The claim is that this is the optimal repetition range for building muscle.
The origin: In 1954, Ian Macqueen, M.D., an English surgeon and competitive bodybuilder, published a scientific paper in which he recommended a moderately high number of repetitions for muscle growth.
The truth: This approach places the muscles under a medium amount of tension for a medium amount of time, making it both effective for and detrimental to maximum muscle gains.
A quick science
The new standard: Vary your repetition range — adjusting the weights accordingly — so that you stimulate every type of muscle growth. Try this method for a month, performing three full-body sessions a week: Do five repetitions per set in your first workout, 10 reps per set in your second workout, and 15 per set in your third workout.