How do Muscles Grow?
How Do Muscles Grow? The Science of Muscle Growth
If you’re a guy in the gym working with weights, not only are you probably trying to lose some fat, but also gain some muscle.
This article discusses the mechanisms of how muscles grow, plus why most women won’t gain large amounts of muscle when working with weights.
Although there are different types of muscles, such as cardiac muscle (your heart), for our concerns, we will talk exclusively about skeletal muscles. Skeletal muscle is composed of thread-like myofibrils and sarcomeres that form a muscle fiber and are the basic units of contraction.
The 650 skeletal muscles in the human body contract when they receive signals from motor neurons, which are triggered from a part of the cell called the sarcoplasmic reticulum. Motor neurons tell your muscles to contract and the better you become at having those signals tell your muscles to contract, the stronger you can get.
When someone like a powerlifter is able to lift very heavy weight despite not looking very muscular, it’s due to their ability to activate those motor neurons and contract their muscles better. This is why some powerlifters can be relatively smaller compared to bodybuilders, but can lift significantly more weight. Motor Unit recruitment also helps to explain why, after practice, certain movements become easier to perform and most of the initial strength gains will be when you first start to lift weights. Muscle growth tends to occur more steadily after this initial period of strength gain because you are more easily able to activate the muscles.
The Physiology Of Muscle Growth
After you workout, your body repairs or replaces damaged muscle fibers through a cellular process where it fuses muscle fibers together to form new muscle protein strands or myofibrils. These repaired myofibrils increase in thickness and number to create muscle hypertrophy (growth).1 Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. This adaption, however, does not happen while you actually lift the weights. Instead, it occurs while you rest.
So how do you actually add muscle to your muscle cells? This is where Satellite cells come in and act like stem cells for your muscles. When activated, they help to add more nuclie to the muscle cells and therefore contribute directly to the growth of myofibrils (muscle cells). Activating these satellite cells may be the difference between what allows certain “genetic freaks” to grow massive muscles and what makes other people “hard-gainers.2”
In one of the most interesting studies in the past 5 years, researchers showed that those who were “extreme responders” to muscle growth, with an incredible 58% myofiber hypertrophy from an exercise, had 23% activation of their satellite cells. Modest responders, who had a 28% growth, had 19% activation of their satellite cells. What is interesting to note, though, is that some people known as “non-responders” in the study had 0% growth and had a concurrent 0% activation of their satellite cells. Therefore, it seems the more you can activate these satellite cells, the more you’ll be able to grow. So then the question becomes, how do you activate these satellite cells to increase muscle growth?
3 Mechanisms That Make Muscles Grow
Underlying all progression of natural muscle growth is the ability to continually put more stress on the muscles. This stress is a major component involved in the growth of a muscle and disrupts homeostasis within your body. The stress and subsequent disruption in homeostasis causes three main mechanisms that spur on muscle growth.
Muscle Growth Mechanism #1: Muscle Tension
In order to produce muscle growth, you have to apply a load of stress greater than what your body or muscles had previously adapted too. How do you do this? The main way is to lift progressively heavier weights. This additional tension on the muscle helps to cause changes in the chemistry of the muscle, allowing for growth factors that include mTOR activation and satellite cell activation.3
Muscular tension also most dramatically effects the connection of the motor units with the muscle cells. Two other factors help to explain why some people can be stronger, but not as big as other people.
Muscle Growth Mechanism #2: Muscle Damage
If you’ve ever felt sore after a workout, you have experienced the localized muscle damage from working out. This local muscle damage causes a release of inflammatory molecules and immune system cells that activate satellite cells to jump into action. This doesn’t mean that you have to feel sore in order for this to happen, but instead that the damage from the workout has to be present in your muscle cells. Typically soreness is attenuated over time by other mechanisms.
Muscle Growth Mechanism #3: Metabolic Stress
If you’ve ever felt the burn of an exercise or had the “pump” in the gym, then you’ve felt the effects of metabolic stress. Scientists used to question bodybuilders when they said the “pump” caused their muscles to become larger. After more investigation, it seems as though they were onto something.
Metabolic stress causes cell swelling around the muscle, which helps to contribute to muscle growth without necessarily increasing the size of the muscle cells. This is from the addition of muscle glycogen, which helps to swell the muscle along with connective tissue growth. This type of growth is known as sarcoplasmic hypertrophy and is one of the ways that people can get the appearance of larger muscles without increases in strength.
So now that you know the three main mechanisms of muscle growth, the next question is: how do hormones affect muscle growth?
How Do Hormones Affect How Muscles Grow?
Hormones are another component largely responsible for muscle growth and repair because of their role in regulating satellite cell activity. Insulin Growth Factor (IGF)-1, in particular Mecho-Growth Factor (MGF) and testosterone are the two most vital mechanisms that promote muscle growth.4
Testosterone is the main hormone that most people think about when working out with weights, and there seems to be some validity to the thought that testosterone increases protein synthesis, inhibits protein breakdown, activates satellite cells, and stimulates other anabolic hormones. Although most testosterone is bound in the body and therefore not available to use (up to 98%), strength training seems to help not only release more testosterone, but also make the receptors of your muscle cells more sensitive to your free testosterone. Testosterone can also stimulate growth hormone responses by increasing the presence of neurotransmitters at the damaged fiber site, which can help to activate tissue growth.
The IGF regulates the amount of muscle mass growth by enhancing protein synthesis, facilitating glucose uptake, repartitioning the uptake of amino acids (the building blocks of protein) into skeletal muscles and once again, activates satellite cells to increase muscle growth.
Why Muscles Need Rest To Grow
If you do not provide your body with adequate rest or nutrition, you can actually reverse the anabolic process and put your body into a catabolic or destructive state. The response of muscle protein metabolism to a resistance exercise bout lasts for 24-48 hours; thus, the interaction between protein metabolism and any meals consumed in this period will determine the impact of the diet on muscle hypertrophy.5 Keep in mind there is a certain limit on how much your muscles can actually grow dependent on gender, age, and genetics. For instance, men have more testosterone than women, which allows them to build bigger and stronger muscles.
Why Rapid Muscle Growth Is Unlikely
Muscle hypertrophy takes time and is relatively slow for the majority of people. People will generally not see visible growth for several weeks or months as most initial changes are due to the ability of your nervous system to activate your muscles.
In addition to that, different people have different genetics, which range from hormonal output, muscle fiber type and number, along with satellite cell activation, that can all limit muscle growth. To ensure you’re doing your best to grow muscle, muscle protein synthesis must exceed muscle protein breakdown. This requires that you take in an adequate source of protein (especially essential amino acids) and carbohydrates to help facilitate the cellular process of rebuilding broken down muscle tissue. Visible muscle growth and evident physical changes in your body’s muscle structure can be highly motivational which is why understanding the science behind how muscles actually grow is important.
How Muscles Grow: Conclusion
For muscle breakdown and growth to occur you must force your muscles to adapt by creating stress that is different than the previous threshold your body has already adapted to. This is can be done by lifting heavier weights, continually changing your exercises so that you can damage more total muscle fibers and pushing your muscles to fatigue while getting a “pump.” After the workout is completed, the most important part begins which is adequate rest and providing ample fuel to your muscles so they can regenerate and grow.
How do muscles grow?
Young sub Kwon, M.S. and Len Kravitz, Ph.D.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of muscle regeneration. Physiological Reviews, Volume 84, 209-238.
Personal trainers and fitness professionals often spend countless hours reading articles and research on new training programs and exercise ideas for developing muscular fitness. However, largely because of its physiological complexity, few fitness professionals are as well informed in how muscles actually adapt and grow to the progressively increasing overload demands of exercise. In fact, skeletal muscle is the most adaptable tissue in the human body and muscle hypertrophy (increase in size) is a vastly researched topic, yet still considered a fertile area of research. This column will provide a brief update on some of the intriguing cellular changes that occur leading to muscle growth, referred to as the satellite cell theory of hypertrophy.
Trauma to the Muscle: Activating The Satellite Cells
When muscles undergo intense exercise, as from a resistance training bout, there is trauma to the muscle fibers that is referred to as muscle injury or damage in scientific investigations. This disruption to muscle cell organelles activates satellite cells, which are located on the outside of the muscle fibers between the basal lamina (basement membrane) and the plasma membrane (sarcolemma) of muscles fibers to proliferate to the injury site (Charge and Rudnicki 2004). In essence, a biological effort to repair or replace damaged muscle fibers begins with the satellite cells fusing together and to the muscles fibers, often leading to increases in muscle fiber cross-sectional area or hypertrophy. The satellite cells have only one nucleus and can replicate by dividing. As the satellite cells multiply, some remain as organelles on the muscle fiber where as the majority differentiate (the process cells undergo as they mature into normal cells) and fuse to muscle fibers to form new muscle protein stands (or myofibrils) and/or repair damaged fibers. Thus, the muscle cells’ myofibrils will increase in thickness and number. After fusion with the muscle fiber, some satellite cells serve as a source of new nuclei to supplement the growing muscle fiber. With these additional nuclei, the muscle fiber can synthesize more proteins and create more contractile myofilaments, known as actin and myosin, in skeletal muscle cells. It is interesting to note that high numbers of satellite cells are found associated within slow-twitch muscle fibers as compared to fast-twitch muscle fibers within the same muscle, as they are regularly going through cell maintenance repair from daily activities.
Growth factors are hormones or hormone-like compounds that stimulate satellite cells to produce the gains in the muscle fiber size. These growth factors have been shown to affect muscle growth by regulating satellite cell activity. Hepatocyte growth factor (HGF) is a key regulator of satellite cell activity. It has been shown to be the active factor in damaged muscle and may also be responsible for causing satellite cells to migrate to the damaged muscle area (Charge and Rudnicki 2004).
Fibroblast growth factor (FGF) is another important growth factor in muscle repair following exercise. The role of FGF may be in the revascularization (forming new blood capillaries) process during muscle regeneration (Charge and Rudnicki 2004).
A great deal of research has been focused on the role of insulin-like growth factor-I and –II (IGFs) in muscle growth. The IGFs play a primary role in regulating the amount of muscle mass growth, promoting changes occurring in the DNA for protein synthesis, and promoting muscle cell repair.
Insulin also stimulates muscle growth by enhancing protein synthesis and facilitating the entry of glucose into cells. The satellite cells use glucose as a fuel substrate, thus enabling their cell growth activities. And, glucose is also used for intramuscular energy needs.
Growth hormone is also highly recognized for its role in muscle growth. Resistance exercise stimulates the release of growth hormone from the anterior pituitary gland, with released levels being very dependent on exercise intensity. Growth hormone helps to trigger fat metabolism for energy use in the muscle growth process. As well, growth hormone stimulates the uptake and incorporation of amino acids into protein in skeletal muscle.
Lastly, testosterone also affects muscle hypertrophy. This hormone can stimulate growth hormone responses in the pituitary, which enhances cellular amino acid uptake and protein synthesis in skeletal muscle. In addition, testosterone can increase the presence of neurotransmitters at the fiber site, which can help to activate tissue growth. As a steroid hormone, testosterone can interact with nuclear receptors on the DNA, resulting in protein synthesis. Testosterone may also have some type of regulatory effect on satellite cells.
Muscle Growth: The ‘Bigger’ Picture
The previous discussion clearly shows that muscle growth is a complex molecular biology cell process involving the interplay of numerous cellular organelles and growth factors, occurring as a result of resistance exercise. However, for client education some important applications need to be summarized. Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. Both, the synthesis and breakdown of proteins are controlled by complimentary cellular mechanisms. Resistance exercise can profoundly stimulate muscle cell hypertrophy and the resultant gain in strength. However, the time course for this hypertrophy is relatively slow, generally taking several weeks or months to be apparent (Rasmussen and Phillips, 2003). Interestingly, a single bout of exercise stimulates protein synthesis within 2-4 hours after the workout which may remain elevated for up to 24 hours (Rasmussen and Phillips, 2003). Some specific factors that influence these adaptations are helpful to highlight to your clients.
All studies show that men and women respond to a resistance training stimulus very similarly. However, due to gender differences in body size, body composition and hormone levels, gender will have a varying effect on the extent of hypertrophy one may possibly attain. As well, greater changes in muscle mass will occur in individuals with more muscle mass at the start of a training program.
Aging also mediates cellular changes in muscle decreasing the actual muscle mass. This loss of muscle mass is referred to as sarcopenia. Happily, the detrimental effects of aging on muscle have been shown be restrained or even reversed with regular resistance exercise. Importantly, resistance exercise also improves the connective tissue harness surrounding muscle, thus being most beneficial for injury prevention and in physical rehabilitation therapy.
Heredity differentiates the percentage and amount of the two markedly different fiber types. In humans the cardiovascular-type fibers have at different times been called red, tonic, Type I, slow-twitch (ST), or slow-oxidative (SO) fibers. Contrariwise, the anaerobic-type fibers have been called the white, phasic, Type II, fast-twitch (FT), or fast-glycolytic (FG) fibers. A further subdivision of Type II fibers is the IIa (fast-oxidative-glycolytic) and IIb (fast-glycolytic) fibers. It is worthy of note to mention that the soleus, a muscle involved in standing posture and gait, generally contains 25% to 40% more Type I fibers, while the triceps has 10% to 30% more Type II fibers than the other arm muscles (Foss and Ketyian, 1998). The proportions and types of muscle fibers vary greatly between adults. It is suggested that the new, popular periodization models of exercise training, which include light, moderate and high intensity training phases, satisfactorily overload the different muscle fiber types of the body while also providing sufficient rest for protein synthesis to occur.
Muscle Hypertrophy Summary
Resistance training leads to trauma or injury of the cellular proteins in muscle. This prompts cell-signaling messages to activate satellite cells to begin a cascade of events leading to muscle repair and growth. Several growth factors are involved that regulate the mechanisms of change in protein number and size within the muscle. The adaptation of muscle to the overload stress of resistance exercise begins immediately after each exercise bout, but often takes weeks or months for it to physically manifest itself. The most adaptable tissue in the human body is skeletal muscle, and it is remarkably remodeled after continuous, and carefully designed, resistance exercise training programs.
What Makes Muscles Grow?
Last updated: May 25, 2016
If you want to develop more muscle, then you better get ready to do some hard work and prepare for slow gains. Learn about resistance training, diet and rest.
I'm sure that most people who have performed some form of resistance training have wanted to increase their muscle mass at one time or another. But the body is just naturally lazy and will only develop enough muscle to deal with the loads placed upon it. So, if you want to develop more muscle, then you better get ready to do some hard work and prepare for slow gains, for the body will only respond to a combination of resistance training, diet and rest.
TWO FORMS OF GROWTH
Muscle growth comes about because of hypertrophy or hyperplasia.
Hypertrophy is an increase in the size of the muscle due to an increase in the size of the muscle fibers, while hyperplasia is an increase in the number of muscle fibers.
Hypertrophy comes in two forms, sarcomere hypertrophy, an increase in the size of the contractile portion of the muscle; and sarcoplasmic hypertrophy, an increase in the non-contractile portion of the muscle.
Sarcomere hypertrophy involves a smaller increase in the diameter of the muscle, but muscle density increases. Sarcoplasmic hypertrophy shows an increase in muscle diameter and a decrease in density. All hypertrophy will involve both processes; the ratio is dependent on training intensity and frequency.
Heavy training on a regular basis will make you feel solid whereas someone who trains for the pump with light weights will feel soft. If you are interested in improving performance, than strive for sarcomere hypertrophy. Sarcoplasmic hypertrophy is only beneficial if an increase in body weight is more important than an improvement in performance.
HYPERPLASIA AND WHAT YOU SHOULD KNOW
Hyperplasia is the splitting of muscle fibers, resulting eventually in a greater number of fibers the same size as the originals. It has been said that the number of muscle cells one is born with is all they will ever have, and the splitting is not new fibers but damage to existing fibers. Russian sports scientists claimed to find hyperplasia in swimmers' shoulders. Swimming is training at a high speed with low resistance.
Other researchers have produced hyperplasia in chickens and grouse by hanging them by their wings for a few days. I do not know if hyperplasia is possible, but if you want to do a little experiment, I would train at high speed as opposed to hanging yourself in a stretched out position for the weekend.
The process of muscle growth is not known for sure, but most theories are based on the idea that lifting breaks down the muscle, and growth results from over-compensating to protect the body from future stress. The human body breaks down and rebuilds all of the muscles every 15 to 30 days. Lifting speeds up the process due to an increased need for fuel. Rebuilding peaks 24 to 36 hours after training and continues at increased rates for as much as 72 hours.
The muscle hypoxia theory is that weight training decreases muscle oxygen content, and that triggers protein synthesis. It came from the fact that lifting loads over 60% of maximum temporarily cuts off blood flow. Restricting blood flow with a tourniquet does not result in muscle growth, and pearl divers are not overly muscular, so the muscle hypoxia theory can be thrown out. Another theory is that training increases blood flow and the increased nutrient supply is responsible for growth.
It sounds good, but running increases blood flow and not too many distance runners have big legs. The ATP debt theory is that muscles use ATP contract, so ATP will break down and the waste products will trigger increased blood flow and growth. It has been discounted because ATP levels are the same following exercising to failure as at rest, and runners still do not have big legs.
The energetic theory that is currently the most accepted is that muscle has a certain amount of energy at any given time, for growth, repair, and movement. Soviet sport scientists have referred to this as an individual's current adaptive reserve. Over time, with a lot of hard work, the body becomes more efficient at breaking down and rebuilding muscle, which shows an increase in the current adaptive reserve. Increased energy supply allows more work to be accomplished, followed by increased growth.
Medvedyev, the long time Soviet weight lifting coach, wrote that training volume is the determining factor in muscle growth. He recommends that if a weightlifter wants to stay in the same weight class, he should train with singles, and if the lifter is trying to go higher a weight class, he should train with sets of three to six reps.
Anthony Ditullio adds that strict exercise performance, minimal rest between sets, and the volume of work in a limited time are the primary factors in muscle growth. Obviously, the more work performed, the greater need for energy.
A maximum single is the most work that can be performed in an instant, but because it lasts a limited time, only a small amount of protein is broken down. High repetition sets with light weight also require a small amount of fuel supply. Medium repetition sets with maximum weights require a larger fuel supply, resulting in an increased protein breakdown, which leads to a bigger rebound in energy stores and increased muscle growth.
DURING YOUR TRAINING SESSION
Lifting weights is a stress, and to counteract the stress and as part of the super compensation, hormones are released. Growth hormone, insulin, insulin-like growth factor, testosterone and cortisol are the hormones released after or during weight training.
Cortisol breaks carbohydrates down for fuel. Growth hormone, insulin, and insulin-like growth factor work together to inhibit the breakdown of muscle by increasing nutrient flow into the muscle. Growth hormone is released in greater amounts following exercise and controls the release of insulin-like growth factor, which is the primary anabolic hormone. Testosterone stimulates the nervous system to send stronger signals and signals the muscle to rebuild.
Growth hormone, insulin, and insulin-like growth factors are now being used as performance enhancers to increase muscle mass, and even though they are banned substances, they are difficult to detect.
LONG-TERM USE WILL HAVE SERIOUS SIDE EFFECTS
Proper nutrition is necessary, most specifically protein! Nutritionists recommend less than one gram of protein per kilogram of body weight or less to maintain health, but more than two to three grams of protein per kilogram have been recommended for weight gain. Protein is used for growth, repair and hormone production. Only twenty to thirty grams of protein can be used at any time except following exercise or fasting; additional protein ends up being stored as fat. This supports eating several small meals throughout the day.
The great Paul Anderson and Doug Hepburn and others famous for their muscle mass put an emphasis on liquid protein because liquid foods are easier to digest in greater quantities. The liquid protein can be consumed during or immediately following exercise. Michael Salvanti, who put a lot of effort into studying what it takes to increase muscle mass, gained twenty pounds in four months, after not gaining any weight in the previous four years, by drinking eight ounces of his protein shake.
Salvanti scheduled his shakes as a part of his training program. Consuming protein and carbohydrates right after lifting has also been shown to increase insulin release and speed up recovery.
THE IMPORTANCE OF STRESS ON THE BODY
BARBELL BENCH PRESS
Research has shown that in order to increase muscle mass, stress must be put on the body, leading to increased hormone release, and increased flow of nutrients into the muscle, and with rest, muscles will grow. If someone asked me what they should do to add some serious amounts of muscle, I would recommend three sets of 8 to 12 reps with all the weight that you can handle for the bench press, bent over rows, combined with a light set of 8x12 reps for full squats, followed by an all out set of 20 in the parallel squat.
I would finish each training session with stretching and cool down, light aerobic work and a large protein added shake. On non-lifting days it is essential that you rest and work out the kinks with a swim, or walk or light bike, but do nothing that involves impact. Best of luck and remember, "luck is when opportunity meets preparation."
When Do Muscles Grow After Working Out With Weights?
Last Updated: Mar 13, 2014 | By Ryan Haas
Lifting weights damages your muscles. Though that may sound strange, pumping iron at the gym actually depletes muscle-building nutrients in your body and creates microscopic tears in your muscle fibers. It is only after you workout that your body begins to repair the damage you inflicted on it and you experience the muscle growth you desire.
Len Kravitz, Ph.D. of the University of New Mexico states that muscle growth occurs in your body when the breakdown of muscle protein is less than the rate of muscle protein synthesis. Every time you lift weights, you are breaking down the proteins in your body, but also increasing their rate of repair for a minimum of two to four hours. Heavy weight training stimulates your protein synthesis for up to 24 hours, thus maximizing your muscle growth until your next workout.
In the book “Sport Nutrition for Health and Performance,” Melinda Manore et. al. describe the anabolic phase as the 45-minute period after your workout when your muscles go into overdrive to repair the damage from weight lifting. While very little actual muscle growth takes place during this phase, it is critical to consume carbohydrates and proteins in a ratio of 3:1 to limit the amount of muscle damage present. A post-workout meal or supplement shake during this window stimulates hormone release and sets the stage for increased protein synthesis.
A majority of the actual muscle growth in your body takes place during the rapid and sustained growth periods after the workout. The rapid growth period begins around the one-hour mark post-workout and lasts up to five hours after the workout. The sustained growth period is anywhere from five to 24 hours after you lift weights. Regularly consuming small meals and snacks of carbohydrates and proteins every two to three hours during the growth phases will help you keep your glycogen, amino acid and nitrogen levels elevated. All of these nutrients contribute to positive protein synthesis.
Doctors advocate sleeping for eight hours every night because this is a critical period for your body to recover and repair itself. Your body, however, runs out of muscle-building glycogen and protein while you sleep and enters a muscle breakdown stage called the catabolic state. In order to offset this breakdown, you should eat a dietary supplement or cottage cheese mixed with whey protein before bed. These products provide you with long-lasting fuel overnight to keep your muscle growth at its highest.