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Why is a chicken's breast "white meat" and its drumstick "dark meat"?

A chicken is a ground-dwelling bird, not a flying one. It uses its leg muscles for unhurried, long-duration movements such as roaming around the barnyard searching for insects and other food. In contrast, a chicken hardly uses its wings except for balance. When it flaps them energetically, it's usually to make a quick escape from a threat. Because the muscle requirements of the chicken leg and breast are different, the two sets of muscles evolve differently. The legs consist predominantly of slow-contraction muscle fibers, while the breast is composed chiefly of fast-contraction muscle fibers to help flap the wings.

The slow-contraction muscle fiber is for the long-duration jobs and the fast-contraction muscle fiber for the quick-energy spurts. The fuel for the slow-contraction muscle fiber is fat and requires oxygen, which is stored in the iron-rich, red-pigmented myoglobin. Consequently, the more slow-contraction fibers in a muscle, the redder the muscle will likely be. Fast-contraction muscle fibers don't require oxygen that much and therefore myoglobin because they use glycogen (a carbohydrate) for fuel. In the absence of myoglobin, the muscles are "white".

Actually, both the chicken leg and breast contain a combination of fast- and slow-contraction muscle fibers. The leg is "dark meat" because the slow-contraction muscle fibers predominate. The opposite is true for the breast.

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