The body or, more specifically, the brain, recruits muscle fibers according to perceived need. Lighter loads on the muscle’used in higher-rep sets’focus on the slow-twitch, or type 1, fibers. Exercises involving heavier weights and lower reps lead to a recruitment of type 2B, or fast-twitch, fibers, which are more conducive to muscular growth than type 1 fibers.
The 100-rep theory says that type 1 fibers dominate initially due to the use of relatively light weights, then, as the set progresses, the type 1 fibers get fatigued, leading to a handoff to the 2B fibers. The theory, however, doesn’t take into account other factors, such as the lactic acid that may build up during an extended set. As the hydrogen ions increase in muscle due to increased lactic acid production, muscle energy enzymes also click off, decreasing exercise efficiency. The point is that you aren’t likely to recruit 2B fibers during 100-rep sets. Such sets will build muscular endurance but add little or nothing to muscular hypertrophy’unless you can somehow use a considerable amount of weight.
Thus, the old advice about using higher reps for muscle endurance and lower reps for promoting muscle size appears to have some solid scientific support, based on the muscle fiber recruitment pattern. Does that mean that using heavy weights for low reps, even as low as one-rep sets, is best for promoting muscular hypertrophy?
No, it doesn’t. Studies show that while the type 2B fibers are most responsive to muscular growth, type 1 fibers also contribute to muscle size increases, though to a lesser extent. Using very heavy weights with a looser style will bypass type 1 muscle fibers, leading to less hypertrophy. Proof of that is seen in many weightlifters and powerlifters, who use enormous weights yet show only moderate muscle size unless they also vary their rep patterns (such as doing sets with moderate rep ranges of eight to 10). Other studies show that much of the muscular size gains in female bodybuilders occur in type 1 fibers, indicating that the endurance fibers do contribute to muscle size gains.
Another reason to favor a rep range of eight to 10 per set has to do with hormone release during exercise. It turns out that using moderate reps leads to the greatest increase in lactic acid. Although, as noted above, the increased acidity associated with higher muscle lactic acid levels turns off muscle energy enzymes, it also signals the release of anabolic hormones, including testosterone and growth hormone.
During higher-rep sets the production of lactic acid is inhibited, since higher reps tend to tap into aerobic metabolism, which produces lower levels of lactic acid. Conversely, performing very low reps’fewer than four’uses the phosphocreatine energy system, which also doesn’t produce lactic acid. Lactic acid increases testosterone during moderate-rep exercise by promoting the release of cyclic-AMP, an ATP by-product. That leads to a biochemical cascade in muscle, resulting in increased testosterone secretion.
Increased muscle lactate causes higher growth hormone levels that remain elevated for at least an hour after training. It also promotes the liver-based synthesis of insulinlike growth factor 1 (IGF-1), another potent anabolic hormone. In addition, if you use an eccentric component, meaning a slow lowering of the weight, during a moderate-rep set, you get greater muscle tension and damage. That leads to increased local production of IGF-1 in the muscle, which promotes muscular growth due to heightened muscle repair processes.
Still another anabolic effect induced by a moderate rep range is the muscle pump effect. The increased arterial blood flow into muscle causes a seepage of plasma out of the capillaries into the areas between the muscle and the blood vessels. That’s the familiar pump effect, which also promotes increased hydration at a cellular level. The upgraded cellular hydration, in turn, relays an anabolic signal to the cell, leading to an increased anabolic boost with a decreased muscle breakdown, an effect that may be increased by creatine use.
The effects of training with eight to 10 reps per set depend on another established principle of training: overload. What that means is, you still need to lift weights heavy enough to barely make the eight-to-10-rep goal on each set. Fooling yourself by using weights that are too light will produce neither increased muscle size nor increased muscle endurance’in short, a wasted set. IM