In this quick read, I am using an example of bodybuilding to explain the rationale behind the 8-12 rep range (i.e. an exercise set consisting of a range of repetitions from 8 to 12). Regardless of your goals, the basic principles that make resistance training effective can be applied to any similar exercise. Even if you are not trying to win any bodybuilding competitions, you still want to build or strengthen your skeletal muscle. Note that the basic principle discussed in this blog are not universal, but span a diverse range of training protocols.
Bodybuilders are athletes whose goal is to achieve hypertrophy, or muscle growth in specific areas of the body. This port is likened to an art form similar to 3-dimensional sculpture, where the medium is the artist's body itself, and the method consists of both addition and subtraction of, in this case, muscle.
A typical bodybuilding resistance set can can consist of failure sets, wherein the muscle in question is exercised until it can no longer perform the work with proper form. With each rep, the muscle tears so that it can later heal into a larger muscle fiber. This tearing and rebuilding into larger muscle fibers is called sarcoplasmic hypertrophy, and is commonly triggered by high repetitions of an exercise. In order to perform a higher number of repetitions, bodybuilders typically start with a lower percentage of their one-rep-max. Given this, you can see why the number of sets matter as much as the number of repetitions, or reps when trying to illicit muscle growth.
The 8-12 rep range is a common part of bodybuilding-type resistance workouts, but the key to understanding why this this range was chosen is the concept of Time Under Tension. Within the exercise, there are three parts: The eccentric (a.k.a "the negative"), isometric hold (a.k.a. "the pause") and the concentric (a.k.a "the lift"). In order to perform an efficient rep, all of these three factors must be optimized. For hypertrophy, or muscle growth, the commonly accepted tempo is 2/1/2. This stands for 2-second eccentric, 1-second pause, and 2-second lift. Each rep, in effect, lasts for 5 seconds.
Consider a workout with 5 sets, where the number of reps decreases by 1 rep for each set completed. The upper range, 12 reps at a 2/1/2 tempo, keeps the muscle under tension for approximately 1 minute per set. Because the muscle is relatively fresh, a heavier weight that will cause fatigue after 12 reps is used. As the muscle fatigues throughout the subsequent sets, a lower weight is selected, while increasing the time under tension slightly.
Finally, at the the tail end of the 5 sets, 8 reps at a 3/1/3 tempo, an even lower weight is selected and the time under tension decreases. The total time under tension for the muscle is 5 minutes. Imagine flexing your bicep as hard as you can for 5 minutes! This is essentially what the 8-12 rep range attempts to simulate. Because 1) rest intervals also play an important role in the exercise, and 2) flexing for 5 minutes straight doesn't seem like something anyone would want to do for fun, the exercises are broke up into sets and reps.
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