Hi Kimberly
Assuming you don't want the Biochemical Mystery Tour, muscle fiber types
are grouped according to the sort of work they do. Slow twitch muscles
are adapted for long, continuous, not-particularly-powerful contractions
without tiring. Postural muscles (the ones that hold you together) and
the heart are examples of the kind of work slow twitch muscles do, also
alot of the muscles for locomotion (though not all of them). Slow
twitch are primarily aerobic and have greater fuel storage capacity, a
larger blood supply, more mitochondria for energy production, and so on
(you said you wanted it simple). The "dark meat" of chicken is
primarily made of slow twitch fibers.
Fast twitch fibers are adapted for short, explosive bursts of
power---for example, a weight lifter's muscles that can lift a huge
weight but only for a very short amount of time. Horses that sprint,
like quarter horses, tend to have much higher populations of fast twitch
fibers. They tend to rely on glycolysis for energy production, which is
not particularly efficient, but can operate in the absence of oxygen
(anaerobically). There are also several intermediate fiber types in
between the two that respond differently according to the type of work
being asked---anaerobic vs. aerobic.
A muscle is always made of of varying populations of fast, slow and
intermediate muscle fibers. Muscles used for aerobic work will have
predominantly slow twitch muscle fibers, while muscles used for short,
explosive bursts of power (like the pectoral muscles) will have a higher
concentration of the fast twitch muscle fibers. Different breeds of
horses tend to have more or less of one type or another depending on
what that breed has been bred to do. Quarter horses have more fast
twitch fibers, while Arabs have more slow twitch, which is why they tend
to excel at endurance activities.
Sorry if I sound a little goofy, it's Finals Week and I think my
eyeballs are falling out of my head. ZZZZZZZzzzzzzz.
Susan Evans
California State Polytechnic University