Muscle hypertrophy

Muscle hypertrophy is an increase in the size of muscle cells. It differs from muscle hyperplasia, which is the formation of new muscle cells.

Hypertrophy stimuli

A range of stimuli can increase the volume of muscle cells. Summarizing, these changes occur as an adaptive response that serves to increase the ability to generate force or resist fatigue in anaerobic conditions.

Strength training

Main article: Strength training

Strength training typically produces a combination of the two different types of hypertrophy: contraction against 80 to 90% of the one repetition maximum for 2–6 repetitions (reps) causes myofibrillated hypertrophy to dominate (as in powerlifters, olympic lifters and strength athletes), while several repetitions (generally 8 – 12 for bodybuilding or 12 or more for muscular endurance) against a sub-maximal load facilitates mainly sarcoplasmic hypertrophy (professional bodybuilders and endurance athletes).^{[citation needed]} The first measurable effect is an increase in the neural drive stimulating muscle contraction. Within just a few days, an untrained individual can achieve measurable strength gains resulting from "learning" to use the muscle.^{[citation needed]} As the muscle continues to receive increased demands, the synthetic machinery is upregulated. Although all the steps are not yet clear, this upregulation appears to begin with the ubiquitous second messenger system (including phospholipases, protein kinase C, tyrosine kinase, and others).^{[citation needed]} These, in turn, activate the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response.^{[citation needed]}

To get the best gains out of training sessions, experts^[who?] agree on some basic principles, however, some are contradicted by other research:

Progressive overload is considered the most important principle behind hypertrophy, so increasing the weight, repetitions (reps), and sets will all have a positive impact on growth. Some experts create complicated plans that manipulate weight, reps, and sets, increasing one while decreasing the others to keep the schedule varied and less repetitive. It is generally believed that if more than 15 repetitions per set is possible, the weight is too light to stimulate maximal growth.^[1]

Anaerobic training

Main article: Anaerobic exercise

Experts and professionals differ widely on the best approaches to specifically achieve muscle growth (as opposed to focusing on gaining strength, power, or endurance); it was generally considered that consistent anaerobic strength training will produce hypertrophy over the long term, in addition to its effects on muscular strength and endurance. As testosterone is one of the body's major growth hormones, on average, men find hypertrophy much easier to achieve than women. Taking additional testosterone, as in anabolic steroids, will increase results. It is also considered a performance-enhancing drug, the use of which can cause competitors to be suspended or banned from competitions. In addition, testosterone is also a medically regulated substance in most countries, making it illegal to possess it without a medical prescription.

Factors affecting hypertrophy

Several biological factors such as age and nutrition can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an increased rate. Natural hypertrophy normally stops at full growth in the late teens. Muscular hypertrophy can be increased through strength training and other short duration, high intensity anaerobic exercises. Lower intensity, longer duration aerobic exercise generally does not result in very effective tissue hypertrophy; instead, endurance athletes enhance storage of fats and carbohydrates within the muscles,^[2] as well as neovascularization.^[3][4] An adequate supply of amino acids is essential to produce muscle hypertrophy.

Changes in Protein synthesis and muscle cell biology associated with stimuli

Protein synthesis

Main article: protein biosynthesis

Ultimately the message filters down to alter the pattern of protein expression. The additional contractile proteins appear to be incorporated into existing myofibrils (the chains of sarcomeres within a muscle cell). There appears to be some limit to how large a myofibril can become: at some point, they split. These events appear to occur within each muscle fiber. That is, hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells. Skeletal muscle cells are however unique in the body in that they can contain multiple nuclei, and the number of nuclei can increase.^[5]

Cortisol decreases amino acid uptake by muscle tissue, and inhibits protein synthesis.^[6] The short-term increase in protein synthesis that occurs subsequent to resistance training returns to normal after approximately 28 hours in adequately fed male youths. ^[7]

A small study performed on young and elderly found that ingestion of 340 grams of lean beef (90 g protein) did not increase muscle protein synthesis any more than ingestion of 113 grams of lean beef (30 g protein). In both groups, muscle protein synthesis increased by 50%. The study concluded that more than 30 g protein in a single meal did not further enhance the stimulation of muscle protein synthesis in young and elderly.^[8] However, this study didn't check protein synthesis in relation to training; therefore conclusions from this research are controversial.

It is not uncommon for bodybuilders to advise a protein intake as high as 2–4 g per kilogram of bodyweight per day.^[9][10] However, scientific literature such as 'Evaluation of protein requirements for trained strength athletes (November 1992)' has suggested this is higher than necessary, as protein intakes greater than 1.8 g per kilogram of body weight showed to have no greater effect on muscle hypertrophy.^[11] A study carried out by American College of Sports Medicine (2002) put the recommended daily protein intake for athletes at 1.2–1.8 g per kilogram of body weight.^[12][13][11] Conversely, Di Pasquale (2008), citing recent studies, recommends a minimum protein intake of 2.2 g/kg "for anyone involved in competitive or intense recreational sports who wants to maximize lean body mass but does not wish to gain weight. However athletes involved in strength events (..) may need even more to maximize body composition and athletic performance. In those attempting to minimize body fat and thus maximize body composition, for example in sports with weight classes and in bodybuilding, it’s possible that protein may well make up over 50% of their daily caloric intake.^[14]

Microtrauma

Main article: Microtrauma

Microtrauma, which is tiny damage to the fibers, may play a significant role in hypertrophy.^{[citation needed]} When microtrauma occurs (from weight training or other strenuous activities), the body responds by overcompensating, replacing the damaged tissue and adding more, so that the risk of repeat damage is reduced. Damage to these fibers have been theorized as the possible cause for the symptoms of delayed onset muscle soreness (DOMS), and is why progressive overload is essential to continued improvement, as the body adapts and becomes more resistant to stress.

Myofibrillar vs. Sarcoplasmic hypertrophy controversy

In the bodybuilding and fitness community and even in some academic books skeletal muscle hypertrophy is described as being in one of two types: Sarcoplasmic or myofibrillar. According to this theory, during sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength, whereas during myofibrillar hypertrophy, actin and myosin contractile proteins increase in number and add to muscular strength as well as a small increase in the size of the muscle. Sarcoplasmic hypertrophy is characteristic of the muscles of certain bodybuilders while myofibrillar hypertrophy is characteristic of Olympic weightlifters.^[15] These two forms of adaptations rarely occur completely independently of one another, one can experience a large increase in fluid with a slight increase in proteins, a large increase in proteins with a small increase in fluid, or a relatively balanced combination of the two. In contrast to this theory it should be noted that when viewed in microscope, muscles are filled entirely by myofibrils, whether or not the muscles from bodybuilders or powerlifters are used. Also, very little actual evidence actually supports that the non-myofibrillar part of the sarcoplasm ever expands. Antagonists to this theory suggest that the cause of this popular notion is twofold: First, it is derived from fractioning of muscle used when measuring protein synthesis. This is a technique in which muscle proteins are separated biochemically into myofibrillar, sarcoplasmic, membrane and mitochondrial fractions for protein synthesis. This validity of this separation is poorly validated and also, the results of this fractionation and the usual following stable isotope protein synthesis measurement does not tell anything about the relative abundance of these protein fractions (as changes in protein synthesis are by definition relative (i.e. a change of 50% in a substance that constitutes 1% of the muscle is still insignificant in a physiological context). Secondly, the sarcoplasmic/myofibrillar proponents use their theory to explain why bodybuilders have less relative strength than strength athletes. But this theory is not necessary to explain these differences. The physiological changes associated with training with very high volume and degrees of muscle fatigue produce difference neuromuscular adaptations that are different from those experienced by strength training with very high mechanical loads and less muscle fatigue.

In sports

Examples of increased muscle hypertrophy are seen in various professional sports, mainly strength related sports such as boxing, bodybuilding, mixed martial arts, rugby, professional wrestling and various forms of gymnastics. These athletes train extensively in strength as well as cardiovascular and muscular endurance training.

See also

• Anabolism
• Muscle atrophy
• Muscle dystrophy
• Myostatin
• Davis's Law

References

1. ^ "Muscle Hypertrophy Training". Muscle Hypertrophy Training. http://www.expert-nutrition.com/hypertrophy-training.html. Retrieved 2010-07-17. 
2. ^ van Loon LJ, Goodpaster BH (2006). "Increased intramuscular lipid storage in the insulin-resistant and endurance-trained state". Pflugers Arch. 451 (5): 606–16. doi:10.1007/s00424-005-1509-0. PMID 16155759. 
3. ^ Soares JM (1992). "Effects of training on muscle capillary pattern: intermittent vs continuous exercise". The Journal of sports medicine and physical fitness 32 (2): 123–7. PMID 1279273. 
4. ^ Prior BM, Yang HT, Terjung RL (2004). "What makes vessels grow with exercise training?". J. Appl. Physiol. 97 (3): 1119–28. doi:10.1152/japplphysiol.00035.2004. PMID 15333630. 
5. ^ Bruusgaard JC, Johansen IB, Egner IM, Rana ZA & Gundersen K. (2010). Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining. Proc Natl Acad Sci U S A 107, 15111-15116.
6. ^ Manchester, K.L., “Sites of Hormonal Regulation of Protein Metabolism. p. 229”, Mammalian Protein [Munro, H.N., Ed.]. Academic Press, New York. p. 273.
7. ^ Am J Physiol Regul Integr Comp Physiol. 2008 Jan;294(1):R172-8. Epub 2007 Nov 21. "Resistance training alters the response of fed state mixed muscle protein synthesis in young men." Exercise Metabolism Research Group, McMaster University, Hamilton, Ontario, Canada.
8. ^ Symons, T. B.; Sheffield-Moore, M.; Wolfe, R. R.; Paddon-Jones, D. (2009). "A Moderate Serving of High-Quality Protein Maximally Stimulates Skeletal Muscle Protein Synthesis in Young and Elderly Subjects". Journal of the American Dietetic Association 109 (9): 1582–1586. doi:10.1016/j.jada.2009.06.369. PMID 19699838.  edit
9. ^ Protein 101: How Much Do You Need & Best Sources of Protein. StrongLifts.com (2009-05-25). Retrieved on 2011-06-19.
10. ^ Bodybuilders and Protein – Part 2. Leehayward.com. Retrieved on 2011-06-19.
11. ^ ^{a b} Tarnopolsky, MA; Atkinson, SA; MacDougall, JD; Chesley, A; Phillips, S; Schwarcz, HP (1992). "Evaluation of protein requirements for trained strength athletes". Journal of applied physiology (Bethesda, Md. : 1985) 73 (5): 1986–95. PMID 1474076. 
12. ^ Rankin, JW (2002). "Weight loss and gain in athletes". Current sports medicine reports 1 (4): 208–13. PMID 12831697. 
13. ^ . doi:10.1080/02640419108729866. 
14. ^ Di Pasquale, Mauro G. (2008). "Utilization of Proteins in Energy Metabolism". In Ira Wolinsky, Judy A. Driskell. Sports Nutrition: Energy metabolism and exercise. CRC Press. p. 79. ISBN 978-0-8493-7950-5. 
15. ^ Kraemer, William J.; Zatsiorsky, Vladimir M. (2006). Science and practice of strength training. Champaign, IL: Human Kinetics. p. 50. ISBN 0-7360-5628-9. http://books.google.com/books?id=QWSn4iKgNo8C&pg=PA50.