forming an actin-myosin crossbridge (complex). but also keeps the ADP and Pi entities bound to it.
Contraction occurs when the head of the myosin molecule in the actin-myosin complex releases the ADP+Pi which it has kept bound to itself, then flexes, pulling the actin filament with it. This flexing and pulling constitute the power stroke" of contraction. ATP is absolutely necessary for this power stroke to be executed.
Work Step by Step
ATP is also required or the recovery stroke. Subsequent to the power stroke, the myosin remains bound to the actin until another ATP binds with the cross bridge complex. The binding of a new ATP with the myosin in the crossbridge complex destabilizes the crossbridge bond between myosin and actin. The recovery stroke can now be executed.
When the new ATP binds to the myosin head, myosin ATPase is released, and hydrolyses the ATP to ADP an Pi molecular entities; the energy released rom ADP hydrolysis recocks the myosin head. This is the recovery stroke. After the recovery stroke, the myosin head is once again ready to attach to an active site in a new position on the actin filament. When this happens a new power stroke will be effected.
