From the HST FAQ, quoting the founder of HST, -Bryan Haycock (a man who knows his stuff!!!): <div></div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">Should I vary exercises?
Should a person “switch” exercises frequently? Well, that depends. If she is going from an exercise that doesn’t stretch the muscle significantly, to an exercise that does, then Yes. You will benefit from switching, but only after the muscle has “adapted” to the exercise with less stretch involved.
Switching exercises that require a high degree of neural skill (most compound exercises) is generally less productive. The hypertrophic response is delayed according to the duration of learning to manifest the neural drive necessary to generate the tension on the muscle fibers for microtrauma to happen. This is generally not a concern with isolation exercises.
As to the value of Weider’s Confusion principle (sorry couldn’t resist), lets consider what we know both about strength, and the physical properties of muscle tissue.
The foundation for the development of strength is neuromuscular in nature. Increases in strength from resistance exercise have been attributed to several neural adaptations including altered recruitment patterns, rate coding, motor unit synchronization, reflex potentiation, prime-mover antagonist activity, and prime-mover agonist activity. Aside from incremental changes in the number of contractile filaments, voluntary force production is largely a matter of "activating" motor units. In order to ascertain the relative contribution of each of these mechanisms, various measurement techniques have been utilized. We can go into this if you wish, but it is largely just an exercise in motor-unit physiology with little applicable value to muscle hypertrophy.
One study might be worth mentioning though. Hakkinen and co-workers have shown that there is an increase in EMG activity with strength training as well as a decrease in EMG activity upon cessation of training (Hakkinen,1983). Male subjects accustomed to weight training went through progressive strength training of combined concentric and eccentric contractions three times per week for 16 wk. The active training period was followed by an 8 week detraining period (not to be confused with SD). The training program consisted mainly of leg extensions with the loads of 80-120% of one maximum concentric repetition (1RM). Significant improvements in muscle function were observed in early conditioning; however, the increase in maximal force during the very late training period was greatly limited. Marked improvements in muscle strength were accompanied by significant increases in the neural activation (EMG) of the quads.
And as you might expect, during detraining, there was a rapid decline in EMG activity patterns.
Now here is something interesting and illustrative at the same time. The relationship between EMG activity and high voluntary forces varied during the training period. The occurrence of these changes varied during the entire course of training. This points out the fact that other neurological factors are involved such as rate coding, motor unit synchronization, reflex potentiation, prime-mover antagonist activity, and prime-mover agonist activity.
Now with respect to the question of changing exercises every 2 weeks or so, you have to ask yourself why? Keep in mind that there are entire textbooks devoted to the field of “motor learning”. It is a very complicated field of study because it involves the whole organism or being. It involves the brain to a much greater extent than the muscle tissue. This of course makes it NOT hypertrophy-specific. Anyway, people are used to asking the question why, they just aren’t accustomed to having to answer it without simply deferring to some widely accepted authoritative source, or simply saying “it makes it harder”. “Harder” is a subjective term related to another subjective term “intensity”. Intensity is only important as it defines the amount of tension applied to the muscle, not how heavy it feels that day or how hard it is to lift when you are really tired.</div>