mATPase

To distinguish between slow and fast-contracting muscle fibers, we use a histochemical assay for myofibrillar ATP (mATP).
mATPase is an enzyme involved in hydrolyzing ATP to enable muscle contraction. High mATPase activity, therefore, correlates with greater contraction speed.

We visualize mATPase activity by letting the transparent reaction product (inorganic phosphate) react with calcium to form calcium phosphate (limestone).
Subsequently, the calcium phosphate is converted to cobalt sulfide, which has a brown-black color we can observe under a light microscope.

Fast muscle fibers appear darker because they hydrolyze ATP at a high pace producing large amounts of inorganic phosphate, compared to slow contracting muscle fibers.

Histochemical assay of muscle fibers using the enzyme mATPase. Muscle fibers are illustrated in different shapes and sizes, laying close to each other. The fast contracting fibers are colored dark brown and the slow contracting fibers are light colored. Most of the fibers in this illustration are colored dark brown and are therefore fast contracting muscle fibers.

Fig.1 mATPase staining of skeletal muscle

Referred from:

Article

Histochemistry staining