ATPase stains are histochemical stains to assess the relative function of myofibrillar ATPase.  This is helpful in distinguishing Type I fibers from Type II fibers.

ATPase stains differently depending on the pH at which the stain is performed.  At lower pH values (e.g., approximately 4), Type I fibers appear dark and Type II fibers appear pale.  At more basic pH values (e.g., approximately 10), Type II fibers are darker than Type I.  The specific pH values used for basic and acidic staining depends on the protocol followed at a given laboratory, and there is no universal standard.

A helpful mnemonic:  At lower pH, the lower-numbered fiber type is dark; at higher pH, the higher-numbered fiber type is dark  (Thanks to Dr. Richard Prayson for that mnemonic).

So why should we care about fiber type staining?  It is helpful in establishing a differential diagnosis.  For example, in myopathy, one would expect to see the normal checkerboard distribution of Type I and Type II fibers.  In a severe chronic neuropathy, on the other hand, it would not be at all unusual to see loss of the normal mosaic, and see "fiber type grouping."  Also, certain disease processes affect the different fiber types differently.  For example, steroid myopathy and disuse atrophy result in preferential atrophy of Type II fibers.