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. When there is a rise in calcium concentrations in the cytoplasm of the skeleton muscle cells, there's also rise in calcium levels in the matrix of the mitochondria. And that calcium will go on and stimulate the phosphatase to actually defosphorylate this molecule, use a water molecule to hydrolyze this bond, releasing that inorganic orthophosphate and activating our Pyruvate dehydrogenase complex and that will stimulate this process. It will basically continually transform the Pyruvate into acetyl coenzyme A and that will commit the molecule to undergoing the citric acid cycle which is ultimately used to generate ATP molecules along the proteins of the inner mitochondria, along that electron transport chain.