Intensity
Intensity (i.e., load) has been shown to have a significant impact on muscle hypertrophy and is arguably the most important exercise variable for stimulating muscle growth (42). Intensity is customarily expressed as a percentage of 1RM and equates to the number of repetitions that can be performed with a given weight. Repetitions can be classified into 3 basic ranges: low (1–5), moderate (6–12), and high (15+). Each of these repetition ranges will involve the use of different energy systems and tax the neuromuscular system in different ways, impacting the extent of the hypertrophic response.
The use of high repetitions has generally proven to be inferior to moderate and lower repetition ranges in eliciting increases in muscle hypertrophy (24,71). In the absence of artificially induced ischemia (i.e., occlusion training), a load less than approximately 65% of 1RM is not considered suffi- cient to promote substantial hypertrophy (115). Although such high rep training can bring about significant metabolic stress, the load is inadequate to recruit and fatigue the highest threshold MUs.
Whether low reps or moderate reps evoke a greater hypertrophic response has been a matter of debate, and both produce significant gains in muscle growth (24). However, there is a prevailing belief that a moderate range of approx- imately 6–12 reps optimizes the hypertrophic response (86,89,205).
The anabolic superiority of moderate repetitions has been attributed to factors associated with metabolic stress. Although low repetition sets are carried out almost exclusively by the phosphocreatine system, moderate repetition schemes rely heavily on anaerobic glycolysis (144). This results in a significant buildup of metabolites. Studies of bodybuilding- style exercise routines performed with multiple sets of 6–12 reps show significant postexercise declines in ATP, creatine phosphate, and glycogen, along with marked increases in blood lactate, intramuscular lactate, glucose and glucose- 6-phosphate (37,178). Buildup of these metabolites has been shown to have a significant impact on anabolic processes (96). It is therefore conceivable that there is a maximum threshold for tension-induced hypertrophy, above which metabolic factors become more important than additional increases in load.