Exercise stimulates a series of metabolic responses that affect the body's anatomy, physiology, and biochemical make-up. How much these change depends on whether the exercise is aerobic or anaerobic. The intensity and the duration of the exercise will determine just how the muscles will respond. Your muscles have both type1 (slow twitch) and type 11 (fast twitch) fibers and how you exercise determines which fibers develop more.

When weight training, using high repetitions (15 or more reps per set), more type1 fibers are used. This promotes muscle endurance, but not maximum muscle growth as the type 1 fiber has limited growth capabilities. The weight used in high repetition work is not enough to activate the higher threshold motor units that are in the muscle.

Using lower repetitions (6-10 reps per set), more of the type11 fibers come into play. This is because they are needed to handle the heavier weight where more force is required. Type11 fibers have the greatest potential for growth. So, when heavier weight is used more complete activation of type11 fibers occurs, which stimulates better size increases. According to the size principal, motor units are recruited in order, according to their recruitment thresholds and firing rates.

Since most muscles contain a range of type1 and type11 fibers, the force they produce can be very low or very high. So, to get to a high threshold motor unit, all of the motor units below it must be recruited in order. Heavy resistance training recruits the higher threshold motor units, so all the motor units below them can undergo hormonal adaptations to the stress of the heavier loads.

Heavy resistance training causes an increase in serum testosterone levels. Since testosterone is the primary hormone that interacts with skeletal muscle tissue, it has both direct and indirect effects on muscle tissue. Using a resistance of 85%-95% of your one-rep max (the amount of weight you can do an exercise with, for one complete repetition) will increase testosterone levels more than other resistance loads.

Attempting to lift near 1 rep max loads for one or two reps to try to gain muscle size is not recommended. Because although heavier loads do activate high threshold motor units, serum testosterone levels are increased best through moderate to high volume exercises. This is best achieved doing multiple sets and a moderate repetition range (around 10 reps), with short rest periods (around 60-90 seconds).

For gains in muscle size, smaller motor units need to be recruited first in each set of exercise. As the set progresses, larger units will be recruited in an orderly fashion. If the low motor units are not allowed to be recruited, such as in explosive movements like power lifting, the low motor units, that were not activated, will not undergo hormonal adaptations.

This is because of the size principal of muscle recruitment. Since motor units are recruited in an orderly fashion from low to high, a moderate rep range must be used to recruit the entire range of fibers. So, an understanding of natural anabolic activity which occurs in your body, is essential to muscular adaptation, proper recovery, progression and ultimately muscle size increases.

Remember, high repetitions during resistance training (15 or more reps), does not recruit high threshold motor units and that limits potential for muscle growth. Power lifting, which does not allow for enough time to activate all motor units in an orderly fashion, diminishes the adaptations of the whole span of muscle fibers in any given motor unit. Exercising with high intensity, (using 8-10 repetitions with heavy resistance and no more than 90 seconds of rest between sets), will maximize serum testosterone levels allowing for better recovery, adaptation and muscle growth.