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,
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.
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.
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.