Why is chicken breast white and dark meat dark? It all has to do with different kinds of muscle. Dark meat is a result of the predominant presence of slow oxidative muscle fibres used for sustained activity by active muscles such as found in the legs and thighs. These fibres have a continuous rich supply of oxygen and generate low levels of force over long periods of time. They contain high levels of a protein called myoglobin that helps facilitate oxygen transport from the blood. This iron-rich, red-pigmented protein, when cooked, turns into metmyoglobin and is what gives dark meat its colour. By contrast, fast glycolytic muscle fibres are mainly found in chicken breast and other muscle regions that are not used actively. These muscle fibres lack myoglobin but are capable of generating a large force over a short time span.