This is an excerpt from Jim Stoppani's Encyclopedia of Muscle & Strength-3rd Edition by Jim Stoppani.
The amount of resistance used for a set is inversely related to the number of repetitions performed. That is, the heavier the weight, the fewer the repetitions that can be performed.
Today, thanks to the many years of trial and error by athletes and the numerous research studies to confirm the original inclinations, it is now well established that using certain resistance intensities provides corresponding results. This information can be used in designing optimal rep ranges as seen in table 2.4. In this table, optimal rep ranges span from 1 to 20+. On the lower end, strength gains are more pronounced, particularly when performing sets of one to six reps using maximal resistances (about 1RM to 6RM). Enhanced muscle hypertrophy is most notable when training with repetition maximums in the 7 to 20 range, which corresponds to about 7RM to 20RM. And muscular endurance benefits occur when repetition maximums of 20 and above, or 20RM+, are used. Evidence suggests that these higher rep ranges are also effective for muscle hypertrophy as long as sets are taken to muscle failure (Burd et al. 2010; Burd et al. 2011; Mitchell et al. 2012).
As you’ll notice in table 2.4, one to six reps is an ideal range for both maximal strength and muscle power, but resistance should be much lighter when training for power since the reps should be performed as fast as possible. When training for power, sets should not be taken to failure. When training for strength, many sets will be taken to failure.
These varied muscle adaptations underscore the importance of periodization for producing the most desirable changes in a muscle, whether the person’s goal is increasing muscle endurance or increasing maximal strength. This is because each adaptation is related to the others. For example, increasing both maximal strength and muscle endurance beneficially affects muscle hypertrophy. So while the person should spend the majority of training time using the repetition range that best fits his or her major goals, the periodic cycling of other intensities will enhance this goal.