Why Do Muscles Grow?

Muscle Growth is based on "overcompensation" concept, where you overcome a real/imagined defect or unwanted trait by overly exaggerating its opposite. When you have a cut, your body repairs the cut and overcompensates by leaving a scar.

When muscles undergo intense exercise, there is trauma to the muscle fibers that is referred to as muscle injury or damage in scientific investigations. A biological effort to repair or replace damaged muscle fibers begins and body overcompensates by increasing the cross-sectional diameter of the muscle cell. So more you use your muscles, the stronger they become. And unused muscles do not remain preserved; neglect causes them to waste away, or atrophy. Each muscle cell contains many small bundles of contractile proteins, called myofibrils. When muscles undergo intense exercise, the muscle cells’ myofibrils will increase in thickness and number.

So when we exercise, especially when using weights, we create micro tearsin our muscle tissue. When you take rest, these tissues repair and regenerate themselves. During this repair process muscle cells adapt by becoming bigger to help your body produce more force making your workout easier. So rest at least 24 hours before training the same muscle group. Try not to rest more than 72 hours as the muscles begin to reverse their progress.

Aging also mediates cellular changes in muscle decreasing the actual muscle mass. Happily, the detrimental effects of aging on muscle have been shown be restrained or even reversed with regular resistance exercise. Importantly, resistance exercise also improves the connective tissue harness surrounding muscle, thus being most beneficial for injury prevention and in physical rehabilitation therapy.

Sources:
http://www.unm.edu/~lkravitz/Article%20folder/musclesgrowLK.html
http://science.nasa.gov/headlines/y2004/10dec_muscles.htm
http://www.naturalphysiques.com/faq/407.html
http://lancebreger.blogspot.com/2007/12/pop-quiz-when-do-your-muscles-grow.html