Muscle hypertrophy, also known as muscle building involves a hypertrophy or increase in size of skeletal muscle through a growth in size of its component cells. Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage; and myofibrillar hypertrophy, which focuses more on increased myofibril size. It is the primary focus of bodybuilding-related activities.
Hypertrophy stimulation A range of stimuli can increase the volume of muscle cells. These changes occur as an adaptive response that serves to increase the ability to generate force or resist fatigue in anaerobic conditions.
Strength training
Strength training (resistance training) causes neural and muscular adaptations which increase the capacity of an athlete to exert force through voluntary muscular contraction: After an initial period of neuro-muscular adaptation, the muscle tissue expands by creating sarcomeres (contractile elements) and increasing non-contractile elements like sarcoplasmic fluid. The majority of strength progression is largely due to neural adaptations such as motor unit recruitment, synchronization, and firing efficiency. These allow for measurable muscle hypertrophy.Damas, F., Phillips, S. M., Vechin, F. C., & Ugrinowitsch, C. (2015). A review of resistance training-induced changes in skeletal muscle protein synthesis and their contribution to hypertrophy. Sports Medicine, 45(6), 801–807.
Muscular hypertrophy can be induced by progressive overload, a strategy of progressively increasing resistance or repetitions over successive bouts of exercise to maintain a high level of effort. However, the precise mechanisms are not clearly understood; the current accepted theory is mechanical tension. Mechanical tension is known to activate pathways such as mTOR, which is responsible for protein synthesis, a mechanism that directly contributes to muscle hypertrophy. Mechanical tension activates mechanosensitive pathways, including mTOR signaling, which increases muscle protein synthesis and contributes directly to hypertrophy.Phillips, S. M., & Winett, R. A. (2010). Uncomplicated resistance training and health-related outcomes: Evidence for a public health mandate. Current Sports Medicine Reports, 9(4), 208–213.
Muscular hypertrophy plays an important role in competitive bodybuilding and strength sports like powerlifting, American football, and Olympic weightlifting.
Blood flow restriction training
Blood flow restriction (BFR) training involves the use of cuffs or bands to partially restrict blood flow to the working muscles during low-load resistance exercise. This method has been shown to induce hypertrophy comparable to traditional high-load training, likely due to mechanical tension and muscle fiber recruitment. BFR training is particularly useful for individuals who cannot tolerate high mechanical loads, such as those recovering from injury or older adults.
Anaerobic training
The best approach to specifically achieve muscle growth (as opposed to focusing on gaining strength, power, or endurance) remains controversial; 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. 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, as well as neovascularization.
Temporary swelling
During a workout, increased blood flow to metabolically active areas causes muscles to temporarily increase in size. This phenomenon is referred to as transient hypertrophy. Longer-term hypertrophy occurs due to more permanent changes in muscle structure.
Causes of muscle swelling may include:"Muscle swelling occurs as a result of the following:
(a) resistance exercise can increase phosphocreatine and hydrogen ion accumulations due to blood lactate and growth hormone production, and
(b) the high lactate and hydrogen ion concentrations may accelerate water uptake in muscle cells according to cell permeability because the molecular weights of the lactate and hydrogen ions are smaller than that of muscle glycogen."
Non-training factors affecting hypertrophy Genetics Biological factors, such as DNA, gender, nutrition, and training variables, can affect muscle hypertrophy. Individual differences in genetics account for a substantial portion of the variance in existing muscle mass. A twin study estimated that about 53% of the variance in lean body mass is heritable, along with about 45% of the variance in muscle fiber proportion.
Testosterone
During puberty in males, hypertrophy occurs at an increased rate. Natural hypertrophy normally stops at full growth in the late teens. As testosterone is one of the body's major growth hormones, on average, males find hypertrophy much easier (on an absolute scale) to achieve than females. On average, men have about 60% more muscle mass than women.
Taking additional testosterone, as in anabolic steroids, will increase results, but also the risks. Drugs such as these can lead to a decrease in natural hormone levels, leaving the user dependent on the drug for continued chemical release and gains. It is considered a performance-enhancing drug, the use of which can cause competitors to be suspended or banned from competitions. Testosterone is also a medically regulated substance in most countries, making it illegal to possess without a medical prescription. Anabolic steroid use can cause testicular atrophy, cardiac arrest, and gynecomastia.
Diet In the long term, a positive energy balance, when more calories are consumed rather than burned, is helpful for anabolism and therefore muscle hypertrophy. An increased requirement for protein can help elevate protein synthesis, which is seen in athletes training for muscle hypertrophy. Protein intakes up to 1.6 grams per kilogram of body weight a day help increase gains in strength and muscle size from resistance training.
Glycogen When performing high intensity exercise, such as weight training, the body will attempt to draw energy from glycogen stores for energy. Glycogen is produced during glycogenesis and is converted from carbohydrates into glycogen and is stored in the muscles and liver for energy. Cyclist studies have shown that when cycling at a VO2 (volume of oxygen) rate of 85%, glycogen was responsible for roughly 66% of the energy expended.Wildman, Robert PhD, RD1; Kerksick, Chad PhD, ATC, CSCS*D, NSCA-CPT*D2; Campbell, Bill PhD, CSCS3. Carbohydrates, Physical Training, and Sport Performance. Strength and Conditioning Journal 32(1):p 21-29, February 2010. | DOI: 10.1519/SSC.0b013e3181bdb161 Having low glycogen stores during a high intensity workout can cause issues with performance, energy, and can even lead to lightheadedness. It is also beneficial to replenish the glycogen that has been used during a workout. Incorporating carbs into a post-workout meal, along with other macro and micronutrients, can aid in recovery.
Training factors affecting hypertrophy Training variables, in the context of strength training, such as frequency, intensity, and total volume all directly affect the increase of muscle hypertrophy. Time under tension and contraction types (eccentric versus concentric) affect hypertrophy as well. A gradual increase in all of these training variables will yield muscular hypertrophy.
Resistance training activates key anabolic pathways. Many crucial ones to hypertrophy include mTORC1 that stimulate satellite cell activity, both of which play central roles in promoting increases in muscle fiber size.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. https://doi.org/10.1519/JSC.0b013e3181e840f3
Range of motion Range of motion (ROM) is also seen as another possible factor to induce hypertrophy. Training through a full ROM, particularly at elongated muscle lengths, has been shown to enhance hypertrophy compared to partial ROM. For example, deep squats and full-ROM deadlifts increase mechanical tension on muscle fibers, particularly in the stretched position, which may stimulate greater muscle growth. Partial ROM training at longer muscle lengths has also been found to promote hypertrophy, potentially due to increased muscle damage.
Time under tension Time under tension (TUT) is the duration of time that the muscle being trained is stressed during a repetition. There are multiple methods to introduce TUT in each repetition of an exercise, either by slowing down the eccentric or concentric phases, or by stopping at certain phases of the exercise. TUT has been proposed to increase muscle hypertrophy because slower repetition tempos increase muscular activity. Research comparing repetition tempos shows mixed results. Burd et al. (2012) reported that slower tempos increased acute mitochondrial and myofibrillar protein synthesis, while other studies found that traditional tempos produced greater hypertrophy in untrained individuals, suggesting that moderate tempos may be most effective.Schoenfeld, B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. This can increase the muscle protein synthesis for an extended period. A study done by Burd et al. (2012) showed that at relatively light loads (30% of best effort) slow contractions (6-second concentric and 6-second eccentric) performed to failure compared to faster contractions (1-second concentric and 1-second eccentric), slow contractions had not only higher rates of acute mitochondrial and sarcoplasmic protein synthesis but also had significant rates of delayed stimulation of myofibrillar protein synthesis 24 to 30 hours after the exercise was done.
However, other research conflicts with this information. In a study done by Shneuke et al., groups of "untrained" individuals underwent slow-speed training (10-second concentric and 4-second eccentric) and normal-speed training. The slow-speed group had some increases in type IIA and IIX fibers, but the greatest increases occurred in the normal-speed group. This shows that although TUT can have some adaptations in fibers, load and intensity are more important for hypertrophy.
The literature suggests that moderate tempos give the best results for hypertrophy (between 2 and 8 seconds), while extremely slow tempos may restrict hypertrophy by limiting the amount of load that can be lifted, limiting progressive overload. On the other hand, very rapid tempos shorten TUT and reduce the stimulus a muscle receives for hypertrophic adaptation.
Overall, while TUT has shown some positive benefits in terms of muscle growth, long-term hypertrophy seems to depend more on total training volume and progressive overload than on repetition duration only.
There has also been a focus on emphasizing the eccentric portion of the repetition to increase muscle growth.
Eccentric contraction emphasis Main Article: Eccentric training
An eccentric contraction occurs when a muscle lengthens under tension. This is different from concentric contraction, which is when the muscle producing force shortens. For example, during the lowering phase of squat or bench press, the external load is greater than the muscle's force output, and so the fibers lengthen under tension. Lifting the weight back up requires the muscles to have a higher force output than the external load, resulting in fibers shortening in the concentric phase.
The primary way that the eccentric contraction promotes hypertrophy is that it produces higher mechanical output at lower metabolic cost when compared to the concentric contraction.