Muscle fibers can be classified as either slow twitch (type I) or fast twitch (type II). Slow-twitch fibers are dependent on slow-oxidative energy pathways and fatigue slowly. Fast-twitch fibers are dependent on fast-oxidative energy pathways and fatigue quickly.


Because all muscle groups have a mix of fast- and slow-twitch fibers, in order to make sure the slow-twitch fibers get a good growth stimulus, you must fatigue them with high reps. Without high-rep sets, slow-twitch fibers will never get fatigued enough to grow.


Hypertrophy isn’t necessarily triggered by fiber fatigue. Mechanical loading is the primary stimulus for muscle growth. If mechanical load is the stimulus for growth, then all fibers should hypertrophy as load goes up.


  • Muscle fibers are “mechanocytes” and respond to mechanical strain biochemically through a process called mechanotransduction.
  • Researchers from Ohio University compared three different rep schemes (high = 20–28 reps, intermediate = 9–11 reps, low = 3–5 reps) for their effects on muscle-fiber hypertrophy. They demonstrated that the degree of hypertrophy of all fiber types was dependent on the weight loads used, with heavier weight producing greater hypertrophy in all fiber types.
  • In the Ohio University study cited above, using reps from 20–28 taken to failure increased slow-twitch fiber growth by 10.3%, whereas reps from 3–5 taken to failure increased slow-twitch fiber growth by 12.4%.


All fiber types will hypertrophy when using weight loads that are sufficient to stimulate growth.


An effective training plan designed to elicit maximum gains in size should include both high and low reps, but fiber type should not be the guiding principle. – FLEX