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