The avian pectoralis: histochemical characterization and distribution of muscle fiber types

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Abstract
The muscle fibers of superficial (ventral) and deep (dorsal) samples from the pectoralis muscle of 43 species of carinate birds are characterized histochemically on the basis of their myofibrillar adenosine triphosphatase (mATPase) activity after acidic and alkaline preincubations. Muscle fibers are described as slow tonic (alkali-labile/acid-stable mATPase activity) or fast twitch (alkali-stable/acid-labile mATPase activity). Three varieties of fast-twitch fibers are recognized histochemically on the basis of their succinate dehydrogenase (SDH) activity: white (low SDH), intermediate (moderate SDH), and red (high SDH). Slow-tonic fibers are restricted to the deep distal area of the muscle in three species studied. All other muscle is fast twitch. In those species studied, there is a significantly (p ≤ 0.0001) higher proportion of red fibers in the deep area of the muscle as compared with the superficial area. The nature and distribution of the fiber types is characteristic of those vertebrate locomotory muscles most specialized for the rapid output of power. The nomenclature of avian extrafusal skeletal muscle fibers is discussed.