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Muscle Hypertrophy: New Insights and Training Recommendations
Aaron Bubbico and Len Kravitz, Ph.D.

Muscle hypertrophy or muscle cell enlargement is a topic of great debate and interest in all fields of health, fitness and sports. How the body responds to muscular overload to elicit muscle growth is still under vast scientific investigation. Many types of training educe muscle hypertrophy. This fact is evidenced by athletes in numerous sports exhibiting wonderful muscular development, yet incorporating different training protocols. The goal of this column is to examine some contemporary understandings of muscle hypertrophy and to highlight some effective training approaches in reaching it.

What is Muscle Hypertrophy?
Muscle hypertrophy is an increase in muscle fiber size, observed as a muscle achieves a larger diameter or cross-sectional area. New muscle fibers are not created during hypertrophy in humans, although Paul and Rosenthal (2002) note this phenomenon has been observed in some animal studies due to unique structural differences in muscle anatomy between species. With muscle hypertrophy at the cellular level in humans, the actin and myosin contractile proteins increase in size and number (Schoenfeld 2010). In addition, Schoenfeld adds there is an increase in the fluid (sarcoplasm) and the non-contractile connective tissues interspersed within muscle, a concept collectively referred to as sarcoplasmic hypertrophy. Proske and Allen continue that with eccentric training, where the muscle is overloaded while lengthening, muscle cells also add sarcomeres (the smallest functional unit of muscle fibers) longitudinally, thus adding length to the muscle fiber too.

It is important to note that strength gains the first couple of months of training are primarily neural adaptations (Schoenfeld 2010). Gabriel, Kamen and Frost (2000) explain that in the early training phases the muscle is acquiring greater neural input, referred to as neural drive. Underlying this greater neural input are the motor unit recruitment patterns of muscle fiber types. Each motor unit represents a single nerve and the many muscle fibers it innervates.

Motor Unit Recruitment
Motor units for the most part are recruited in order of increasing size, because the size (in diameter) of the motor unit is directly related to its force producing capability. A lighter force demand placed upon the muscle will emphasize activation of the slow twitch type 1 fibers. As the force on muscle increases the fast twitch type IIA fibers are activated with the help of the type I fibers. With the most challenging demands of muscle, the most powerful (and largest) type IIB fibers are triggered to fire, with the help of the type I and type IIA fibers. Source: (Enoka, 1995).

Satellite Cells, Cytokines, and Hormones: Exploring New Insights into Muscle Hypertrophy Mechanisms

Satellite cells are the 'stem' cells of skeletal muscle (Schoenfeld 2010). Like stem cells, satellite cells have unique physiologic characteristics and functions. Satellite cells, which can be characterized as small mononuclear (one nucleus) cells are located between the basement membrane of a muscle fiber (called the basal lamina) and the sarcolema (the polarized plasma membrane). They function to repair damaged muscle tissue and trigger skeletal muscle growth after any type of overload. Once satellite cells are stimulated by a muscular overload, they fuse to the muscle fiber and facilitate muscle hypertrophy by forming a new nucleus. Uniquely to human physiology, muscle fibers have numerous nuclei. Schoenfeld continues that each nucleus is responsible for a finite area of volume and tissue within the muscle fiber, a concept called myonuclear domain.

The exercise-induced stimulus from resistance exercise activates a complex response of cellular messaging pathways, cytokines and hormones that set in motion muscle hypertrophy. Three very distinctive messaging pathways (calcium-dependent pathway, mitogen-activated protein-kinase pathway, and rapamycin pathway) shift the cell into a protein synthesis condition while also inhibiting protein breakdown (Schoenfeld, 2010). Messaging proteins called cytokines from the immune system interact with specialized receptors on muscle to promote tissue growth. And prominently, some anabolic (muscle growth promoting) hormones including insulin-like growth factor, testosterone and growth hormone play a primary role in promoting hypertrophy (Schoenfeld, 2010).

What are Some Distinctive Training Methods that Promote Hypertrophy?
Shoenfeld (2011) explains that even though direct research is lacking, several body building training methods appear to promote muscle growth because they incite one or all of the factors that activate hypertrophy in muscle.
Descending Weight Sets or Drop Sets
There are countless variations of descending weight or drop sets. For example, an exerciser may do 8 repetitions of a dumbbell lateral raises with 35 lb to failure and then put the dumbbells down and complete 8 repetitions with 25 lbs to failure and then drop to 8 repetitions of 15 lbs to failure. Depending on the exerciser a sequential drop of 10-25% in weight would be appropriate with this technique.
Alternating Rest Periods Between Multiple Sets
Typically, three main rest periods are used in resistance exercise: short (30 seconds or less), moderate (60-90 seconds), and long (3 minutes plus) (Willardson, 2006). Short rests periods may cause a significant amount of metabolic stress, which is now believed to be a potent stimulator for hypertrophy. However, with shorter rest periods (between multiple sets) it is harder to obtain higher workloads needed to really overload muscle. So by alternating rest periods (between multiple sets) the exerciser may be able to create more metabolic stress on some sets and more mechanical tension on other sets, both of which promote hypertrophy.
Eccentric Training
As highlighted by Schoenfeld (2011), a significant amount of research has been completed that shows eccentric training results in great gains in muscle hypertrophy (for a complete review of eccentric training see 2010 IFJ volume 7, #9) One popular eccentric training technique is the 'supramaximal technique,' where the client lifts a weight (with the aid of a personal trainer) that is 105% to 125% of their normal load and then lowers the weight eccentrically in 3-4 seconds.
Forced Repetitions
With the spotting aid of the personal trainer, have the client complete 2-4 extra repetitions after reaching momentary muscular fatigue on a set.
Super Sets
Super sets or paired sets are any two sets that are performed in sequence (with no rest between exercises). Schoenfeld (2011) notes that the metabolic stress this training technique induces may be responsible for the hypertrophic gains. Possible super set strategies (and examples of each) include agonist/antagonist (biceps curl and triceps extension), opposite action (chest fly and seated row), upper body/lower body (chest press and leg press), lower body only (lunge and heel raise) and upper body only (fly and chest press).
Concluding Comments
It is remarkable to see how scientists are now beginning to understand and explain many hypertrophy techniques that have been used for decades. The physiological responses to training vary according to a person's age, fitness level, hormonal levels, gender, and tolerances for mechanical overload. It is always a major responsibility of the personal trainer to assess what strategies to utilize and how much training is best to help clients attain their muscular fitness goals.

Side Bar 1. Five Questions About Muscle Hypertrophy?
1. What are the most important factors that promote hypertrophy?
Schoenfeld (2011) highlights that mechanical tension, muscle damage and metabolic stress are the three primary factors that promote hypertrophy from exercise. The mechanical tension is directly related to intensity of the exercise, which is the key to stimulating muscle growth. Muscle damage, that leads to muscle soreness, from exercise training initiates an inflammatory response, which activates satellite cells growth processes. Thirdly, metabolic stress that is a result of the byproducts of anaerobic metabolism (i.e., hydrogen ions, lactate, inorganic phosphates) is now also believed to promote hormonal factors leading to muscle hypertrophy (Schoenfeld).

2. Which tends to show hypertrophy faster, the upper body or the lower body?
The upper extremities tend to show more growth earlier then the lower body (Schoenfeld, 2011).

3. What is the ideal intensity to elicit the best hypertrophy effects?
Maximal growth occurs with loads between 80-95% of 1 repetition maximum (1RM) (Fry 2004).

4. How do weightlifters, powerlifters and body builders differ in their hypertrophic responses to training?
According to Fry (2004), weightlifters and powerlifters show more favorable hypertrophy of type II (fast twitch) muscle fibers whereas body builders appear to have comparable hypertrophy in both the type I (slow twitch) and type II muscle fibers.

5. Which is better for developing hypertrophy: single joint or multiple joint exercises?
Multi-joint exercises have been shown to produce larger increases of anabolic hormones than single-joint exercises and thus should be prioritized accordingly (Hansen et al., 2001).

Enoka, R.M. (1995). Morphological features and activation patterns of motor units. Journal of Clinical Neurophysiology, 12(6), 538-559.
Fry, A.C. (2004). The role of resistance exercise intensity on muscle fibre adaptations. Sports Medicine, 34(10), 663-679.
Gabriel, D.A., Kamen, G. and Frost, G. (2000). Neural adaptations to resistive exercise: Mechanisms and recommendations for training practices. Sports Medicine, 36(2), 131-149.
Hansen, S., Kvorning, T., Kjaer, M., and Sjogaard, G. (2001). The effect of short-term strength training on human skeletal muscle: the importance of physiologically elevated hormone levels. Scandinavian Journal of Medicine & Science in Sports, 11(6), 347-354.
Paul, A.C. and Rosenthal, N. (2002). Different modes of hypertrophy in skeletal muscle fibers. The Journal of Cell Biology, 156(4), 751-760.
Proske, U. and Allen, T.J. (2005). Damage to skeletal muscle from eccentric exercise. Sport Science Reviews, 33(2), 98-104.
Schoenfeld, B.J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. Journal of Strength and Conditioning Research, 24(10), 2857-2872.
Schoenfeld, B. (2011). The use of specialized training techniques to maximize muscle hypertrophy. Strength and Conditioning Journal, 33(4), 60-65.
Willardson, J.M. (2006). A brief review: Factors affecting the length of the rest interval between resistance exercise sets. Journal of Strength Conditioning Research, 20(4), 978-984.

Aaron Bubbico has a bachelors' degree in Exercise Science and is working on his Masters degree in Exercise Science at the University of New Mexico (Albuquerque). He is owner and operator of Quality Fitness Training in Albuquerque, NM. His research interests include eccentric load training, hypertrophic adaptations and mechanisms, and periodization.
Bio:Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and was chosen as the American Council on Exercise 2006 Fitness Educator of the Year.