A review of more than 60 training studies indicates training with weight loads between 65–85% of your 1RM is the sweet spot for hypertrophy.
Trained subjects were divided into high-resistance and low-resistance groups. The high group used 70–80% of its 1RM. The low group used 30–50% of its 1RM for three sets of seven exercises targeting all major muscles. Ninety seconds of rest was taken between sets. The groups trained three times per week for eight weeks, with all sets taken to failure.
Both high and low groups produced increases in thickness of the biceps (5.3% vs. 8.6%, respectively), triceps (6% vs. 5.2%, respectively), and quadriceps (9.3% vs. 9.5%, respectively). Improvements in strength were different, however, with the high group showing better improvements in strength.
This study shows that lighter-weight, high-rep training taken to failure can produce equivalent gains in size as more traditional weight loads and rep ranges, even in trained subjects.
Although this study was able to demonstrate equal gains in overall muscle size with 25- to 35-rep sets as well as eight- to 12-rep sets, it did not identify which fibers contributed most to the growth. It is likely that the high-rep training resulted in more slow-twitch fiber growth, and the low-rep training produced fast-twitch fiber growth. Ideally, you would want to stimulate the growth of both types of fibers to maximize gains. So incorporate both high- and low-rep training into your routine.