Perhaps the biggest question among fitness enthusiasts centers on what constitutes the most ideal rep range. First off, you need to understand that there is no magic rep scheme. The best rep scheme is really one that incorporates a variety of low, moderate, and high reps, but the majority of your reps should stay in a certain range dependent on whether your goals are to burn fat or increase strength. Different rep ranges tend to provide different results. Let me explain how each rep range has its own benefit.

Low Reps

Low reps are typically defined as anywhere from 1-5 reps (a corresponding weight used where you reach momentary muscular failure at that rep range). These low reps place the most load on the body since it’s the heaviest amount of weight. This is the rep range most often used by powerlifters because it tends to put on the most power and strength in the quickest time possible. This range puts the most demand on our central nervous system and uses ATP and creatine phosphate almost exclusively for fuel, which explains the lack of muscular pumps. The time under tension isn’t great enough for the muscle to be gorged with blood to get a blood pump. It is often believed that low reps will stimulate the fast twitch muscle fibers while the high reps will stimulate the slow twitch muscle fibers. The truth is that low reps will stimulate all the muscle fibers, from slow to intermediate to fast. When a load is placed on a muscle, the slow twitch fibers are used first. If the slow twitch fibers cannot generate enough force to lift the weight then intermediate fibers are activated. If you still can’t lift the weight then the fast twitch fibers will finally be recruited.

Low reps also stimulate myofibrillar hypertrophy, which is when the actin and myosin contractile proteins increase in number and add to muscular strength as well as a small increase in the size of the muscle fibers. This is important because progressive overload is one of the primary necessities for continued long-term increases in strength. This low rep range will certainly build some serious strength but it won’t put on as much muscle as the other rep ranges due to the fact that the time under tension is limited to create a blood pump and hypertrophy.

Moderate Reps

Moderate reps are typically defined as anywhere from 6-12 reps. Numerous studies have confirmed that moderate rep ranges lead to the greatest amount of muscular growth. This range is often referred to as the “hypertrophy” or muscle building range. This range taxes the nervous system, though not nearly as much as with the power rep range. This is usually the amount of reps that bodybuilders do due to the propensity for putting on muscle mass faster. This faster muscle growth is realized because the time under tension is significantly increased. In other words, the weight is heavy enough to create a stimulus and the muscle is placed under stress for a longer period of time, meaning the total amount of work on the muscle is enhanced. This increased time under tension will stimulate sacroplasmic hypertrophy, which is when the volume of sarcoplasmic fluid and other non-contractile proteins within the muscle cell increases with no accompanying increase in muscular strength. This is primarily induced by lifting light loads for higher reps, which is great for muscle size and increasing definition, but not as beneficial for gains in strength.

So in conclusion, since doing low reps with heavy weight are best at stimulating myofibrillar hypertrophy, and higher reps are best at stimulating sacroplasmic hypertrophy, moderate reps are the perfect balance between both forms of hypertrophy. This is a huge reason why most people see the best gains with this rep range. This rep range uses ATP, creatine phosphate, and glucose as fuel. Using glucose as fuel can explain the muscular pumps. This rep range is the perfect blend of size and strength and can really put on lean muscle.

High Reps

High reps are typically defined as anything around or above 15 reps. Since low reps stimulate all the muscle fibers and result in the best strength gains and moderate reps results in the most hypertrophy, there may seem like no place for doing high reps. However, this is not the case. High reps cause a large glycogen depletion during your workout, which will actually increase the amount of glycogen the muscle stores and lead to great fat loss. As a result, this extra glycogen will pull even more water into the cell. This results in the cells stretching and leading to greater overall muscle growth and the release of anabolic hormones.

High rep training is also terrific for increasing muscular endurance, or the amount of time your muscles are able to perform work with light to moderate weights. In addition, greater occlusion happens with high rep training. Higher rep training can prevent blood from leaving the area being trained, which can help spur even more muscle growth.

High Reps vs. Low Reps | The Routine

Since we learned all about the different rep ranges, let’s put it all together and figure out what is best for your specific goals.

Primary Goal – Increasing Strength:
Strength – Under 6 reps (80-100% of exercise volume)
Hypertrophy – 6-15 reps (0-20% of exercise volume)
Endurance – 15+ reps (0-10% of exercise volume)

Primary Goal – Burning Fat:
Strength – Under 6 reps (0-15% of exercise volume)
Hypertrophy – 6-15 reps (70-85% of exercise volume)
Endurance – 15+ reps (15% of exercise volume)

Source

http://www.muscleandstrength.com/articles/truth-rep-ranges-muscle-growth

http://www.mensfitness.com/training/your-ideal-rep-range

http://www.builtlean.com/2012/07/19/high-reps-vs-low-reps/