Modern technology changes the information we have access to at such a rapid pace one would think that when it came to muscle growth we would know almost everything there is to know. There are still certain areas dealing with the functioning of the human body such as the brain and muscle development that still eludes us.
Although we have made significant advances in these areas, the truth of the matter is that we still really don't have an exact understanding of what makes the brain work or how muscles hypertrophy.
Many of you are thinking, "What's he talkin' `bout? I know exactly what makes muscles grow, weight training." You're correct but we still haven't been able to identify all the signals or all the substances involved in placing the growth mechanism into motion.
Currently Researched Theories
Currently there are a couple of well-researched theories that attempt to explain what is involved in the process resulting in muscle hypertrophy. 
Muscle Damage Theory
The first is called the Muscle Damage Theory. This theory states that when muscles are damaged through exercise; the resulting damage turns on a cascading sequence of signals that result in muscle hypertrophy. Although this appears to be the most widely and supported theory; it doe not appear to be the only game in town.
Substrate Accumulation Theory
The other theory is called The Substrate Accumulation Theory. The foundation of this theory is that several substances accumulate during exercise that may cause the muscle growth mechanism to activate through the direct or indirect stimulation of the release of anabolic hormones.
Although both theories have merit, one without the other leaves us with an incomplete explanation of the subject. It is important to examine each of these theories and their possible stimulus to be able to better understand and utilize their principles to produce greater NATURAL muscle growth.
Explanation And Understanding
We all know that it is possible to cause damage to muscles. This can be done is different ways. Muscle damage may occur from trauma and result in an injury such as a muscle pull, separation or tear. This type of damage is of course detrimental to the muscle and can result in long-term weak points if not allowed to heal properly.
However, there is also good trauma, which induces micro-tears in muscle fibers because of exercise, specifically resistance training. Remember the last time you worked legs with maximum intensity. You were really sore for a couple of days afterwards, weren't you.
It wasn't to long ago that we thought the soreness was the result of a build-up of lactic acid in the muscle, but we now know that isn't the case. Lactic acid causes the muscle contraction process to signal a stop in order to protect the muscle from damage. The liver, the heart and slow-twitch (red muscle) fibers all acts as processors to metabolize the lactic acid so it can be quickly removed from our system. The resulting soreness we feel is actually a result of the muscle damage that occurred during exercise.
Other Findings
Several studies have taken muscle biopsies before and after exercise and examined them under an electron microscope. The myofibers before exercise have a normal arrangement but the post exercise myofibers show a completely disrupted pattern. This disruption is referred to as "micro injury" or "micro tears". These tears appear to effect only a small percentage of fibers in the muscle. It requires a considerable amount of force to cause these tears such as those from heavy weight training.
It is also interesting that these injuries result from the eccentric (lowering) contraction portion of the exercise. This explains why it is so important to pump out those last couple of reps and make a concentrated effort to lower the weight slowly in strict form to maximize the stimulation of growth through maximum fiber recruitment.
Process Of Muscle Damage
It is extremely important to visualize what is happening in the muscle to fully understand how it is functioning during the execution of resistance exercises. There are several components required for a muscle to contract. In the most basic terms, the muscle requires ATP, Calcium, and water. Muscle damage occurs in two different phases. The first phase begins immediately after the exercise is completed.
Phase One
In this phase, the tension the muscle undergoes during the contractions causes channels to open in the muscle fibers. Especially during the eccentric portion of the exercise when the muscles are lengthening that these channels allow Calcium to pour into the interior of the muscle cell. As a result of this influx, there is an activation of structures known as lysosomes. It is these guys' job to specialize in the digestion of proteins. This results in the beginning of the destruction of the muscle.
The Calcium also activates these other guys called Phospholipase A2. These guys' job is to punch little holes in the membrane of the muscle. The result of their work also causes an increase in the number of free radicals. Basically, what is happening here is that once Calcium enters the muscle cells it activates a self-destruct mechanism within the muscle.
Phase Two
The next phase begins after three or four hours have passed. As a result of the first phase damage, it is this second phase's job to begin to mount an assault on the damaged area to clear away the remaining debris. The release of inflammatory chemicals induces swelling in the damaged area that results in more soreness. You also notice an increase in soreness known as "second day lag" which occurs as a result of this phase's efficient and increased handy work.
The result of all this destruction to the cell membrane opens microscopic holes allowing several other substances like growth factors to leak out of the cell. It is theorized that through the release of these substances that the growth factors have an affect on the synthesis of protein in the surrounding cells. This process may result in satellite cell proliferation.
Satellite Cell Proliferation
Satellite cells consist of just a nucleus and it is within the nucleus of the cell that protein synthesis takes place. Growth factors can cause the satellite cells to combine with the damaged cells and assist in the damaged cells ability to synthesize protein more efficiently. The satellite cells can fuse to each other and create new muscle cells in a process know as hyperplasia. Studies have proven this true.
What all this means is that when cells are damaged through exercise, several processes take place to release growth factors. This results in satellite cells combining with other cells to create new muscle cells. They may also merge with existing muscle fibers to promote the repair process through the enhancement of protein synthesis.
The theory of Muscle Damage is very popular and has many supporters; it is only fair to point out that is not without its weak points. One point that seems to stand out is that assuming that the signal for muscle growth is muscle damage than anything that reduces the amount of muscle damage should also reduce the corresponding muscle hypertrophy.
HMB Effects 
Recent studies have shown that through the supplementation of HMB, a supplement that reduces exercise induced muscle damage, actually increased muscle hypertrophy to some extent. It seems that there is more going on here than meets the eye. There appears to be many different and inter-related chemical and metabolic processes working in unison that produces the desired muscle growth.
The information gained through scientific research and the utilization of different stimulus and training techniques will further increase our knowledge. This information will lead us to the more specific requirements needed to trigger the growth mechanism into motion. Once the process is set into motion, we have to then begin to further investigate the elements that the process utilizes to remain in motion.
The resulting interactions will then need to be further studied in greater detail in order to decipher their role that results in the actual muscle growth.
Conclusion
This is truly an interesting and relatively virgin area of research that should eventually lead us to better training techniques, faster and more complete recovery response and superior supplementation formulations to further enhance NATURAL musculature development processes beyond normal limitations. The human body has so many unyielded mysteries that we may never truly realize all its' wondrous complexities and capabilities.
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