When you train with low reps (1 – 5), the adaptations that make you stronger are mostly neurological: You develop an increased ability to recruit more muscle fibers, you stimulate the higher threshold fibers that are not activated with high rep, low weight sets, you decrease neuromuscular inhibition, and there is increased coordination between the muscle groups. However, with low reps, the hypertrophy (size increase) of the muscle fibers is minimal.
In other words, reps under 6 make you stronger, but they don’t necessarily make you bigger because the strength gains come from adaptations in the nervous system – the muscle fibers and other muscle cell structures do not hypertrophy (enlarge). This explains why certain athletes, powerlifters and Olympic lifters can be wicked strong but they don’t look as strong as they are.
When you train with medium reps (6-12) the adaptations are more metabolic and cellular and only moderately neurological. This is why 6-12 reps is the range most often recommended for bodybuilding and hypertrophy. You get bigger and stronger in this rep range, but your strength gains are not maximal. This explains why some bodybuilders look stronger than they are (and why they are often the brunt of jokes made by powerlifters and weight lifters; i.e. “big, weak, slow, useless muscles”, ha ha).
When you train with higher reps (13-20+), the adaptations are mostly metabolic and cellular. This rep range produces local muscular endurance, a small degree of hypertrophy in certain cellular components such as the mitochondria and the capillaries, and very little strength.
