Adenosine triphosphatase (ATPase) is a hydrolytic enzyme that catalyzes the decomposition of ATP to produce energy, and is divided into membrane ATPase, myosin ATPase, and mitochondrial ATPase according to the activator, inhibitor, and localization of the enzyme. Myosin ATPase has an optimal pH of 9.2~9.4 and is activated by calcium ions and inhibited by magnesium ions. The energy produced when it breaks down ATP is used for muscle contraction. This enzyme is localized in skeletal muscle and is often used to distinguish between the two types of muscle fibers. type I muscle fibers, also known as red muscle, are slow fibers, i.e., the contraction of muscle fibers is slow and the activity is long-lasting, with high enzyme activity and dark staining; type II muscle fibers, also known as white muscle, are fast fibers, i.e., the contraction of muscle fibers is fast but the activity is not long-lasting, with low enzyme activity and light staining. the reaction of hydrolysis of ATP by ATPase is as follows: A-P-P-P + H2O→ A-P-P-P + H3PO A-P-P + H3PO4 + energy

The principle of ATPase staining solution (calcium activation method) for myofibers is that ATPase hydrolyzes ATP to ADP and phosphoric acid, and phosphoric acid combines with calcium ions in the enzyme active site to form a colorless calcium phosphate precipitate, and the calcium phosphate is processed by cobalt chloride to form cobalt phosphate, and then processed by the sulfide solution, it will form a brownish-black cobalt sulfide precipitate in the enzyme active site. This enzyme uses acid or alkali for pre-incubation treatment. When alkali is used for pre-incubation treatment, only white muscle fibers show positive reaction, which is specific for myosin ATPase; when acid is used for pre-incubation treatment, red muscle fibers are firstly doubly stained. Therefore, ATPase staining by the calcium-activated method is mainly used to differentiate between red and white muscle, which is valuable in the diagnosis of neurogenic and myogenic myasthenia gravis.