Muscle hypertrophy
Muscle hypertrophy or 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
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 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.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.
Muscular hypertrophy plays an important role in competitive bodybuilding and strength sports like powerlifting, American football, and Olympic weightlifting.
Blood flow restriction (BFR) training
Another form of training that has been researched in terms of inducing muscle hypertrophy is blood flow restriction training. 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 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, or more commonly known as being "pumped up" or getting "a pump". About two hours after a workout and typically for seven to eleven days, muscles swell due to an inflammation response as tissue damage is repaired. Longer-term hypertrophy occurs due to more permanent changes in muscle structure. After a workout involving eccentric contractions, muscle swelling peaks around the 4th to 5th day post-exercise and then gradually returns to baseline over about 7 to 11 days.Hirono et al. explained the causes of muscle swelling:
"Muscle swelling occurs as a result of the following:
resistance exercise can increase phosphocreatine and hydrogen ion accumulations due to blood lactate and growth hormone production, and
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."
Factors affecting hypertrophy
Biological factors, 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 classical twin study design estimated that about 53% of the variance in lean body mass is heritable, along with about 45% of the variance in muscle fiber proportion.
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 to achieve than females, and, on average, have about 60% more muscle mass 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. 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.
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.
Training variables, in the context of strength training, such as frequency, intensity, and total volume also directly affect the increase of muscle hypertrophy. A gradual increase in all of these training variables will yield muscular hypertrophy. Range of motion is also seen as another possible factor to induce hypertrophy. Training through a full Range of Motion, 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. 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. and contraction types.
Time under tension (TUT)
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. 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. This can increase the muscle protein synthesis for an extended period. A study done by Burd et al. showed that at relatively light loads. Slow contractions performed to failure compared to faster contractions, slow contractions had not only higher rates of acute mitochondrial and sarcoplasmic protein synthesis but also had significant rates of delayed simulation 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 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, 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 trainingAn 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. This higher mechanical tension is considered essential for growth. Additionally, Eccentric exercise causes a significant increase in exercise-induced muscle damage, as seen by the microlesions in muscle fibers, sarcolemmal disruption, and an inflammatory response that leads to delayed-onset muscle soreness. There is also evidence to support that Eccentric Contractions activate specific molecular pathways, which cause greater anabolic signaling and gene expression than concentric contractions. This is shown in the structural remodeling differences in the muscle when comparing the two training contractions. Eccentric training seems to cause a greater increase in fascicle length, while concentric training leads to an increase in the pennation angle.
One 8-week study found that subjects training with the same intensity, one with primarily eccentric contractions, increased muscle fiber mass by approximately 40%, while the concentric contraction group showed no change. However, this difference might not be the same when the total load is matched between training types. When matched for load, the increase in muscle volume seems to be the same between concentric and eccentric training.
Other applications for Eccentric contractions are worth noting. The concept of eccentric overload, where the eccentric phase is loaded with more weight than the concentric phase, is a strategy that advanced lifters use to maximize hypertrophic stimulus. This method works because of the unique properties of the eccentric phase mentioned above. Due to the low metabolic cost relative to the high force production, eccentric exercise is also used for rehabilitation training, especially for elderly patients with chronic conditions who are unable to perform strenuous activity.
Collectively, the evidence suggests that eccentric contractions can produce substantial muscle hypertrophy due to the high force production and unique molecular signaling. It might not be superior to Concentric Training if matched for total load and reps.