Postnatal Development of Muscle Fiber Types in Domestic Animals

Histochemical evaluation of muscles from piglets, lambs and calves allows the conclusion that in these species, as was previously reported for chick muscle (Ashmore and Doerr, 1971), there are two basic muscle fiber types. These have been termed a and β based on myosin ATPase activity. Initially, a fibers are “red”, but have the capacity to transform from an aerobic state of metabolism (aR) to an anerobic state (aW). β fibers remain “red” throughout their lifespan. Transformation is primarily concerned with changes in energy-producing enzymes, and is accompanied by a rapid increase in fiber size. The majority of muscle fibers are a. Since the number of fibers in a muscle is relatively fixed at birth, it is suggested that increasing degree of muscularity in domestic animals is achieved by practices which favor transformation of aR fibers to aW fibers. Selection of heavily muscled animals for breeding stock, and progressive elimination of requirements which utilize sustained muscular activity could be examples of such practices.