muscled

Double-muscled cattle are breeds of cattle that carry one of seven known mutations that limits and reduces the activity of the myostatin protein. Normally, myostatin limits the number of muscle fibers present at birth, and interfering with activity of this protein causes animals to be born with higher numbers of muscle fibers, consequently augmenting muscle growth. Additionally, these mutations reduce the superficial and internal fat deposits, causing the meat to be less marbled and lower in fat content. Animals homozygous for myostatin mutation (inheriting a mutant copy of myostatin from both sire and dam) also have improved meat tenderness in some cuts of meat. The enlarged muscles of dam and calf at birth leads to increased difficulty of calving, and in some breeds frequently necessitates birth by cesarean section.
Double-muscling historically has also been known as myofibre hyperplasia, doppellender, muscular hypertrophy, a groppa doppia, and culard.

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