Hypertrophy is Optimal at 8-12 Repetitions

1-5 reps primarily develop strength, with more impact on muscle size and none on endurance. 6-12 reps develop a balance of strength, muscle size and endurance. 13-20 reps develop endurance, with some increases to muscle size and limited impact on strength.

Total mechanical tension, which is a key hypertrophy stimulus, can be maximized through moderate relative loads (e.g. 75% of 1RM) and high volume training (e.g. 10 sets of 10 reps) in comparison with high relative loads (e.g. 90% of 1RM) and moderate volume training (6 sets of 4 reps).