Low reps are usually categorized as reps in the 1-5 range. It is often said that low reps will stimulate fast twitch muscle fibers while high reps stimulate the slow twitch muscle fibers. This is yet another false fact about rep ranges. The truth is that low reps will stimulate ALL muscle fibers from slow to intermediate to fast and everything in between.
The body calls fibers into play on an as needed basis in order from slow to intermediate to fast. When a load is placed on a muscle, the slow twitch fibers will be recruited first. If the slow twitch fibers cannot generate enough force to lift the weight then the body will call the intermediate fibers into action.
If the slow and intermediate fibers cannot handle the weight or tire out then the fast twitch fibers will finally be recruited. When fibers are recruited they are never recruited half way or partially. When a fiber contracts, it will contract maximally (Saladin, 2007), so this means when you lift a heavy load you will fully stimulate slow and intermediate muscle fibers.
Low reps are also effective for stimulating myofibrillar hypertrophy. Myofibrillar hypertrophy is an increase in the number and size of the actin and myosin filaments within muscle tissue. This type of hypertrophy is accompanied by strength gains since it involves an increase in the contractile tissue (Zatsiorsky, 2006). This is important because progressive overload is one of the primary necessities for continued long term growth. So you can see that very heavy weight for low reps is vitally important for maximum growth.
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.