Muscle hypertrophy
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Muscle hypertrophy is the increase of the size of muscle cells. It differs from muscle hyperplasia, which is the formation of new muscle cells.
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[edit] Muscular 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, although those kind of exercises have little effect strengthening the muscles involved in respiration. 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,[1] as well as neovascularization.[2][3] Though an adequate supply of amino acids is essential to produce muscle hypertrophy and the consumption of carbohydrates.
[edit] Types of hypertrophy
There are two different types of muscular hypertrophy: sarcoplasmic hypertrophy and myofibrillar hypertrophy. During sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength. During myofibrillar hypertrophy, the myofibrils, comprised of the 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.
[edit] Types of myofibrillar hypertrophy
Myofibrillar hypertrophy can, in theory, arise through two processes:
- Increase in the number of nuclei within each muscle fiber, or
- Increase in the amount of contractile material supported by each nucleus.
The latter is the usual means of muscle hypertrophy.
[edit] 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 repetitions (reps) causes myofibrillated hypertrophy to dominate (as in powerlifters, olympic lifters and strength athletes), while several repetitions (generally 12 or more) against a sub-maximal load facilitates mainly sarcoplasmic hypertrophy (professional bodybuilders and endurance athletes). 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]
Muscle hypertrophy due to strength training does not occur for everyone and is not necessarily well correlated with gains in actual muscle strength: it is possible for muscles to grow larger without becoming much stronger.[4]
[edit] Protein synthesis
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.
Cortisol decreases amino acid uptake by muscle, and inhibites protein synthesis.[5]
[edit] Anaerobic training
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, but the psychological and physiological side-effects can cause serious health issues. 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 without a medical prescription.
In order to get the best gains out of training sessions, experts 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, reps, and sets will all have a positive impact on growth. Some experts create complicated plans that play around with weight, reps, and sets, increasing one while decreasing the others to constantly shock the body into growing. Keeping the sets and reps the same while just increasing weight will lead to growth but will focus more on developing muscular strength. Keeping the weight the same but doing more sets, or doing a few extra reps, may be more effective at stimulating growth for a few weeks, before a rise in weight is necessary.[citation needed] It is generally believed that with more than 15 repetitions per set, the weight will be too light to stimulate growth.
[edit] Microtrauma
Microtrauma, which is tiny damage to the fibres, is seen as the basis for 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. This is why progressive overload is essential to continued improvement, as the body adapts and becomes more resistant to stress.
[edit] References
- ^ 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:. PMID 16155759.
- ^ 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.
- ^ Prior BM, Yang HT, Terjung RL (2004). "What makes vessels grow with exercise training?". J. Appl. Physiol. 97 (3): 1119–28. doi:. PMID 15333630.
- ^ Hubal, MJ; Gordish-Dressman H, Thompson PD, Price TB, Hoffman EP, Angelopoulos TJ, Gordon PM, Moyna NM, Pescatello LS, Visich PS, Zoeller RF, Seip RL, Clarkson PM (June 2005). "Variability in muscle size and strength gain after unilateral resistance training.". Medicine and Science in Sports and Exercise 37 (6): 964–972. PMID 15947721. http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=15947721&dopt=Citation. Retrieved on 2007-07-17. Lay summary – The New York Times (February 12, 2002).
- ^ Manchester, K.L., “Sites of Hormonal Regulation of Protein Metabolism. p. 229”, Mammalian Protein [Munro, H.N., Ed.]. Academic Press, New York. On p273.
