Vary your rep ranges to build muscles.

To Build Muscle, Vary Your Reps – The optimal number of reps for hypertrophy is a source of ongoing debate. But evidence indicates that a moderate rep range (about 6-12 reps per set) is best for maximizing muscle growth. This is often referred to as “bodybuilding-style training” since it provides the ideal combination of mechanical tension, muscle damage, and metabolic stress – the 3 primary factors involved in hypertrophic gains. The problem is, most lifters seem to think this means all training should be carried out in this rep range and thus they rigidly adhere to the same loading patterns. Wrong assumption.
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Maximal muscular development is built on a foundation of strength. This mandates that at least some of your sets need to be carried out in the lower rep ranges (1-5 reps per set). Stronger muscles allow you to use heavier weights and generate greater muscle tension in those moderate rep ranges. By increasing muscle tension without compromising metabolic stress, you’re setting the stage for enhanced growth.
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On the other end of the spectrum, high-rep sets (15-20 per set) also have a place in a hypertrophy-oriented routine. Provided that you train at or near your sub-rep max, these sets help to increase your lactate threshold, the point at which lactic acid rapidly begins to accumulate in working muscles. The problem with lactic acid is that beyond a certain point, its accumulation interferes with muscle contraction, reducing the number of reps you can do.
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Here’s the good news: Higher-rep training increases capillary density and improves muscle buffering capacity, both of which help to delay lactic buildup. The upshot is, you’re able to maintain a greater time under tension at a given hypertrophy-oriented workload. In addition, you develop a greater tolerance for higher volumes of work – an important component for maximizing hypertrophy.
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Optimal muscle growth is achieved by varying your rep range over time. Sure, hypertrophy training is probably best attained with moderate-rep sets, but higher and lower intensities are important for optimizing muscular development.