PROTEIN SYNTHESIS IN MUSCLES
MUSCLE PROTEIN SYNTHESIS
The synthesis of muscle protein is essential to the body’s ongoing growth, repair, and maintenance of its skeletal muscle groups. The other types of human muscle tissue, cardiac muscle and the smooth muscles that are part of internal organ structure, are constituted through different cellular processes.
Proteins are the compounds comprised of amino acids—the building blocks of tissue formation within the body. The synthesis of protein is the method by which muscles are constructed. The human body synthesizes protein from diet at a rapid rate while the body is growing through adolescence and into young adulthood. The rate at which protein is synthesized slows significantly after age 20. It is for this reason that even among active, highly trained adults, the actual rate of muscle growth will be far less in relative terms to that of a healthy teenager.
In an adult athlete, the synthesis of muscle protein is also related to how the muscles are being exercised. Other than the ongoing repair and maintenance of existing muscle tissues that may be damaged through the course of daily living, human skeletal muscle will never get larger or stronger through either sedentary activities or the consumption of particular foods or supplements. Muscular activity is a prerequisite of meaningful muscle development, built on protein synthesis.
All forms of physical activity will direct specific stress into a muscle. In a sport such as distance running or cycling, the stresses are cumulative, the combined effect of repetitive movements that are at a relatively lower level of intensity. In activities that involve explosive and powerfully focused movements (such as weight training) the forces directed against the muscle are much more significant, and they occur over a much shorter time span.
In each circumstance, the muscle will naturally break down, a process known as “catabolism.” The breakdown includes the physical separation of the fibers that comprise the muscle structure. The subsequent repair of the damaged muscle is “anabolism,” the building up and the growth of the existing and previously damaged fiber. Anabolic steroids take their name due to their contribution to the building up of muscles. Protein synthesis is the mechanism by which the body affects this repair and muscle growth: as a very general proposition, when the body produces more synthesized protein than it consumes through its catabolic processes, muscle will be developed.
There are measurable ways to determine the balance in the body on an ongoing basis between its catabolic and its anabolic states. The essential amino acid, leucine, is used as an indicator of the state of this balance in more sophisticated sports science analyses. A positive leucine balance is evidence that this acid is present in the cells, a condition consistent with protein anabolism. Leucine is a component of numerous commercial protein supplements taken to stimulate further protein synthesis in the body.
Muscle protein synthesis is also considered in the context of sore, damaged, or overused muscles. Sports research in this area has focused not so much on whether the ingestion of protein is likely to be useful to the athlete, but at what point after the exercise should the protein be consumed so that the body can derive a maximum restorative and muscle-building effect. The combined effects of carbohydrate consumption and protein consumption have also been thoroughly considered in recent years.
The sports science community supports the usefulness of carbohydrate replacement immediately after exercise. Such replacement tends to deliver glycogen to the affected muscles more quickly; in addition, the entire body has a replenished supply of carbohydrates from which the whole of the musculoskeletal system can be restored. When proteins are consumed along with carbohydrates immediately after exercise, the catabolic process is not stopped within the affected muscles; the process of protein synthesis is immediately stimulated (a kick-start is an expression commonly employed in the research to describe the effect). This action leads to the prevention of further protein loss in the muscle. As the degradation of the muscle due to strenuous exercise will not reach its peak for approximately three days after the exercise that affected the muscle, it is important to continue the ingestion of protein. The maintenance of consistent dietary practices is essential to the body’s ability to respond on an ongoing basis to the demand for muscle protein synthesis.
The body has a need to ensure effective muscle protein synthesis throughout the course of an athletic career. With the rise in masters level participation in a wide variety of sports (generally defined as competitions for athletes aged 40 years and older), older athletes are affected by catabolic and anabolic processes. The body’s response to the increased consumption of protein after exercise does not significantly vary with age, for either men or women.
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.
Exercise has a profound effect on muscle growth, which can occur only if muscle protein synthesis exceeds muscle protein breakdown; there must be a positive muscle protein balance. Resistance exercise improves muscle protein balance, but, in the absence of food intake, the balance remains negative (i.e., catabolic). 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. Amino acid availability is an important regulator of muscle protein metabolism. The interaction of postexercise metabolic processes and increased amino acid availability maximizes the stimulation of muscle protein synthesis and results in even greater muscle anabolism than when dietary amino acids are not present. Hormones, especially insulin and testosterone, have important roles as regulators of muscle protein synthesis and muscle hypertrophy. Following exercise, insulin has only a permissive role on muscle protein synthesis, but it appears to inhibit the increase in muscle protein breakdown. Ingestion of only small amounts of amino acids, combined with carbohydrates, can transiently increase muscle protein anabolism, but it has yet to be determined if these transient responses translate into an appreciable increase in muscle mass over a prolonged training period.
PROTEIN SYNTHESIS IN MUSCLE GROWTH
Muscles grow through protein synthesis. Despite what supplement companies will tell you, there is no magic formula that will supersede the basic science of protein synthesis. Once you understand how protein synthesis creates muscle growth, you can achieve your athletic goals, whether they are bodybuilding or fitness.
Muscles grow by repairing small micro-tears that occur on a cellular level during exercise, making exercise a key component of muscle growth, according to World of Sport Science. Resistance training is generally considered the best type of exercise to promote muscle growth. When the muscle experiences small micro-tears, blood flow to the area increases, bringing with it the necessary components for repair through protein synthesis. In this specific case, the repaired muscle is then stronger and larger than it was before.
For protein synthesis and muscle growth to occur, a number of components must be present. First, the muscle must have exercise-induced micro-injury. Second, naturally occurring hormones, including testosterone and growth hormones produced by the pituitary, must be present. Finally, you must have a diet containing sufficient protein. Protein is the basic building block of all of the body’s tissues, especially muscle. Proteins are made from amino acids, some of which the body can synthesize and some of which must be consumed in the diet.
Protein synthesis does not create new muscle cells. Instead, protein synthesis creates a state of hypertrophy. In hypertrophy, individual muscle cells increase in size. Bigger muscle cells are stronger and may give you a more esthetically pleasing appearance.
Recommendations for the best time to consume protein before and after a workout to increase muscle growth and protein synthesis vary from researcher to researcher. A 2001 article in the “International Journal of Sport Nutrition and Exercise Metabolism” noted a 24- to 48-hour window following resistance exercise when meals can affect muscle hypertrophy.
If muscle growth is your goal, you may want to avoid taking a post-workout over-the-counter pain reliever and soothe your sore muscles with a warm bath instead. According to a 2001 article published in the “American Journal of Physiology, Endocrinology and Metabolism,” over-the-counter doses of acetaminophen and ibuprofen can interfere with post-exercise protein synthesis.
HOW DO MUSCLES GROW?
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.