This rep range is typically defined as the 6-12 rep range. Moderate rep ranges have consistently been proven in study after study to lead to the greatest amount of growth. The reason that this rep range is so effective for building muscle is because it does a little bit a everything.
This means that it provides many of the benefits of low rep training combined with the benefits high rep training by allowing for relatively heavy loads to be used while increasing time under tension. The heavy loads allow for myofibrillar protein synthesis to take place which, as discussed, will increase the size of the contractile proteins. The increased time under tension will stimulate sarcoplasmic hypertrophy.
Sarcoplasmic hypertrophy is an increase of the sarcoplasm and other non-contractile proteins within muscle cells and is primarily induced by lifting light loads for higher reps. This type of growth, although not typically accompanied by any strength gains, is the primary reason why bodybuilders tend to be more muscular than strength and power athletes.
Moderate rep training also induces an excellent muscle pump. While the pump is often thought of as a short-term training effect, it may possibly result in greater growth. Studies show that cellular swelling causes both an increase in protein synthesis and a decrease in protein breakdown (Grant et al., 2000; Stoll et al., 1992; Millar et al., 1997).
So while low reps with heavy weight is best at stimulating myofibrillar hypertrophy, and high reps with light weight is best at stimulating sarcoplasmic hypertrophy, moderate reps seem to strike a balance between inducing significant amounts of both myofibrillar and sarcoplasmic hypertrophy. The proven track record of the moderate rep range makes it so that it cannot be ignored in your training routine.
High reps are usually considered to be any set that contains 15 reps or more. There are many that argue, since low reps stimulate all the muscle fibers and moderate reps induce sarcoplasmic protein synthesis, that there is really no need to do high rep sets. At first this sounds like sound reasoning, but it leaves out one very important factor. This important factor is the effect of glycogen on protein synthesis.
Glycogen is essentially stored carbohydrate within muscle tissue. Glycogen is hydrophillic, it causes muscles to swell since every gram of glycogen stores 2.7 grams of water along with it (Chan et al. 1982). I know many of your are thinking, “why would I want my muscles packed with water?” Besides the fact that this added water will increase the size of your muscles, it will also increase protein synthesis.
Many people do not realize that cellular hydration is an extremely strong anabolic trigger.Protein synthesis is often directly related to a muscles cells state of hydration. In response to increased cellular hydration, the cell initiates a signaling cascade that causes the muscle to grow larger to protect itself.
So what does this all have to do with high rep training? High rep training will drastically deplete glycogen stores. At first this may sound counterproductive but the body will react to this depletion by increasing muscular glycogen stores. In the long run this will allow cells to stretch and lead to greater overall muscle growth and release of anabolic hormones.
In addition to all of the above benefits, greater occlusion is associated with higher rep training. This prevents blood from leaving the area being trained, which can induce growth through increases in growth factor production and possibly satellite cell fusion (Vierck et al., 2000).