Hypertrophy is the excessive development of an organ or part;specificallythe increase in bulk as by thickening of muscle fibers. The best rep range to build muscle (hypertrophy) depends on which body parts you are training. Both upper body and lower body exercises require different rep ranges to build muscle. The best rep range to build muscle in the upper body is 8-12 reps. The best rep range to build muscle in the lower body is 15-25 reps. Now let me explain why and I'll start with the upper body. Low rep ranges of 2-5 reps are great for building strength but not muscle. I know you are probably wondering dont the two go hand and hand if I get stronger aren't I building muscle. Yes, but lets look at the physiques of those who compete in the World Strongest Man competitions and those who compete in body building competitions. Participants of the World's strongest man are the strongest individuals in the world yet many of them do not have the muscularity of bodybuilding champions.
World Strongest Man Competitor
This is because they utilize low reps ranges in their routines because low reps help you put on enormous amounts of strength. But low rep ranges fail to stimulate the muscle in a way that brings about muscle hypertrophy. To build muscle effectively you must stimulate your muscles in a way that brings about hypertrophy and this is done by doing sets of 8-12 reps. So this means you will choose a weight that only allows you to perform a max of 12 reps but no less than 8 reps. If you do more reps than 12 the weight is too light and you are building muscular endurance more than muscle hypertrophy. If you are doing less than 8 reps the weight is to heavy. Now when it comes to lower body exercises and the correct range of 15-25 reps this is due to the fact our legs have adapted to us being up on them all the time. To effectively build muscle in the quads, hamstrings, glutes, calves you must do 15-25 reps. So keep your rep ranges in your lifting routines in the ranges mentioned and you are well on your way to building muscle and strength fast.