This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
The energy which produces the calcium flow in the muscle fibers comes from
"mitochondria", the part of the muscle cell that converts glucose (blood
sugar) into energy. Different types of muscle fibers have different amounts
of mitochondria. The more mitochondria in a muscle fiber, the more energy
it is able to produce. Muscle fibers are categorized into "slow-twitch
fibers" and "fast-twitch fibers". Slow-twitch fibers (also called "Type 1
muscle fibers") are slow to contract, but they are also very slow to
fatigue. Fast-twitch fibers are very quick to contract and come in two
varieties: "Type 2A muscle fibers" which fatigue at an intermediate rate,
and "Type 2B muscle fibers" which fatigue very quickly. The main reason the
slow-twitch fibers are slow to fatigue is that they contain more
mitochondria than fast-twitch fibers and hence are able to produce more
energy. Slow-twitch fibers are also smaller in diameter than fast-twitch
fibers and have increased capillary blood flow around them. Because they
have a smaller diameter and an increased blood flow, the slow-twitch fibers
are able to deliver more oxygen and remove more waste products from the
muscle fibers (which decreases their "fatigability").
These three muscle fiber types (Types 1, 2A, and 2B) are contained in all
muscles in varying amounts. Muscles that need to be contracted much of the
time (like the heart) have a greater number of Type 1 (slow) fibers.
According to `HFLTA':
When a muscle begins to contract, primarily Type 1 fibers are activated
first, then Type 2A, then 2B. This sequence of fiber recruitment allows
very delicate and finely tuned muscle responses to brain commands. It
also makes Type 2B fibers difficult to train; most of the Type 1 and 2A
fibers have to be activated already before a large percentage of the 2B
`HFLTA' further states that the the best way to remember the difference
between muscles with predominantly slow-twitch fibers and muscles with
predominantly fast-twitch fibers is to think of "white meat" and "dark
meat". Dark meat is dark because it has a greater number of slow-twitch
muscle fibers and hence a greater number of mitochondria, which are dark.
White meat consists mostly of muscle fibers which are at rest much of the
time but are frequently called on to engage in brief bouts of intense
activity. This muscle tissue can contract quickly but is fast to fatigue
and slow to recover. White meat is lighter in color than dark meat because
it contains fewer mitochondria.