The above derivative aspects deal with the specificity of adaptation. On a broader scale, the modality of adaptation alters throughout a training career, subsuming three distinct stages: 1) beginner (neuromuscular), 2) intermediate (hypertrophic), and 3) advanced (neuromuscular).
Beginner Trainees
During the initial few months of strength training, as with any non-practiced physical activity, adaptation is largely neurological. Uncoordinated bodies * unaccustomed to lifting weights * do not efficiently lift loaded barbells, do not isolate the intended targeted area, using more musculature and energy than necessary. It is not unusual, for example, for beginners to feel more tension in their forearms than in their shoulders while performing lateral raises, or experience sore abdominals after performing triceps pushdowns. Eventually coordination and motor skills improve, shifting a greater burden on the targeted muscles.
Intermediate Trainees
Subsequently, once the nervous system adequately adapts to skillfully lift weights, it is then obligatory for muscles to hypertrophy in order to confront future overloads. Although several hypotheses exist as to why and how muscles adapt via hypertrophy, it appears a key factor is overcompensation of protein synthesis or an increase in myofibrillar proteins. Greg Bradley-Popovich explained this process in the book Rational Strength Training: Principles & Casebook (available at
www.i-a-r-t.com) as follows:
"During the process of protein synthesis in any cell, energy is consumed in the form of adenosine triphosphate (ATP), the body's energy currency. Muscle contraction, like protein synthesis and the vast majority of other physiological processes, also consumes ATP. The moments of ATP shortage during protein synthesis (as a result of ATP being used for intense muscular contractions) are hypothesized to be of paramount importance. Supposedly, an unknown signal would report to the muscle cell nuclei ordering them to send more protein-building instructions back out into the cell. It is proposed that when a trained muscle attempts to "catch up" on its protein synthesis during rest, it inadvertently overshoots resulting in a supercompensation, or net increase, in the amount of muscle protein. The concept of this competition for energy, called the ATP Deficit Theory, is somewhat analogous to the glycogen supercompensation characteristic of trained muscles."
Evidently, to promote muscle hypertrophy, the activity must be demanding enough to produce this effect. Low intensity endurance training will not sufficiently deplete ATP stores available for protein synthesis. ATP can actually regenerate itself that quickly. Conversely, if the tension time of a set is too brief * apparently under 45 seconds (although this is debatable) while implementing very heavy weights * ATP is taken up quickly, but not in proportion to muscle fatigue; again resulting in lack of ATP store depletion allocated for protein synthesis. This may explain why power and Olympic lifters increase strength (via skill acquisition/adaptive coordination) without the characteristic hypertrophy of bodybuilders. Hence, in order to maximize muscle growth, it is best that sets last at least 45 seconds, but not so long (over 2 minutes) as to promote a high endurance capacity and potential overuse atrophy.
Advanced Trainees
After 18-24 months of proper training (a rarity, indeed), realizing optimum hypertrophic adaptation becomes quite probable. However, this is only true relative to the individual. It would be true of a thirty-year old trainee, whose testosterone levels are slowly declining, but not applicable to a 15-year old teenage boy. However, with all factors being ideal, two years is more than sufficient to realize optimum hypertrophy in the young adult.
Thereafter,
hypertrophy slowly relinquishes its role as a fundamental factor in progressive strength gains, with neurological and psychological factors taking over. Trainees become so used to lifting that improvement continues by implementing better methods of leverage while increasing the participatory rate of surrounding muscle groups to help lift progressively heavier weights * also known as adaptive coordination. Trainees also acquire the mental focus and discipline to better use emotions (e.g., anger) to volitionally 'will' weight up * the psychological factor.
It then becomes a trainee's greatest challenge to eke out a few final pounds of muscle mass, to realize full genetic potential. For a novice, the mere inclusion of resistance training is new and unusual. Consequently, nearly any program - regardless of how poorly designed - elicits a positive effect. (This does not suggest that those with six months or less training experience should refrain from the hyper-training suggestions in this article, but is unnecessary and should remain in reserve for when progress slows considerably.)
However, for the advanced trainee, it is not as simple as performing the same workouts and the same exercises incessantly. Adding a repetition or a few pounds to the bar inevitably increases strength * if sufficient recovery exists - but is only part of a synergistic totality necessary to stimulate muscle hypertrophy beyond current levels.
Repetition of a mundane stimulus, regardless of the seemingly positive outcome of strength increases from workout-to-workout, results in over-adaptation of a stressor. What is occurring are strength increases due to neuromuscular coordination and volitional effort. As previously stated, the objective is muscle adaptation to exercise stress * to increase functional ability * not adaptation to the method of exercise stress - to the element stimulating growth. To make further progress, to fulfill one's ultimate genetic potential, workouts must undertake unusual events * events to which the body is not accustomed.
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