The premise for most training models, especially those that incorporate periodization, is founded on the research of Hans Selye from the 1930s. Selye studied the effects of chronic stress on rats. He found that exposing rats to toxic levels of “noxious agents” and even swimming to exhaustion caused a systemic reaction (mainly thymus involution and adrenal hyperplasia), which he termed the stress response. He divided this stress response into three stages: alarm, resistance, and exhaustion, and called it the General Adaptation Syndrome (GAS). The GAS model states that chronic exposure to a particular stressor may lead to an exhaustion phase in which adaptation is no longer possible. Therefore, the objective of periodization is to alter volume, intensity, and training frequency to maximize performance and reduce the odds of overtraining. Thus, GAS and periodization eventually found their way into bodybuilding training.
Contrary to what is commonly believed, there is little evidence that a periodized program leads to greater muscle growth compared with a nonperiodized program that includes progressive overload.
Some initial research comparing periodized with nonperiodized training reported an advantage for periodized programs, but these studies used underwater weighing and other imprecise methods to determine changes in muscle mass.
When very precise methods of measuring muscle mass (i.e., magnetic resonance imaging) were used, there have been no differences observed in at least seven studies between periodized and non-periodized programs measuring muscle growth.
Periodization as either linear or undulating should not be expected to produce any better gains than a nonperiodized program.
Unlike traditional periodization based on the GAS model, HST’s structure is based on principles of progressive load and the “repeated bout effect,” not stress. The repeated bout effect refers to the adaptation whereby a single bout of resistance exercise protects against muscle damage from subsequent exercise bouts. Over time, the repeated bout effect inhibits not only muscle damage but also muscle growth. The cyclical nature of HST incorporates continually increasing loads followed by a period of “strategic deconditioning” to undo some of the repeated bout effect.