Hypertrophy

Hypertrophy is an increase in the size of muscle tissue.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 18.} With conventional training, most growth comes from adding sarcomeres in parallel within existing fibers, making each fiber thicker.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 18.} New fibers contribute little, if anything, to the increase in whole-muscle size.^{2Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 59.}

Resistance training enlarges both type I and type II muscle fibers, though type II fibers usually grow more and shrink more when a client stops training.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 91.}

In practice

Loads as light as 30% of a one rep max can produce similar muscle growth to heavier loads, provided the sets are hard.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 422.} Israetel and colleagues define a hard set as one stopped 5 or fewer reps short of failure, using about 30% to 85% of the client’s one-rep max.^{5Israetel et al., Scientific Principles of Hypertrophy Training, p. 60.}