Muscle cells share one thing in common with the fat cells—they rarely multiply. The number of muscle cells you had at birth is the same number you have now. And there’s probably nothing you can do to change that.
Fortunately, you don’t need to increase the number of your muscle cells to gain strength—you simply need to beef up the ones you have. Oddly enough, the trick to building stronger muscles is to damage them, and the best tool for inflicting the gentle trauma you need is exercise.
Here’s how it works. When you exercise, the vigorous muscle contractions create microscopic tears in your muscle fibers. As your body repairs these tears, it stuffs in a bit more protein to make the muscle a little more resilient the next time. Repeat this process over the course of a year, and you have a recipe that gradually bulks up your muscle, making it stronger along the way.
Muscles don’t necessarily need to get bigger to get stronger. Studies find that exercised muscles develop a better blood supply, which gives them improved access to oxygen and lets them work longer before tiring out. They also respond more readily to the signals your brain sends them, springing into action more easily. Exercised muscle cells also get more mitochondria, which are the power plants of your body. They carry out the energy-producing chemical reactions that muscles need to contract. The net effect is that an exercised body gets a larger and more easily accessible energy supply.
There is one case when your body creates new muscle cells—if an existing muscle cell dies because of damage or disease. But this process has strict limits. For example, your body can’t repair certain types of muscle tissue or patch up extensive damage, and it may fill gaps with useless scar tissue. (This is what happens if you suffer a heart attack, in which case your heart is never the same again.)
Source of Information : Oreilly - Your Body Missing Manual