Muscles can be divided into three main types (Cardiac, Smooth, and Skeletal). Cardiac muscles are responsible for such tasks as breathing and the beating of your heart. Smooth muscles cover or wrap around internal organs and blood vessels, and skeletal muscles are used in everyday activities, like walking, swimming, and sitting. For the purpose of this article, we’ll be discussing only skeletal muscles and how they respond to exercise.
Skeletal muscles are attached to your bones, which is why they are called skeletal. They’re also the muscles that you look at in the mirror when you’re feeling a little bit vain. These are the muscles that you consciously use to perform your exercises.
Skeletal muscles are further divided into two groups: Type I and Type II muscle fibers. Type I (or slow-twitch) fibers, are used for less powerful, longer-lasting actions (like swimming, distance running, aerobics, etc.). Type II (or fast-twitch) fibers, are used for strenuous, powerful actions (like lifting a very heavy weight or sprinting).
Not everyone has the same number of type I and type II fibers. Some people have a lot of type I fibers, resulting in an easier time getting lean and running marathons. Others have more type II fibers, resulting in an easier time gaining muscle mass and excelling at sports that require quick bursts of power.
When you perform an action, a particular muscle fiber either gets involved or it doesn’t. A fiber doesn’t give an effort of 50%. It either goes all-out or not at all. When it does get activated, it breaks down a little bit, and then waits to get repaired by incoming protein that you consume.
When a muscle contracts, the smallest of its fibers go to work first. If that isn’t enough, more and bigger fibers will get involved. Your type I muscle fibers are the smallest, while your type II are the biggest. So in any activity you do, you activate your type I fibers first. Since they are small, the potential for their growth is small.
Since your fast-twitch type II fibers are the biggest (and used as a last resort), you’ll have to work them extremely hard to activate them in your exercises. That’s why bodybuilders work out with heavy weights, so that they can finally activate those large type II muscle fibers. It’s also important for body builders to occasionally lift their absolute maximums, so that those very last, biggest fibers get activated.
If the only exercises you do are endurance-type activities, you are never really going to bulk up your muscles. You might bulk them up a little, but nothing like that of a body builder. Simply put, the body builder routinely activates his larger, type II fibers, resulting in bigger muscle gains.



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