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The term “white muscle” refers to seldom-used muscle tissue lacking in pigment due to the absence of myoglobin molecules. Myoglobin, a unique protein found exclusively in the frequently-used muscles — which are known as red muscles —, attracts and stores iron and oxygen. Much like in blood, the combination of oxygen and iron in myoglobin imparts a crimson tinge to muscles of regular use. Since white muscle tissue is seldom used, it does not require ready stores of oxygen and, thus, has little distinguishable myoglobin or its associated pigment. White muscles are capable of movement — specifically quick and short explosive actions — and are noted for being fast-contracting muscles that return to rest very quickly; red muscles, on the other hand, contract slowly.
Not every creature has white muscle in the same location. White muscles can occur in different parts of the body, depending on the species and the behavioral tendencies of that species. For example, in birds that don’t fly, such as chickens, the breast muscle is seldom used and is, therefore, comprised of white muscle. In birds of flight, the muscles supporting the breast are red muscle.
For the typical human, the Achilles tendon, latissimus dorsi and rectus abdominus are types of white muscle. These muscles support seldom-contracted muscles such as those in the heel, back and stomach, respectively. Other white muscles in humans include the trapezius, which is a set of muscles at the base of the neck and behind the shoulders just above the deltoids. Muscles used constantly by humans, such as those in the thighs and arms, are not part of the white muscle group.
Beyond the fact that white muscles use less oxygen than red muscle, white muscle is further distinguished by its size and composition. Compared to red muscles, white muscles have fewer capillaries and mitochondria; mitochondria are energy hubs that fuel the body through the production of adenosine triphosphate. White muscles also have less oxidative enzymes and more glycolytic enzymes. In terms of size, white muscles are not as large as red muscles; they also have fewer muscle fibers.
Studies of animals with a high percentage of white muscle reveal that the amount of light muscle may be linked to happiness and reproduction. For example, studies of crickets and birds suggest that females with white muscles exhibit cheerier dispositions and have a greater ovum production. No similar correlations have been found in humans. Animals with light muscle might also be more likely to suffer white muscle disease, which is the death and decay of white muscle in the heart and along the skeleton due to a lack of oxygen and nutrients. This phenomenon is limited to animals and does not occur in humans.