How Muscles Grow

After you workout, your body repairs or replaces damaged muscle fibers through a cellular process where it fuses muscle fibers together to form new muscle protein strands or myofibrils. These repaired myofibrils increase in thickness and number to create muscle hypertrophy (growth). Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. This adaption, however, does not happen while you actually lift the weights. Instead, it occurs while you rest.
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So how do you actually add muscle to your muscle cells? This is where Satellite cells come in and act like stem cells for your muscles. When activated, they help to add more nuclei to the muscle cells and therefore contribute directly to the growth of myofibrils (muscle cells). Activating these satellite cells may be the difference between what allows certain “genetic freaks” to grow massive muscles and what makes other people “hard-gainers.
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In one of the most interesting studies in the past 5 years, researchers showed that those who were “extreme responders” to muscle growth, with an incredible 58% myofiber hypertrophy from an exercise, had 23% activation of their satellite cells. Modest responders, who had a 28% growth, had 19% activation of their satellite cells. What is interesting to note, though, is that some people known as “non-responders” in the study had 0% growth and had a concurrent 0% activation of their satellite cells. Therefore, it seems the more you can activate these satellite cells, the more you’ll be able to grow. – 
So then the question becomes, how do you activate these satellite cells to increase muscle growth? 
The answer – Muscle damage