Whether you make your own training plans, or you follow the plans of a trainer, you have seen things like 3 sets of 10 reps, 4 sets of 3 reps, or eve 3 sets of 20 reps. However, you may not understand why we are completing different rep ranges. Understanding why you are training different rep ranges can be crucial in understanding what you are trying to accomplish, as well as how rep ranges can be manipulated to target specific goals. Some reps ranges are great for power and strength, while others can be good for endurance. One of the biggest keys to understand is that these rep ranges are not black and white but exists on a continuum. Just because some targets strength most does not mean it can not have a positive effect on endurance. All rep ranges can be useful if used at the right place and right time, but it is important to understand what the main effect of certain rep ranges are.
This range of repetitions is the main driver for increased muscle strength and power. The main metabolism being used is the small amount of ATP stored within the muscle and Creatine Phosphate. Metabolisms used in rep ranges play a role in what the training effects will be, and the metabolism being used in the rep range to the type of energy that producing strong, powerful muscle contractions. When training low reps, such as 5 reps and below, you are likely using heavy weight, which causes the body to recruit the largest muscle fibers it has, resulting in the strongest contractions it can create. The secondary purpose of this rep range can be muscle growth as your muscles will likely grow larger to help promote strength improvement.
6-12 repetitions are the main range used by those trying to grow larger muscles. This rep range relies more on glycolysis for energy, as each set normally lasts longer than a set of 5 will. 6-12 reps will also recruit large muscle fibers, potentially not as large as the fibers used for strength range (unless you go to complete failure), causing your large muscles to grow larger. If size is your goal, this is the main rep range you should be training. You will also increase in strength in this rep range as your muscles grow larger, because larger muscles (by cross sectional area/width) can lift heavier weights.
12 Reps and More
When you begin training 15, 20, or maybe 30 reps, your main goal is muscle endurance. Your body is using an oxidative metabolism, meaning you are using oxygen to burn fat, resulting in weaker contractions that will not cause your body to get as tired. You will be using lighter weights which will recruit smaller muscles, these muscles are more resistant to getting tired and will result in longer lasting energy. There is the possibility of putting on muscle mass with this rep range, if you use a slightly heavier weight and take your sets to failure, as the smaller muscles begin to tire, your body may recruit larger ones to finish out those last couple reps.
BS Exercise Science