Even if your body is more Bill Gates than Mr. Universe, it still boasts about 650 skeletal muscles. Each muscle is built out of long, tough fibers that are grouped into rope-like bundles. When you throw a javelin (or just lift your posterior off the couch), these fibers pull in unison, generating the force you need.
There are actually two types of fibers at work in any skeletal muscle:
• Fast-twitch. These fibers contract with brief bursts of explosive force. However, they can’t keep contracting for long. Fast twitch fibers are a light whitish color.
• Slow-twitch. These fibers contract more slowly and with less force. However, they can sustain their contraction for longer periods of time. Slow-twitch muscle fibers are reddish and darker in color than fasttwitchers because they have more mitochondria—the cellular factories that generate energy. Your muscles have a different mix of fast- and slow-twitch fibers, depending on their purpose. For example, the neck muscles you use to hold your head upright perform slow, steady work all day, so you’re likely to find that they have more slow-twitch fibers.
If you examine the muscles of elite athletes, you usually find that their mix of muscle fibers matches the requirements of their sport. In other words, the muscles of sprinters are fast-twitch white meat, while the muscles of marathon runners are slow-twitch dark meat. Much of the difference between the amount of fast-twitch and slow-twitch fiber you have is genetic. Even hard-core exercise seems unable to change your ratio of fast-twitch to slowtwitch muscle fibers. So if you’re a white-meat sort of person, all the crosstraining in the world won’t give you dark-meat muscles. But don’t shelve your gym shoes just yet. Most people have a wealth of underused muscle fibers. When you exercise, you trigger a raft of beneficial changes that alter the way your muscles work in ways both straightforward and subtle. And if you train for a particular sport, your body adapts to become more and more efficient at it. So the bottom line is this—just about anyone can develop the muscles of a proficient marathon runner, but you’ll need an extra genetic gift to become a world champion.
To let this all sink in, try reflecting over a takeout chicken. The breast meat is mostly white because it’s made up of fast-twitch fibers that (in better times) helped the chicken flap its way into the air with short bursts of contractions. The leg meat is a darker color because it’s rich in slow-twitch fibers, which the chicken used to use to amble around all day.
Source of Information : Oreilly - Your Body Missing Manual