"How many reps should I do each set to build the most muscle?" — Ben T., Santa Fe, NM
The number of reps you need to perform should change over time, just like the exercises themselves.
Sets of one to five reps do the most to build pure strength, but they also activate your biggest muscle fibers—so even if getting strong isn’t something you care about, don’t neglect them.
Sets of six to 12 reps maximize muscle size and can be used on most of your lifts most of the time. Higher reps—more than 12—should make up a smaller portion of your training until you’re experienced. At that point, sets of 20 or so reps on leg exercises (done with perfect form) can shock the quads and hamstrings into new growth.
Finally, low-rep sets, because they imply heavy weights, should be used sparingly—two or three times a week on only a couple of exercises.