The different rep ranges cause a different type of stress to our body and it responds with adaptation on a various levels. To be able to reach our specific goals we have to understand the way our body works and what stimulation we need to achieve them.When you train with lower reps (1-5) the body responds with adaptation primarily on neurological level: Your body develops its ability to activate more motor units (muscle fibers) by increasing the frequency of the neural impulses send from the brain. In the same time is improving the innermuscle cordination and the coordination between the muscle groups. All these changes are making us stronger but the hypertrophy of the muscles (muscle gains) is minimal. That explains why some athletes like powerlifters or the Olympic weightlifters can be enormously strong without having the musculature of professional bodybuilder.

When you train with medium reps (6-12) the adaptation is more metabolic and cellular and only moderately neurological. In other words when you train in this rep range you will get bigger and stronger, but the strength gains are not maximal like in the training with lower reps (1-5).

This is why that rep range is the most recommendable for bodybuilding and hypertrophy.

When you train with higher reps (13-20+) the adaptation is mostly on metabolic and cellular level. This rep range stimulates the local muscle endurance and small hypertrophy in a certain cellular components (sarcoplasmic hypertrophy) like the mitochondria and the capillaries and very little strength.

There is no clear dividing line where neural adaptations end and metabolic/structural begin.

For example when you train in 6-8 rep range the adaptation is somewhere neurological and metabolic/structural. In this rep range you get great strength gains and also great hypertrophy. In the 8-12 rep there is still neurological adaptation but less from the 6-8 range and much less than 1-5 range. This is the rep range that will give you maximum pure hypertrophy. You will also get stronger, but not so much like with the lower rep ranges.

Rep range	Percent of 1 rep max	Training Effect	Goal desired
1-5 reps	85-100%	Neurologic	Strength & power, little hypertrophy
6-8 reps	75-85%	Neurologic & metabolic/Structural	Strength & Hypertrophy
9-12 reps	70-75%	Metabolic/Structural & Neurologic	Hypertrophy & some strength
13-20+ reps	60-70%	Metabolic/Structural	local endurance, some hypertrophy, little strength

So if your goal is to develop your strength and power the larger part of your training must be in 1-5 rep range.

If your goal is bigger muscle mass and more size the majority of your training must be 6-12 rep range, but you should also do a little bit of training in the 3-5 rep range to build strength and power which will later help you to build more size by increasing the working weights and the stress on your muscles. You should also do a little bit of training in 13-20+ rep range to build mitochondrial density and increase capillarization (sarcoplasmic hypertrophy) and to stimulate the slow twitch muscle fibers to grow.

If you want to increase your endurance stay in the 13-20 + rep range and emphasize on aerobic training.