A chicken has white wing meat and dark leg meat, and humans are much the same in having dark leg muscles and white arm muscles. These differences in color are due to the use of and demands on the limbs. Dark muscle is specialized for endurance and its color comes from a rich blood supply and high myoglobin content. Endurance in dark muscle is at the expense of speed. Your legs can carry you all day, but they cannot move with the speed of a magician’s hand. White muscle specializes in very fast contractions and movements, such as wildly clapping hands or swinging a tennis racquet. White muscle tires quickly because it is less well supplied with blood.