This is an excerpt from Science and Development of Muscle by Brad Schoenfeld.
Hypertrophy can be achieved in all loading zones. Low-load training emphasizes metabolic stress and promotes the greatest increases in local muscular endurance, whereas low-repetition, high-load training requires high mechanical tension and enhances the ability to lift heavier loads as a result of greater neural adaptations. There appears to be a fiber type - specific response in which heavy-load training produces greater cross-sectional area increases in Type II fibers and light loads have a preferential effect on Type I hypertrophy. Thus, if the primary goal is maximizing hypertrophy without regard to strength-related factors, then training across a wide spectrum of repetition ranges (1 through 20+) is recommended to exploit all possible avenues for the complete development of the whole muscle. There is merit to focusing on a medium-repetition range (6- to 12RM), because it provides high levels of mechanical tension sufficient to stimulate the full array of fiber types while allowing for sufficient training volumes. Incorporating heavy loading (1- to 5RM) enhances strength, which ultimately allows the use of heavier loads during medium-repetition lifting. Additionally, light-load training should be included both to ensure the optimal development of Type I fibers and to improve the buffering capacity of muscle so that additional repetitions can be performed at a given medium intensity of load.
On the other hand, if the goal is to promote hypertrophy to maximize strength, there appears little reason to employ loads less than approximately 70% of 1RM. The compelling body of research indicates the presence of a strength - endurance continuum, in which lighter loads promote the ability to carry out submaximal resistive efforts at the expense of maximal force production. Increases in Type I fiber hypertrophy, as would be expected when training with low loads, have limited transfer to strength-related improvements.
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