Unlike time under tension—the amount of time a muscle spends under load, which actually does nothing for hypertrophy—mechanical tension kicks in the moment your reps begin to naturally slow down as you approach muscle failure. “Effective reps are defined as the last five or so repetitions in a set that brings the muscle to failure, and these reps are believed to provide the most stimulus for hypertrophy due to the mechanical tension being experienced by the active fibers,” says Welch. “The closer you get to failure, the more motor units and muscle fibers you recruit, and the greater chance you have to increase the size of your muscle,” says Carlson.