Muscle fibres come in two major types – fast-twitch and slow-twitch. Fast-twitch fibres are geared for bursts of energy (turkeys use their breast and wing muscles to escape predators), while slow-twitch fibres are adapted for sustained use (leg and thigh muscles are used constantly for standing and walking).
The colour of meat is related to which type of muscle fibre it contains. Slow-twitch fibres need a steady supply of oxygen, so they are packed with a protein called myoglobin. This binds tightly to oxygen and provides a ready-made store of it for the muscle. Myoglobin is a deep red colour so this meat appears dark. Fast-twitch fibres contain very little myoglobin, so the meat appears paler.