Skeletal muscle is attached to the bones and is responsible for their movement, it is characterized by a hierarchy of smaller and smaller parallel units. A skeletal muscle consists of a bundle of long fibers running the length of the muscle. Each fiber is a single cell with many nuclei, reflecting its formation by the fusion of many embryonic cells. Each fiber is itself a bundle of smaller myofibrils arranged longitudinally. The myofibrils, in turn, are composed of two kinds of myofilaments. Thin filaments consist of two strands of actin and one strand of regulatory protein coiled around one another, while thick filaments are staggered arrays of myosin molecules.

Now, Resistance training (for example, free weights, jump-training and isometric training), mostly increases the size of muscle fibers; increase in myofibrils, more actin and myosin filaments and more connective tissue (hypertrophy). It is not clear whether training can increase the number of muscle fibers (hyperplasia) . Muscle fibers get bigger by having more muscle protein content, and that is achieved by making new protein and decreasing the rate at which existing proteins are broken down. These proteins include contractile proteins as well as the enzymes that are involved in various metabolic reactions. By increasing the strength of muscles, resistance training can also increase the power of muscles.

Steroids are a class of lipids characterized by a carbon skeleton consisting of four rings with various functional groups attached. The use of steroids has been associated with a wide range of adverse side effects ranging from some that are physically unattractive, such as acne and breast development in men, to others that are life threatening, such as heart attacks and liver cancer. People who abuse steroids, disrupt the normal production of hormones in the body causing adverse reactions which can include:
In boys and men, reduced sperm production, shrinking of the testicles, impotence, difficulty or pain in urinating, baldness, and irreversible breast enlargement.
In males and females of all ages, potentially fatal liver cysts and liver cancer; blood clotting, cholesterol changes, and hypertension, each of which can promote heart attack and stroke; and acne.

Although not all scientists agree, some interpret available evidence to show that anabolic steroid abuse-particularly in high doses promotes aggression that can manifest itself as fighting, physical and sexual abuse, armed robbery, and property crimes such as burglary and vandalism. Upon stopping anabolic steroids, some abusers experience symptoms of depressed mood, fatigue, restlessness, loss of appetite, insomnia, reduced sex drive, headache, muscle and joint pain, and the desire to take more anabolic steroids.

Muscles consist of a large number of parallel fibers, which contract when stimulated. This pulls the bones that they attach to together, and through using the bones as levers, they can push as well. Using a muscle causes the muscle fibers to become more responsive to signals sent from the nervous system to trigger them to pull. Thus, a practiced muscle will have a larger number of fibers pull when stimulated, and thus will generate more force.

In a person who is growing, if a muscle is used, it will grow more of these fibers as well, further increasing the force that it can exert.

Your muscles adapt to the type of work that you make them do. Bodybuilding usually involves short bursts of intense strain on your muscles. Your muscles, which are made of many fibers that run along the length of the muscle, adapt to this in three ways. First, they increase the amounts of proteins in them that make them contract (actin and myosin). More actin and myosin means that each muscle contraction will be stronger. Second, each fiber increases in width, so the overall muscle gets larger. (This also gives each fiber more room for all the new actin and myosin that was made.) Third, some researchers think that more fibers are formed. This is in addition to the enlargement of the existing fibers. If this third adaptation plays a role, though, it's a minor role. Mostly what matters is that the fibers thicken and that more actin and myosin are made.

Without the weightlifting, your muscles would not adapt by going through these changes, so they wouldn't get bigger. This is all different than the kind of adaptations caused by longer, less intense workouts, like swimming or running...but that's another story!

When you put steroids in your body how does it effect your body? There's a lot about steroids that is not known, which is one of the reasons they're dangerous to use. Many steroids mimic natural hormones that your body makes. The problem is that these natural hormones do a lot of different things. For example, anabolic steroids are the ones people use to make their bodies bigger and stronger. These steroids mimic the natural hormone called testosterone, which you could think of as the male sex hormone. Testosterone does a number of things: it is what made us boys instead of girls when we were fetuses, it's why men usually have bigger muscles than women, it's what causes boys to get deeper voices and grow facial hair during puberty, etc. The fact that anabolic steroids can give you bigger muscles, like testosterone can, is why people take them--but they don't realize that they're messing around with their normal hormones when they do that. It can cause a lot of problems in men, because their bodies often can't tell the difference between the steroids and the normal testosterone. Their bodies suddenly think there's too much testosterone around, so they stop making it.

Without normal testosterone, you might lose your facial hair, you might stop making sperm cells (so you wouldn't be able to have children), and you might suffer from uncontrollable mood swings. (Such mood swings have caused some people to act violently, hurting other people and sometimes even ending up in jail.) In women, anabolic steroids can cause other problems: they can decrease breast size, cause facial hair to grow, and prevent some women from being able to have children. These reasons are why steroids are illegal without a prescription from a doctor, not to mention all the things steroids and hormones do that we don't really understand yet. Plus, for anabolic steroids to work, you would still have to work out a whole lot, so lots of athletes think it's better to just do the workouts and not take the steroids. Even though steroids may make your muscles bigger and stronger(sometimes they don't, though), they can cause a lot of other serious problems.