Sports Medicine:

Heavy weights make you bigger? Lighter weights get you define?

The average beginning trainee knows that high repetitions (15 or more) are best for muscular endurance, but are not conducive to gaining muscular mass. The higher loads used in high-repetitions work are not enough to innervate higher threshold motor units in a muscle. The key is that only muscle fibers activated by the resistance training will respond to increased levels of your natural anabolic hormones. When heavier weights (lower reps) are used in resistance training, more muscle fibers are recruited. The more muscle fibers recruited for an exercise, the greater the extend of remodeling in the entire muscle. There is another reason that light weight and high repetitions are not optimal for stimulating muscular hypertrophy. The majority of the work done in high-repetition sets is accomplished by slow-twitch Type I muscle fibers. Type I muscle fibers have a limited ability to hypertrophy. Type IIB fibers are activated when more force is required, and thus have the greatest potential for growth. Heavier weights accomplish more complete activation of type IIB muscle fibers. According to the size principle, motor units are recruited in order according to their recruitment thresholds and firing rates. Since most muscles contain a range of , force production can be very low or very high.

Therefore, to get to a high-threshold motor unit, all of the motor units below must be sequentially recruited. Heavy resistance training recruits these high-threshold motor units, therefore all the units below it can undergo hormonal adaptations to the stress of the heavy loads. An increase in serum testosterone levels is one of the result of heavy resistance training. Since testosterone is the principal hormone that interacts with skeletal muscle tissue, it has both direct and indirect effects on muscle tissue. Resistance truing utilizing large muscle groups of the lower body can increase serum testosterone concentrations more than other types of exercises.

Using a resistance of 85% to 95% of one-rep maximum will also increase testosterone levels more than other resistance loads. Although heavy resistance does innervate high-threshold motor units, serum testosterone levels are increased through moderate to high volume of exercise. This is achieved through multiple sets, exercises, and a moderate repetition range (around 10), with short rest intervals (30 seconds to 1 minute). After a muscle has been subjected to intense high stress through maximal force contractions over moderate repetition range, hormones begin the growth process and muscle remodeling. Growth hormone plays a vital role in adapting to the stress of resistance training. Growth hormone levels can be increased with resistance training though high intensity (Example: 10 repetitions coupled with heavy resistance), threes sets of each exercise (high total workload) and short, one-minute rest periods.

Once the levels are elevated, a cascade of events occur: decreased glucose utilization, increased amino acid transport across cell membranes, increased protein synthesis, increased utilization of fatyy acids, increased lipolysis (fat breakdown), enhanced immune functions, and compensatory renal hypertrophy promotion. An understanding of natural anabolic activity is essential to muscular adaptation, successful recovery, training progression and muscular gains. High-repetition resistance training does not innervate high threshold motor units and therefore limits the potential for Type IIB muscle fiber hypertrophy.

From The Body to the Soul, by Randy Rivero. Orthopedic Corner | Leon Mead MD Orthopedic Doctor | 730 Goodlette Road North, Suite 201  Naples Florida 34102 | Phone: (239) 262-1119