The Accidental Scientist: Science of Cooking Exploratorium
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Muscles that are used for extended periods of activity, such as standing or walking, are made up of muscles with fibers that are called slow-twitch. Since these muscles are constantly being used, they need a consistent energy source. The protein myoglobin stores oxygen in muscle cells, which use oxygen to extract the energy needed for constant activity. The more myoglobin there is in the cells, the redder, or darker, the meat.

Muscles that are used for situations where quick bursts of activity are needed, such as fleeing from danger, are made up of fibers called fast-twitch. These muscles get energy from glycogen, which is also stored in the muscles.

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  Chickens spend a lot of time roaming around or standing. Their thigh and leg muscles are used constantly, and so the meat from these parts is dark. Since they rarely fly, and then only for very short distances, the meat that comes from the breast and wings is white. In contrast, wild birds such as ducks fly a lot; the meat from their breasts and wings is dark.
  Cattle spend a lot of time standing, and so their muscles are constantly being used. Therefore, beef has a fairly high concentration of myoglobin and is dark red.
  Pigs also can spend quite a bit of time standing and roaming around. The pink color of pork is due to myoglobin, but because the animals used for pork are young and small, their muscles are less developed and do less work. So pigs have a lower concentration of myoglobin in their muscles than do cows.
  Fish float in water and don't need constant muscle energy to support their skeletons. Most fish meat is white, with some red meat around the fins and tail, which are used for swimming. The red color of some fish, such as salmon and trout, is due to astaxanthin, a naturally occurring pigment in the crustaceans they eat.

Humans have both types of fibers as well. However, unlike animals and fish, humans' fast- and slow-twitch fibers can't be delineated quite so neatly. Both types are interspersed throughout the body.

The average human has about 50% slow-twitch and 50% fast-twitch fibers. Professional athletes can have a higher percentage of one or the other type. For instance, Olympic sprinters may have as much as 80% fast-twitch fibers and long-distance runners may have as much as 80% slow-twitch. Weight-lifters need fast-twitch fibers for quick bursts of strength, and long-distance swimmers need the constant movement provided by slow-twitch fibers. When you roll over the diagram of the human at the top of the page, you get a very simple view of which muscles are more prevalent in sprinters and in long-distance runners. Research is ongoing, but it seems that there is a genetic predisposition for having more of one fiber than another, and that you can't drastically alter the ratio of fibers you are born with.



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