Now, just because we're no longer phosphorylating this molecule doesn't actually mean the molecule will spontaneously go back into this state. We actually have to activate the phosphatase. Now, what activates the phosphatase to reverse these effects of the kinase? Well, it's the calcium. So remember in contracting muscles, when we contract our skeleton muscles, what happens is the calcium that is stored in the sarcoplasm reticulum will be released into the cytoplasm and the calcium is not only used to actually contract those actin myosin filaments. But the calcium is also used by this enzyme here.